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Assignment operators assign values to JavaScript variables.

Shift Assignment Operators

Bitwise assignment operators, logical assignment operators, the = operator.

The Simple Assignment Operator assigns a value to a variable.

Simple Assignment Examples

The += operator.

The Addition Assignment Operator adds a value to a variable.

Addition Assignment Examples

The -= operator.

The Subtraction Assignment Operator subtracts a value from a variable.

Subtraction Assignment Example

The *= operator.

The Multiplication Assignment Operator multiplies a variable.

Multiplication Assignment Example

The **= operator.

The Exponentiation Assignment Operator raises a variable to the power of the operand.

Exponentiation Assignment Example

The /= operator.

The Division Assignment Operator divides a variable.

Division Assignment Example

The %= operator.

The Remainder Assignment Operator assigns a remainder to a variable.

Remainder Assignment Example

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The <<= Operator

The Left Shift Assignment Operator left shifts a variable.

Left Shift Assignment Example

The >>= operator.

The Right Shift Assignment Operator right shifts a variable (signed).

Right Shift Assignment Example

The >>>= operator.

The Unsigned Right Shift Assignment Operator right shifts a variable (unsigned).

Unsigned Right Shift Assignment Example

The &= operator.

The Bitwise AND Assignment Operator does a bitwise AND operation on two operands and assigns the result to the the variable.

Bitwise AND Assignment Example

The |= operator.

The Bitwise OR Assignment Operator does a bitwise OR operation on two operands and assigns the result to the variable.

Bitwise OR Assignment Example

The ^= operator.

The Bitwise XOR Assignment Operator does a bitwise XOR operation on two operands and assigns the result to the variable.

Bitwise XOR Assignment Example

The &&= operator.

The Logical AND assignment operator is used between two values.

If the first value is true, the second value is assigned.

Logical AND Assignment Example

The &&= operator is an ES2020 feature .

The ||= Operator

The Logical OR assignment operator is used between two values.

If the first value is false, the second value is assigned.

Logical OR Assignment Example

The ||= operator is an ES2020 feature .

The ??= Operator

The Nullish coalescing assignment operator is used between two values.

If the first value is undefined or null, the second value is assigned.

Nullish Coalescing Assignment Example

The ??= operator is an ES2020 feature .

Test Yourself With Exercises

Use the correct assignment operator that will result in x being 15 (same as x = x + y ).

Start the Exercise

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JavaScript Variables – A Beginner's Guide to var, const, and let

Variables are a fundamental concept in any programming language. In JavaScript, you can declare variables by using the keywords var, const, or let.

In this article, you’ll learn why we use variables, how to use them, and the differences between const, let and var.

What are variables used for in JavaScript?

In the context of coding, data is information that we use in our computer programs. For example, your Twitter username is a piece of data.

Much of programming is about manipulating or displaying data. In order to do this, programmers need a way of storing and keeping track of data. Let's demonstrate this with an example.

First we’ll open up our JavaScript console. To launch your JavaScript console on Chrome, you can use the shortcut Ctrl + Shift + J on Windows and Linux. For Mac, use Cmd + Option + J.

Once the console has launched, think of your dog or cat’s current age (or any similar number if you don't have pets) and enter it into the console.

Now what if we want to refer to that number again? We’ll have to type it out for a second time.

We need a way to refer to this piece of data so we can reuse it throughout our program.

Introducing variables in JavaScript

A helpful analogy is to think of variables as labels for our values. Think of a container of blueberries with a label on it marked blueberries. In this example, the variable, blueberries , points to a value, which is the blueberries themselves.

Let's declare a variable, age, and use the assignment operator (the equals sign) to assign our value, 4, to this variable. We’ll use the var keyword.

Variables are how programmers give a name to a value so that we can reuse it, update it, or simply keep track of it. Variables can be used to store any JavaScript type.

Now that we’ve assigned this value to the variable age, we can refer back to this value later. If you now type in the variable age in your console, you’ll have the value of 4 returned back to you.

How to use the var keyword in JavaScript

Keywords in JavaScript are reserved words. When you use the var keyword, you’re telling JavaScript you’ll be declaring a variable.

When using the var keyword, variables can be reassigned. We’ll demonstrate this by first declaring a new variable, name, and assigning it to the value of Madison.

Next, we’ll reassign this variable to point to the value of a different name, Ben.

Now if you run console.log(name) you’ll get the output of Ben.

When using the var keyword, variables can also be declared with no initial value.

Here, we’ve declared a variable year but it does not point to any value. Later on if we want it to point to a value, we can use the assignment operator to do so.

Now our variable year will point to the value of 2020.

When JavaScript was first created, the only way to declare a variable was with the var keyword.

In recent updates to JavaScript (ECMAScript2015), const and let were created as other keywords to declared variables.

To explain why they were needed, we’ll look at problems with the var keyword. In order to look at these problems, we’ll learn about what scope is.

What is scope?

Scope refers to where in our code variables are available for use. When a variable is globally scoped , that means it is available anywhere in your program. Let’s look at an example.

Take the following code and enter it into your console.

Here we’ve created and called a function, printName, that will print the value of the name var, Madison . You’ll see this printed in your console.

Because our var was created outside of the function, it is globally scoped. This means that it is available anywhere in your code, including inside of any function. This is why our function, printName, has access to the name var.

Let’s now create a variable that is function-scoped. This means that the variable is only accessible inside the function it was created in. This next example will be very similar to the code above, but with a different placement of the variable.

Now in our console we’ll get an error: year is not defined. This is because the var year is function-scoped. That is, it only exists inside of the function it was created in. We don’t have access to it outside of the function, which is where we’re trying to access it when we run our console.log.

Function-scoped variables are helpful to programmers because we often want to create variables that are only useful or needed inside a certain function. Creating global variables can also lead to errors or mistakes.

Now that we have a basic understanding of scope, we can return to our discussion of problems with the var keyword.

Problems with the var keyword in JavaScript

Let's look at another example.

We'll create a variable, age . Next we’ll write an if statement that checks if age has a value, and if it does, returns a console.log of the number that is double that age.

This is a simplified example, but we’ll first check if age has a value because we want to make sure we are adding to a valid value.

Now in our console, you’ll see Double your current age is 47 .

Our variable, doubleAge , is now a global variable. If you enter doubleAge into your console, you’ll see that you have access to it.

As previously discussed, variables created with the var keyword are function-scoped. Function-scoped variables only exist inside of the function they were created in.

But since the doubleAge variable is not inside a function, that means it has been scoped globally. That is, the doubleAge variable is now available anywhere in our code.

The problem is, doubleAge is just a variable we used once inside of our if statement , and we don’t necessarily need it available everywhere in our code. It has “leaked” outside of the if statement it was created in, even though we didn’t need it to.

It would be great if we had a way of creating a variable that *only* existed inside the if statement it was created in. In other words, the block of code that exists in between the curly brackets.

To help fix this problem, the const and let keywords were introduced in JavaScript.

How to use the const keyword in JavaScript

const works similarly to var, but with a few big differences.

First, const is block -scoped, whereas var is function -scoped.

What is a block ?

A block refers to any space between an opening and closing bracket. This might seem confusing at first. Let's write out our previous example, but this time using const instead of let when declaring our doubleAge variable.

Now, type doubleAge into your console and hit enter. You should get an error, doubleAge is not defined. This is because const is block-scoped: it only exists in the block it was defined.

The   doubleAge variable is ‘trapped’ inside the two curly brackets it was defined in. Code that is also inside those brackets can access doubleAge, but no code outside of it can.

By using const instead of var , our previous problem is fixed. Our doubleAge var is no longer “leaking” into our global scope unnecessarily. Instead, it only exists inside of the block it was created in.

How do block-scoped variables work within the context of functions? To learn about this, let's create and then call a function, returnX.

By calling this function returnX , we can see that our function returns the value of x, which is 1.

If we next type in x , we’ll get back referenceError: x is not defined . This is because functions are also considered blocks, so our const x will exist only within the function.

The next thing to know about const is that it can only ever be declared once. Type this code into your console:

You should see an error,   Identifier 'x' has already been declared.

This is a difference between var and const. While const will give you an error, letting you know that you’ve already declared this variable, the var keyword won’t.

The variable x will point to the value of 2 without an error. This can cause bugs for you as a programmer, as perhaps you did not mean to reassign your value to a new variable. Thus, using const can help you as you’ll receive an error if you accidentally try to reassign a variable.

This is a strength of the const keyword that was introduced as an updated and better way of creating variables in JavaScript. However, what about the times when you do want to update your variable?

Let's look at an example that shows why we would want to do this.

Let's declare a variable, adult , and set it to false . We’ll also create an age variable and set it to 20 .

const adult = false

const age = 20.

Say we want to check a user’s age, and set our adult variable to false if age is over 18. We can write an if statement to do this.

What happens when we run this code?

Here we’ll see Error: Assignment to constant variable.

This is because, in accordance with the rules of const , we cannot redeclare this variable. That is, our variable age is already pointing to the value of true, and we cannot now point it to something else.  

If we print out adult again, we can see that it has stayed the same and still holds the value of false .

We cannot reassign our age variable, and const is working as expected. However, what if we do want to reassign this variable?

Often times programmers will want to be able to redeclare their variables.

This is where our third keyword, let, comes in.

How to use the let keyword in JavaScript

First let’s go over how let is similar to const .

Let , like const , is block-scoped. If you replaced const with let in our above doubleAge example, it would work the same.

However, let differs from const in a fundament way. Variables declared with the let keyword can be redeclared, while variables created with the const keyword cannot. Let’s go over an example.

Using our same example above, replace const with let. We’ll keep our age variable as a const with the value of 20 .

Now if we type out adult , instead of getting an error as we previously did, we’ll see the output of true .

By using the let keyword, we’ve updated our variable to point to the value of true as we wanted to. Sometimes in programming we’ll want to update our variable depending on certain data that we receive. We can use let to do this.

Wrapping up

In summary, we’ve learned that variables are used to keep track of and reuse data in our computer programs. Scope refers to where in our code variables are available for use.

Variables can be declared using var, const, or let. Var is function-scoped, while const and let are block-scoped. Const variables cannot be reassigned, while let variables can be.  

Var, const, and let can be confusing at first. It can help to read different tutorials on them, as well as test out your own code in different ways to solidify your understanding.

Having a strong foundation of var, const, and let will help you not just at the start of your JavaScript career but throughout its entirety.

A real-life analogy

We can easily grasp the concept of a “variable” if we imagine it as a “box” for data, with a uniquely-named sticker on it.

For instance, the variable message can be imagined as a box labelled "message" with the value "Hello!" in it:

We can put any value in the box.

We can also change it as many times as we want:

When the value is changed, the old data is removed from the variable:

We can also declare two variables and copy data from one into the other.

A variable should be declared only once.

A repeated declaration of the same variable is an error:

So, we should declare a variable once and then refer to it without let .

It’s interesting to note that there exist so-called pure functional programming languages, such as Haskell , that forbid changing variable values.

In such languages, once the value is stored “in the box”, it’s there forever. If we need to store something else, the language forces us to create a new box (declare a new variable). We can’t reuse the old one.

Though it may seem a little odd at first sight, these languages are quite capable of serious development. More than that, there are areas like parallel computations where this limitation confers certain benefits.

Variable naming

There are two limitations on variable names in JavaScript:

• The name must contain only letters, digits, or the symbols $ and _ .
• The first character must not be a digit.

Examples of valid names:

When the name contains multiple words, camelCase is commonly used. That is: words go one after another, each word except first starting with a capital letter: myVeryLongName .

What’s interesting – the dollar sign '$' and the underscore '_' can also be used in names. They are regular symbols, just like letters, without any special meaning.

These names are valid:

Examples of incorrect variable names:

Variables named apple and APPLE are two different variables.

It is possible to use any language, including Cyrillic letters, Chinese logograms and so on, like this:

Technically, there is no error here. Such names are allowed, but there is an international convention to use English in variable names. Even if we’re writing a small script, it may have a long life ahead. People from other countries may need to read it sometime.

There is a list of reserved words , which cannot be used as variable names because they are used by the language itself.

For example: let , class , return , and function are reserved.

The code below gives a syntax error:

Normally, we need to define a variable before using it. But in the old times, it was technically possible to create a variable by a mere assignment of the value without using let . This still works now if we don’t put use strict in our scripts to maintain compatibility with old scripts.

This is a bad practice and would cause an error in strict mode:

To declare a constant (unchanging) variable, use const instead of let :

Variables declared using const are called “constants”. They cannot be reassigned. An attempt to do so would cause an error:

When a programmer is sure that a variable will never change, they can declare it with const to guarantee and communicate that fact to everyone.

Uppercase constants

There is a widespread practice to use constants as aliases for difficult-to-remember values that are known before execution.

Such constants are named using capital letters and underscores.

For instance, let’s make constants for colors in so-called “web” (hexadecimal) format:

• COLOR_ORANGE is much easier to remember than "#FF7F00" .
• It is much easier to mistype "#FF7F00" than COLOR_ORANGE .
• When reading the code, COLOR_ORANGE is much more meaningful than #FF7F00 .

When should we use capitals for a constant and when should we name it normally? Let’s make that clear.

Being a “constant” just means that a variable’s value never changes. But some constants are known before execution (like a hexadecimal value for red) and some constants are calculated in run-time, during the execution, but do not change after their initial assignment.

For instance:

The value of pageLoadTime is not known before the page load, so it’s named normally. But it’s still a constant because it doesn’t change after the assignment.

In other words, capital-named constants are only used as aliases for “hard-coded” values.

Name things right

Talking about variables, there’s one more extremely important thing.

A variable name should have a clean, obvious meaning, describing the data that it stores.

Variable naming is one of the most important and complex skills in programming. A glance at variable names can reveal which code was written by a beginner versus an experienced developer.

In a real project, most of the time is spent modifying and extending an existing code base rather than writing something completely separate from scratch. When we return to some code after doing something else for a while, it’s much easier to find information that is well-labelled. Or, in other words, when the variables have good names.

Please spend time thinking about the right name for a variable before declaring it. Doing so will repay you handsomely.

Some good-to-follow rules are:

• Use human-readable names like userName or shoppingCart .
• Stay away from abbreviations or short names like a , b , and c , unless you know what you’re doing.
• Make names maximally descriptive and concise. Examples of bad names are data and value . Such names say nothing. It’s only okay to use them if the context of the code makes it exceptionally obvious which data or value the variable is referencing.
• Agree on terms within your team and in your mind. If a site visitor is called a “user” then we should name related variables currentUser or newUser instead of currentVisitor or newManInTown .

Sounds simple? Indeed it is, but creating descriptive and concise variable names in practice is not. Go for it.

And the last note. There are some lazy programmers who, instead of declaring new variables, tend to reuse existing ones.

As a result, their variables are like boxes into which people throw different things without changing their stickers. What’s inside the box now? Who knows? We need to come closer and check.

Such programmers save a little bit on variable declaration but lose ten times more on debugging.

An extra variable is good, not evil.

Modern JavaScript minifiers and browsers optimize code well enough, so it won’t create performance issues. Using different variables for different values can even help the engine optimize your code.

We can declare variables to store data by using the var , let , or const keywords.

• let – is a modern variable declaration.
• var – is an old-school variable declaration. Normally we don’t use it at all, but we’ll cover subtle differences from let in the chapter The old "var" , just in case you need them.
• const – is like let , but the value of the variable can’t be changed.

Variables should be named in a way that allows us to easily understand what’s inside them.

Working with variables

• Declare two variables: admin and name .
• Assign the value "John" to name .
• Copy the value from name to admin .
• Show the value of admin using alert (must output “John”).

In the code below, each line corresponds to the item in the task list.

Giving the right name

• Create a variable with the name of our planet. How would you name such a variable?
• Create a variable to store the name of a current visitor to a website. How would you name that variable?

The variable for our planet

That’s simple:

Note, we could use a shorter name planet , but it might not be obvious what planet it refers to. It’s nice to be more verbose. At least until the variable isNotTooLong.

The name of the current visitor

Again, we could shorten that to userName if we know for sure that the user is current.

Modern editors and autocomplete make long variable names easy to write. Don’t save on them. A name with 3 words in it is fine.

And if your editor does not have proper autocompletion, get a new one .

Uppercase const?

Examine the following code:

Here we have a constant birthday for the date, and also the age constant.

The age is calculated from birthday using someCode() , which means a function call that we didn’t explain yet (we will soon!), but the details don’t matter here, the point is that age is calculated somehow based on the birthday .

Would it be right to use upper case for birthday ? For age ? Or even for both?

We generally use upper case for constants that are “hard-coded”. Or, in other words, when the value is known prior to execution and directly written into the code.

In this code, birthday is exactly like that. So we could use the upper case for it.

In contrast, age is evaluated in run-time. Today we have one age, a year after we’ll have another one. It is constant in a sense that it does not change through the code execution. But it is a bit “less of a constant” than birthday : it is calculated, so we should keep the lower case for it.

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A Guide to Variable Assignment and Mutation in JavaScript

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Variable Assignment

Variable reassignment, variable assignment by reference, copying by reference, the spread operator to the rescue, are mutations bad, frequently asked questions (faqs) about javascript variable assignment and mutation.

Mutations are something you hear about fairly often in the world of JavaScript, but what exactly are they, and are they as evil as they’re made out to be?

In this article, we’re going to cover the concepts of variable assignment and mutation and see why — together — they can be a real pain for developers. We’ll look at how to manage them to avoid problems, how to use as few as possible, and how to keep your code predictable.

If you’d like to explore this topic in greater detail, or get up to speed with modern JavaScript, check out the first chapter of my new book Learn to Code with JavaScript for free.

Let’s start by going back to the very basics of value types …

Every value in JavaScript is either a primitive value or an object. There are seven different primitive data types:

• numbers, such as 3 , 0 , -4 , 0.625
• strings, such as 'Hello' , "World" , `Hi` , ''
• Booleans, true and false
• symbols — a unique token that’s guaranteed never to clash with another symbol
• BigInt — for dealing with large integer values

Anything that isn’t a primitive value is an object , including arrays, dates, regular expressions and, of course, object literals. Functions are a special type of object. They are definitely objects, since they have properties and methods, but they’re also able to be called.

Variable assignment is one of the first things you learn in coding. For example, this is how we would assign the number 3 to the variable bears :

A common metaphor for variables is one of boxes with labels that have values placed inside them. The example above would be portrayed as a box containing the label “bears” with the value of 3 placed inside.

An alternative way of thinking about what happens is as a reference, that maps the label bears to the value of 3 :

If I assign the number 3 to another variable, it’s referencing the same value as bears:

The variables bears and musketeers both reference the same primitive value of 3. We can verify this using the strict equality operator, === :

The equality operator returns true if both variables are referencing the same value.

Some gotchas when working with objects

The previous examples showed primitive values being assigned to variables. The same process is used when assigning objects:

This assignment means that the variable ghostbusters references an object:

A big difference when assigning objects to variables, however, is that if you assign another object literal to another variable, it will reference a completely different object — even if both object literals look exactly the same! For example, the assignment below looks like the variable tmnt (Teenage Mutant Ninja Turtles) references the same object as the variable ghostbusters :

Even though the variables ghostbusters and tmnt look like they reference the same object, they actually both reference a completely different object, as we can see if we check with the strict equality operator:

When the const keyword was introduced in ES6, many people mistakenly believed that constants had been introduced to JavaScript, but this wasn’t the case. The name of this keyword is a little misleading.

Any variable declared with const can’t be reassigned to another value. This goes for primitive values and objects. For example, the variable bears was declared using const in the previous section, so it can’t have another value assigned to it. If we try to assign the number 2 to the variable bears , we get an error:

The reference to the number 3 is fixed and the bears variable can’t be reassigned another value.

The same applies to objects. If we try to assign a different object to the variable ghostbusters , we get the same error:

Variable reassignment using let

When the keyword let is used to declare a variable, it can be reassigned to reference a different value later on in our code. For example, we declared the variable musketeers using let , so we can change the value that musketeers references. If D’Artagnan joined the Musketeers, their number would increase to 4:

This can be done because let was used to declare the variable. We can alter the value that musketeers references as many times as we like.

The variable tmnt was also declared using let , so it can also be reassigned to reference another object (or a different type entirely if we like):

Note that the variable tmnt now references a completely different object ; we haven’t just changed the number property to 5.

In summary , if you declare a variable using const , its value can’t be reassigned and will always reference the same primitive value or object that it was originally assigned to. If you declare a variable using let , its value can be reassigned as many times as required later in the program.

Using const as often as possible is generally considered good practice, as it means that the value of variables remains constant and the code is more consistent and predictable, making it less prone to errors and bugs.

In native JavaScript, you can only assign values to variables. You can’t assign variables to reference another variable, even though it looks like you can. For example, the number of Stooges is the same as the number of Musketeers, so we can assign the variable stooges to reference the same value as the variable musketeers using the following:

This looks like the variable stooges is referencing the variable musketeers , as shown in the diagram below:

However, this is impossible in native JavaScript: a variable can only reference an actual value; it can’t reference another variable . What actually happens when you make an assignment like this is that the variable on the left of the assignment will reference the value the variable on the right references, so the variable stooges will reference the same value as the musketeers variable, which is the number 3. Once this assignment has been made, the stooges variable isn’t connected to the musketeers variable at all.

This means that if D’Artagnan joins the Musketeers and we set the value of the musketeers to 4, the value of stooges will remain as 3. In fact, because we declared the stooges variable using const , we can’t set it to any new value; it will always be 3.

In summary : if you declare a variable using const and set it to a primitive value, even via a reference to another variable, then its value can’t change. This is good for your code, as it means it will be more consistent and predictable.

A value is said to be mutable if it can be changed. That’s all there is to it: a mutation is the act of changing the properties of a value.

All primitive value in JavaScript are immutable : you can’t change their properties — ever. For example, if we assign the string "cake" to variable food , we can see that we can’t change any of its properties:

If we try to change the first letter to “f”, it looks like it has changed:

But if we take a look at the value of the variable, we see that nothing has actually changed:

The same thing happens if we try to change the length property:

Despite the return value implying that the length property has been changed, a quick check shows that it hasn’t:

Note that this has nothing to do with declaring the variable using const instead of let . If we had used let , we could set food to reference another string, but we can’t change any of its properties. It’s impossible to change any properties of primitive data types because they’re immutable .

Mutability and objects in JavaScript

Conversely, all objects in JavaScript are mutable, which means that their properties can be changed, even if they’re declared using const (remember let and const only control whether or not a variable can be reassigned and have nothing to do with mutability). For example, we can change the the first item of an array using the following code:

Note that this change still occurred, despite the fact that we declared the variable food using const . This shows that using const does not stop objects from being mutated .

We can also change the length property of an array, even if it has been declared using const :

Remember that when we assign variables to object literals, the variables will reference completely different objects, even if they look the same:

But if we assign a variable fantastic4 to another variable, they will both reference the same object:

This assigns the variable fantastic4 to reference the same object that the variable tmnt references, rather than a completely different object.

This is often referred to as copying by reference , because both variables are assigned to reference the same object.

This is important, because any mutations made to this object will be seen in both variables.

So, if Spider-Man joins The Fantastic Four, we might update the number value in the object:

This is a mutation, because we’ve changed the number property rather than setting fantastic4 to reference a new object.

This causes us a problem, because the number property of tmnt will also also change, possibly without us even realizing:

This is because both tmnt and fantastic4 are referencing the same object, so any mutations that are made to either tmnt or fantastic4 will affect both of them.

This highlights an important concept in JavaScript: when objects are copied by reference and subsequently mutated, the mutation will affect any other variables that reference that object. This can lead to unintended side effects and bugs that are difficult to track down.

So how do you make a copy of an object without creating a reference to the original object? The answer is to use the spread operator !

The spread operator was introduced for arrays and strings in ES2015 and for objects in ES2018. It allows you to easily make a shallow copy of an object without creating a reference to the original object.

The example below shows how we could set the variable fantastic4 to reference a copy of the tmnt object. This copy will be exactly the same as the tmnt object, but fantastic4 will reference a completely new object. This is done by placing the name of the variable to be copied inside an object literal with the spread operator in front of it:

What we’ve actually done here is assign the variable fantastic4 to a new object literal and then used the spread operator to copy all the enumerable properties of the object referenced by the tmnt variable. Because these properties are values, they’re copied into the fantastic4 object by value, rather than by reference.

Now any changes that are made to either object won’t affect the other. For example, if we update the number property of the fantastic4 variable to 5, it won’t affect the tmnt variable:

The spread operator also has a useful shortcut notation that can be used to make copies of an object and then make some changes to the new object in a single line of code.

For example, say we wanted to create an object to model the Teenage Mutant Ninja Turtles. We could create the first turtle object, and assign the variable leonardo to it:

The other turtles all have the same properties, except for the weapon and color properties, that are different for each turtle. It makes sense to make a copy of the object that leonardo references, using the spread operator, and then change the weapon and color properties, like so:

We can do this in one line by adding the properties we want to change after the reference to the spread object. Here’s the code to create new objects for the variables donatello and raphael :

Note that using the spread operator in this way only makes a shallow copy of an object. To make a deep copy, you’d have to do this recursively, or use a library. Personally, I’d advise that you try to keep your objects as shallow as possible.

In this article, we’ve covered the concepts of variable assignment and mutation and seen why — together — they can be a real pain for developers.

Mutations have a bad reputation, but they’re not necessarily bad in themselves. In fact, if you’re building a dynamic web app, it must change at some point. That’s literally the meaning of the word “dynamic”! This means that there will have to be some mutations somewhere in your code. Having said that, the fewer mutations there are, the more predictable your code will be, making it easier to maintain and less likely to develop any bugs.

A particularly toxic combination is copying by reference and mutations. This can lead to side effects and bugs that you don’t even realize have happened. If you mutate an object that’s referenced by another variable in your code, it can cause lots of problems that can be difficult to track down. The key is to try and minimize your use of mutations to the essential and keep track of which objects have been mutated.

In functional programming, a pure function is one that doesn’t cause any side effects, and mutations are one of the biggest causes of side effects.

A golden rule is to avoid copying any objects by reference. If you want to copy another object, use the spread operator and then make any mutations immediately after making the copy.

Next up, we’ll look into array mutations in JavaScript .

Don’t forget to check out my new book Learn to Code with JavaScript if you want to get up to speed with modern JavaScript. You can read the first chapter for free. And please reach out on Twitter if you have any questions or comments!

What is the difference between variable assignment and mutation in JavaScript?

In JavaScript, variable assignment refers to the process of assigning a value to a variable. For example, let x = 5; Here, we are assigning the value 5 to the variable x. On the other hand, mutation refers to the process of changing the value of an existing variable. For example, if we later write x = 10; we are mutating the variable x by changing its value from 5 to 10.

How does JavaScript handle variable assignment and mutation differently for primitive and non-primitive data types?

JavaScript treats primitive data types (like numbers, strings, and booleans) and non-primitive data types (like objects and arrays) differently when it comes to variable assignment and mutation. For primitive data types, when you assign a variable, a copy of the value is created and stored in a new memory location. However, for non-primitive data types, when you assign a variable, both variables point to the same memory location. Therefore, if you mutate one variable, the change is reflected in all variables that point to that memory location.

What is the concept of pass-by-value and pass-by-reference in JavaScript?

Pass-by-value and pass-by-reference are two ways that JavaScript can pass variables to a function. When JavaScript passes a variable by value, it creates a copy of the variable’s value and passes that copy to the function. Any changes made to the variable inside the function do not affect the original variable. However, when JavaScript passes a variable by reference, it passes a reference to the variable’s memory location. Therefore, any changes made to the variable inside the function also affect the original variable.

How can I prevent mutation in JavaScript?

There are several ways to prevent mutation in JavaScript. One way is to use the Object.freeze() method, which prevents new properties from being added to an object, existing properties from being removed, and prevents changing the enumerability, configurability, or writability of existing properties. Another way is to use the const keyword when declaring a variable. This prevents reassignment of the variable, but it does not prevent mutation of the variable’s value if the value is an object or an array.

What is the difference between shallow copy and deep copy in JavaScript?

In JavaScript, a shallow copy of an object is a copy of the object where the values of the original object and the copy point to the same memory location for non-primitive data types. Therefore, if you mutate the copy, the original object is also mutated. On the other hand, a deep copy of an object is a copy of the object where the values of the original object and the copy do not point to the same memory location. Therefore, if you mutate the copy, the original object is not mutated.

How can I create a deep copy of an object in JavaScript?

One way to create a deep copy of an object in JavaScript is to use the JSON.parse() and JSON.stringify() methods. The JSON.stringify() method converts the object into a JSON string, and the JSON.parse() method converts the JSON string back into an object. This creates a new object that is a deep copy of the original object.

What is the MutationObserver API in JavaScript?

The MutationObserver API provides developers with a way to react to changes in a DOM. It is designed to provide a general, efficient, and robust API for reacting to changes in a document.

How does JavaScript handle variable assignment and mutation in the context of closures?

In JavaScript, a closure is a function that has access to its own scope, the scope of the outer function, and the global scope. When a variable is assigned or mutated inside a closure, it can affect the value of the variable in the outer scope, depending on whether the variable was declared in the closure’s scope or the outer scope.

What is the difference between var, let, and const in JavaScript?

In JavaScript, var, let, and const are used to declare variables. var is function scoped, and if it is declared outside a function, it is globally scoped. let and const are block scoped, meaning they exist only within the block they are declared in. The difference between let and const is that let allows reassignment, while const does not.

How does JavaScript handle variable assignment and mutation in the context of asynchronous programming?

In JavaScript, asynchronous programming allows multiple things to happen at the same time. When a variable is assigned or mutated in an asynchronous function, it can lead to unexpected results if other parts of the code are relying on the value of the variable. This is because the variable assignment or mutation may not have completed before the other parts of the code run. To handle this, JavaScript provides several features, such as promises and async/await, to help manage asynchronous code.

Darren loves building web apps and coding in JavaScript, Haskell and Ruby. He is the author of Learn to Code using JavaScript , JavaScript: Novice to Ninja and Jump Start Sinatra .He is also the creator of Nanny State , a tiny alternative to React. He can be found on Twitter @daz4126.

Home » JavaScript Tutorial » JavaScript Variables

JavaScript Variables

Summary : in this tutorial, you’ll learn about JavaScript variables and how to use variables to store values in the application.

A variable is a label that references a value like a number or string. Before using a variable, you need to declare it.

Declare a variable

To declare a variable, you use the var keyword followed by the variable name as follows:

A variable name can be any valid identifier. By default, the message variable has a special value undefined if you have not assigned a value to it.

Variable names follow these rules:

• Variable names are case-sensitive. This means that the message and Message are different variables.
• Variable names can only contain letters, numbers, underscores, or dollar signs and cannot contain spaces. Also, variable names must begin with a letter, an underscore ( _ ) or a dollar sign ( $) .
• Variable names cannot use the reserved words.

By convention, variable names use camelcase like message , yourAge , and myName .

JavaScript is a dynamically typed language. This means that you don’t need to specify the variable’s type in the declaration like other static-typed languages such as Java or C# .

Starting in ES6, you can use the let keyword to declare a variable like this:

It’s a good practice to use the let keyword to declare a variable. Later, you’ll learn the differences between var and let keywords . And you should not worry about it for now.

Initialize a variable

Once you have declared a variable, you can initialize it with a value. To initialize a variable, you specify the variable name, followed by an equals sign ( = ) and a value.

For example, The following declares the message variable and initializes it with a literal string "Hello" :

To declare and initialize a variable at the same time, you use the following syntax:

For example, the following statement declares the message variable and initializes it with the literal string "Hello" :

JavaScript allows you to declare two or more variables using a single statement. To separate two variable declarations, you use a comma ( , ) like this:

Since JavaScript is a dynamically typed language, you can assign a value of a different type to a variable. Although, it is not recommended. For example:

Change a variable

Once you initialize a variable, you can change its value by assigning a different value. For example:

Undefined vs. undeclared variables

It’s important to distinguish between undefined and undeclared variables.

An undefined variable is a variable that has been declared but has not been initialized with a value. For example:

In this example, the message variable is declared but not initialized. Therefore, the message variable is undefined.

In contrast, an undeclared variable is a variable that has not been declared. For example:

In this example, the counter variable has not been declared. Hence, accessing it causes a ReferenceError .

A constant holds a value that doesn’t change. To declare a constant, you use the const keyword. When defining a constant, you need to initialize it with a value. For example:

Once you define a constant, you cannot change its value.

The following example attempts to change the value of the workday constant to 4 and causes an error:

Later, you’ll learn that the const keyword actually defines a read-only reference to a value in the constants tutorial.

• A variable is a label that references a value.
• Use the let keyword to declare a variable.
• An undefined variable is a variable that has been declared but not initialized while an undeclared variable is a variable that has not been declared.
• Use the const keyword to define a readonly reference to a value.

11 Variables and assignment

11.1  let, 11.2.1  const and immutability, 11.2.2  const and loops.

• 11.3  Deciding between const and let
• 11.4.1  Shadowing variables
• 11.5  (Advanced)
• 11.6.1  Static phenomenon: scopes of variables
• 11.6.2  Dynamic phenomenon: function calls

11.7.1  globalThis [ES2020]

11.8.1  const and let : temporal dead zone.

• 11.8.2  Function declarations and early activation
• 11.8.3  Class declarations are not activated early

11.8.4  var : hoisting (partial early activation)

• 11.9.1  Bound variables vs. free variables
• 11.9.2  What is a closure?
• 11.9.3  Example: A factory for incrementors
• 11.9.4  Use cases for closures

These are JavaScript’s main ways of declaring variables:

• let declares mutable variables.
• const declares constants (immutable variables).

Before ES6, there was also var . But it has several quirks, so it’s best to avoid it in modern JavaScript. You can read more about it in Speaking JavaScript .

Variables declared via let are mutable:

You can also declare and assign at the same time:

11.2  const

Variables declared via const are immutable. You must always initialize immediately:

In JavaScript, const only means that the binding (the association between variable name and variable value) is immutable. The value itself may be mutable, like obj in the following example.

You can use const with for-of loops, where a fresh binding is created for each iteration:

In plain for loops, you must use let , however:

11.3 Deciding between const and let

I recommend the following rules to decide between const and let :

• const indicates an immutable binding and that a variable never changes its value. Prefer it.
• let indicates that the value of a variable changes. Use it only when you can’t use const .

exercises/variables-assignment/const_exrc.mjs

11.4 The scope of a variable

The scope of a variable is the region of a program where it can be accessed. Consider the following code.

• Scope A is the (direct) scope of x .
• Scopes B and C are inner scopes of scope A.
• Scope A is an outer scope of scope B and scope C.

Each variable is accessible in its direct scope and all scopes nested within that scope.

The variables declared via const and let are called block-scoped because their scopes are always the innermost surrounding blocks.

11.4.1 Shadowing variables

You can’t declare the same variable twice at the same level:

eval() delays parsing (and therefore the SyntaxError ), until the callback of assert.throws() is executed. If we didn’t use it, we’d already get an error when this code is parsed and assert.throws() wouldn’t even be executed.

You can, however, nest a block and use the same variable name x that you used outside the block:

Inside the block, the inner x is the only accessible variable with that name. The inner x is said to shadow the outer x . Once you leave the block, you can access the old value again.

See quiz app .

11.5 (Advanced)

All remaining sections are advanced.

11.6 Terminology: static vs. dynamic

These two adjectives describe phenomena in programming languages:

• Static means that something is related to source code and can be determined without executing code.
• Dynamic means at runtime.

Let’s look at examples for these two terms.

11.6.1 Static phenomenon: scopes of variables

Variable scopes are a static phenomenon. Consider the following code:

x is statically (or lexically ) scoped . That is, its scope is fixed and doesn’t change at runtime.

Variable scopes form a static tree (via static nesting).

11.6.2 Dynamic phenomenon: function calls

Function calls are a dynamic phenomenon. Consider the following code:

Whether or not the function call in line A happens, can only be decided at runtime.

Function calls form a dynamic tree (via dynamic calls).

11.7 Global variables and the global object

JavaScript’s variable scopes are nested. They form a tree:

• The outermost scope is the root of the tree.
• The scopes directly contained in that scope are the children of the root.

The root is also called the global scope . In web browsers, the only location where one is directly in that scope is at the top level of a script. The variables of the global scope are called global variables and accessible everywhere. There are two kinds of global variables:

• They can only be created while at the top level of a script, via const , let , and class declarations.
• They are created in the top level of a script, via var and function declarations.
• The global object can be accessed via the global variable globalThis . It can be used to create, read, and delete global object variables.
• Other than that, global object variables work like normal variables.

The following HTML fragment demonstrates globalThis and the two kinds of global variables.

Each ECMAScript module has its own scope. Therefore, variables that exist at the top level of a module are not global. Fig.  5 illustrates how the various scopes are related.

The global variable globalThis is the new standard way of accessing the global object. It got its name from the fact that it has the same value as this in global scope.

For example, in browsers, there is an indirection . That indirection is normally not noticable, but it is there and can be observed.

11.7.1.1 Alternatives to globalThis

Older ways of accessing the global object depend on the platform:

• Global variable window : is the classic way of referring to the global object. But it doesn’t work in Node.js and in Web Workers.
• Global variable self : is available in Web Workers and browsers in general. But it isn’t supported by Node.js.
• Global variable global : is only available in Node.js.

11.7.1.2 Use cases for globalThis

The global object is now considered a mistake that JavaScript can’t get rid of, due to backward compatibility. It affects performance negatively and is generally confusing.

ECMAScript 6 introduced several features that make it easier to avoid the global object – for example:

• const , let , and class declarations don’t create global object properties when used in global scope.
• Each ECMAScript module has its own local scope.

It is usually better to access global object variables via variables and not via properties of globalThis . The former has always worked the same on all JavaScript platforms.

Tutorials on the web occasionally access global variables globVar via window.globVar . But the prefix “ window. ” is not necessary and I recommend to omit it:

Therefore, there are relatively few use cases for globalThis – for example:

• Polyfills that add new features to old JavaScript engines.
• Feature detection, to find out what features a JavaScript engine supports.

11.8 Declarations: scope and activation

These are two key aspects of declarations:

• Scope: Where can a declared entity be seen? This is a static trait.
• Activation: When can I access an entity? This is a dynamic trait. Some entities can be accessed as soon as we enter their scopes. For others, we have to wait until execution reaches their declarations.

Tbl.  1 summarizes how various declarations handle these aspects.

import is described in §27.5 “ECMAScript modules” . The following sections describe the other constructs in more detail.

For JavaScript, TC39 needed to decide what happens if you access a constant in its direct scope, before its declaration:

Some possible approaches are:

• The name is resolved in the scope surrounding the current scope.
• You get undefined .
• There is an error.

Approach 1 was rejected because there is no precedent in the language for this approach. It would therefore not be intuitive to JavaScript programmers.

Approach 2 was rejected because then x wouldn’t be a constant – it would have different values before and after its declaration.

let uses the same approach 3 as const , so that both work similarly and it’s easy to switch between them.

The time between entering the scope of a variable and executing its declaration is called the temporal dead zone (TDZ) of that variable:

• During this time, the variable is considered to be uninitialized (as if that were a special value it has).
• If you access an uninitialized variable, you get a ReferenceError .
• Once you reach a variable declaration, the variable is set to either the value of the initializer (specified via the assignment symbol) or undefined – if there is no initializer.

The following code illustrates the temporal dead zone:

The next example shows that the temporal dead zone is truly temporal (related to time):

Even though func() is located before the declaration of myVar and uses that variable, we can call func() . But we have to wait until the temporal dead zone of myVar is over.

11.8.2 Function declarations and early activation

In this section, we are using functions – before we had a chance to learn them properly. Hopefully, everything still makes sense. Whenever it doesn’t, please see §25 “Callable values” .

A function declaration is always executed when entering its scope, regardless of where it is located within that scope. That enables you to call a function foo() before it is declared:

The early activation of foo() means that the previous code is equivalent to:

If you declare a function via const or let , then it is not activated early. In the following example, you can only use bar() after its declaration.

11.8.2.1 Calling ahead without early activation

Even if a function g() is not activated early, it can be called by a preceding function f() (in the same scope) if we adhere to the following rule: f() must be invoked after the declaration of g() .

The functions of a module are usually invoked after its complete body is executed. Therefore, in modules, you rarely need to worry about the order of functions.

Lastly, note how early activation automatically keeps the aforementioned rule: when entering a scope, all function declarations are executed first, before any calls are made.

11.8.2.2 A pitfall of early activation

If you rely on early activation to call a function before its declaration, then you need to be careful that it doesn’t access data that isn’t activated early.

The problem goes away if you make the call to funcDecl() after the declaration of MY_STR .

11.8.2.3 The pros and cons of early activation

We have seen that early activation has a pitfall and that you can get most of its benefits without using it. Therefore, it is better to avoid early activation. But I don’t feel strongly about this and, as mentioned before, often use function declarations because I like their syntax.

11.8.3 Class declarations are not activated early

Even though they are similar to function declarations in some ways, class declarations are not activated early:

Why is that? Consider the following class declaration:

The operand of extends is an expression. Therefore, you can do things like this:

Evaluating such an expression must be done at the location where it is mentioned. Anything else would be confusing. That explains why class declarations are not activated early.

var is an older way of declaring variables that predates const and let (which are preferred now). Consider the following var declaration.

This declaration has two parts:

• Declaration var x : The scope of a var -declared variable is the innermost surrounding function and not the innermost surrounding block, as for most other declarations. Such a variable is already active at the beginning of its scope and initialized with undefined .
• Assignment x = 123 : The assignment is always executed in place.

The following code demonstrates the effects of var :

11.9 Closures

Before we can explore closures, we need to learn about bound variables and free variables.

11.9.1 Bound variables vs. free variables

Per scope, there is a set of variables that are mentioned. Among these variables we distinguish:

• Bound variables are declared within the scope. They are parameters and local variables.
• Free variables are declared externally. They are also called non-local variables .

Consider the following code:

In the body of func() , x and y are bound variables. z is a free variable.

11.9.2 What is a closure?

What is a closure then?

A closure is a function plus a connection to the variables that exist at its “birth place”.

What is the point of keeping this connection? It provides the values for the free variables of the function – for example:

funcFactory returns a closure that is assigned to func . Because func has the connection to the variables at its birth place, it can still access the free variable value when it is called in line A (even though it “escaped” its scope).

Static scoping is supported via closures in JavaScript. Therefore, every function is a closure.

11.9.3 Example: A factory for incrementors

The following function returns incrementors (a name that I just made up). An incrementor is a function that internally stores a number. When it is called, it updates that number by adding the argument to it and returns the new value.

We can see that the function created in line A keeps its internal number in the free variable startValue . This time, we don’t just read from the birth scope, we use it to store data that we change and that persists across function calls.

We can create more storage slots in the birth scope, via local variables:

11.9.4 Use cases for closures

What are closures good for?

For starters, they are simply an implementation of static scoping. As such, they provide context data for callbacks.

They can also be used by functions to store state that persists across function calls. createInc() is an example of that.

And they can provide private data for objects (produced via literals or classes). The details of how that works are explained in Exploring ES6 .

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• Assignment operators

An assignment operator assigns a value to its left operand based on the value of its right operand.

The basic assignment operator is equal ( = ), which assigns the value of its right operand to its left operand. That is, x = y assigns the value of y to x . The other assignment operators are usually shorthand for standard operations, as shown in the following definitions and examples.

Simple assignment operator which assigns a value to a variable. The assignment operation evaluates to the assigned value. Chaining the assignment operator is possible in order to assign a single value to multiple variables. See the example.

Addition assignment

The addition assignment operator adds the value of the right operand to a variable and assigns the result to the variable. The types of the two operands determine the behavior of the addition assignment operator. Addition or concatenation is possible. See the addition operator for more details.

Subtraction assignment

The subtraction assignment operator subtracts the value of the right operand from a variable and assigns the result to the variable. See the subtraction operator for more details.

Multiplication assignment

The multiplication assignment operator multiplies a variable by the value of the right operand and assigns the result to the variable. See the multiplication operator for more details.

Division assignment

The division assignment operator divides a variable by the value of the right operand and assigns the result to the variable. See the division operator for more details.

Remainder assignment

The remainder assignment operator divides a variable by the value of the right operand and assigns the remainder to the variable. See the remainder operator for more details.

Exponentiation assignment

This is an experimental technology, part of the ECMAScript 2016 (ES7) proposal. Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future version of browsers as the spec changes.

The exponentiation assignment operator evaluates to the result of raising first operand to the power second operand. See the exponentiation operator for more details.

Left shift assignment

The left shift assignment operator moves the specified amount of bits to the left and assigns the result to the variable. See the left shift operator for more details.

Right shift assignment

The right shift assignment operator moves the specified amount of bits to the right and assigns the result to the variable. See the right shift operator for more details.

Unsigned right shift assignment

The unsigned right shift assignment operator moves the specified amount of bits to the right and assigns the result to the variable. See the unsigned right shift operator for more details.

Bitwise AND assignment

The bitwise AND assignment operator uses the binary representation of both operands, does a bitwise AND operation on them and assigns the result to the variable. See the bitwise AND operator for more details.

Bitwise XOR assignment

The bitwise XOR assignment operator uses the binary representation of both operands, does a bitwise XOR operation on them and assigns the result to the variable. See the bitwise XOR operator for more details.

Bitwise OR assignment

The bitwise OR assignment operator uses the binary representation of both operands, does a bitwise OR operation on them and assigns the result to the variable. See the bitwise OR operator for more details.

Left operand with another assignment operator

In unusual situations, the assignment operator (e.g. x += y ) is not identical to the meaning expression (here x = x + y ). When the left operand of an assignment operator itself contains an assignment operator, the left operand is evaluated only once. For example:

Specifications

Browser compatibility.

• Arithmetic operators

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• Warning: -file- is being assigned a //# sourceMappingURL, but already has one
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• Warning: Date.prototype.toLocaleFormat is deprecated
• Warning: JavaScript 1.6's for-each-in loops are deprecated
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• Warning: expression closures are deprecated
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• JavaScript technologies overview
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JavaScript Variables

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• JavaScript Global Variables
• JavaScript var
• JavaScript Tutorial
• JavaScript Let
• How to unset JavaScript variables?
• Variables in TypeScript
• JavaScript Strings
• Variable Shadowing in JavaScript
• JavaScript Rest parameter
• Variables and Datatypes in JavaScript
• What is Variable Scope in JavaScript ?
• How to pass JavaScript variables to PHP ?
• Objects in Javascript
• Uses of JavaScript
• Global and Local variables in JavaScript
• ES6 | Variables
• Javascript Scope
• JavaScript Set object key by variable
• How to calculate the number of days between two dates in JavaScript ?
• Convert a String to an Integer in JavaScript
• How to append HTML code to a div using JavaScript ?
• How to Open URL in New Tab using JavaScript ?
• Difference between var and let in JavaScript
• How do you run JavaScript script through the Terminal?
• Remove elements from a JavaScript Array
• How to read a local text file using JavaScript?
• JavaScript console.log() Method
• JavaScript Number toString() Method

Variables are used to store data in JavaScript. Variables are used to store reusable values. The values of the variables are allocated using the assignment operator(“=”).

Examples of JavaScript Variables

Javascript assignment operator.

JavaScript assignment operator is equal (=) which assigns the value of the right-hand operand to its left-hand operand.

JavaScript Identifiers

JavaScript variables must have unique names. These names are called Identifiers.

Basic rules to declare a variable in JavaScript:

• These are case-sensitive
• Can only begin with a letter, underscore(“_”) or “$” symbol
• It can contain letters, numbers, underscore, or “$” symbol
• A variable name cannot be a reserved keyword.

JavaScript is a dynamically typed language so the type of variables is decided at runtime. Therefore there is no need to explicitly define the type of a variable. We can declare variables in JavaScript in three ways:

• JavaScript var keyword
• JavaScript let keyword
• JavaScript const keyword  

Note: In JavaScript, variables can be declared automatically.

All three keywords do the basic task of declaring a variable but with some differences Initially, all the variables in JavaScript were written using the var keyword but in ES6 the keywords let and const were introduced.

Example 1: In this example, we will declare variables using var.

Example 2: In this example, we will declare variables using let.

To learn more about JavaScript let check this article JavaScript Let

Example 3: In this example, we will declare the variable using the const keyword.

Explanation: const keyword is used when we assign a value permanently to a variable. So when we try to change the value of a variable declared with the const keyword it will throw an error. The variables declared with var and let are mutable that is their value can be changed but variables declared using const are immutable. 

To learn more about JavaScript const check this article JavaScript Const

Note: The newly introduced keywords let and const are block scoped whereas var is function scoped. 

Let us see an example to understand the difference:

Example: In this example, we are trying to access the block scoped variables outside the block that’s why we are getting error.

Explanation: Since variables “a” and “c” are block scoped so we were not able to access them outside their block.

When to Use var, let, or const

• We declare variables using const if the value should not be changed
• We use const if the type of the variables should not be changed such as working with Arrays and objects
• We should use let if we want mutable value or we can not use const
• We use var only if we support old browser.

To learn more about the scope of variables refer to this article Understanding variable scopes in JavaScript

Comparison of properties of let, var, and const keywords in JavaScript:

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