Ultimate Difference Between let const and var in JavaScript (ECMAScript 6)

If you’re a developer, you’ve likely heard of the terms let, const, and var when working with JavaScript. These three keywords are used to declare variables in JavaScript, and understanding their differences is crucial to writing efficient and effective code.

In this article, we’ll explore the difference between let const and var, and when it’s best to use each keyword.

var vs let vs const

Before we dive into the difference between let const and var, let’s quickly go over what each keyword does.

  • var – declares a variable that is globally scoped or function-scoped.
  • let – declares a variable that is block-scoped.
  • const – declares a variable that is block-scoped and cannot be reassigned.

What is var in JavaScript?

In JavaScript, var was the original way of declaring variables. It is still commonly used today, although let and const have become increasingly popular.

Variables declared with var are function-scoped or globally scoped, depending on where they are declared. This means that they can be accessed and modified throughout the entire function or global scope.

For example, consider the following code:

function example() {
var x = 10;
if (true) {
var x = 20;
console.log(x); // Output: 20
}
console.log(x); // Output: 20
}

In this example, the variable x is declared twice, both times using the var keyword. Because var is function-scoped, the variable x can be accessed and modified throughout the entire example function.

What is let in JavaScript?

let was introduced in ECMAScript 6 as a new way to declare variables. Unlike var, variables declared with let are block-scoped, which means that they can only be accessed within the block they were declared in.

Consider the following example:

function example() {
  let x = 10;
  if (true) {
    let x = 20;
    console.log(x); // Output: 20
  }
  console.log(x); // Output: 10
}

In this example, the variable x is declared twice, both times using the let keyword. Because let is block-scoped, the x variable declared inside the if statement can only be accessed within the if block.

What is const in JavaScript?

const is another way to declare variables in JavaScript. Like let, variables declared with const are also block-scoped. However, unlike let, const variables cannot be reassigned once they are declared.

Consider the following example:

function example() {
  const x = 10;
  x = 20; // Error: Assignment to constant variable.
  console.log(x);
}

In this example, the variable x is declared using const, and then an attempt is made to reassign it to a different value. However, because const variables cannot be reassigned, an error is thrown.

Difference between let const and var

Now that we understand what let const and var are, let’s go over some key differences between them.

Scope

In JavaScript, variables declared with var have function scope. This means that the variable is accessible within the function it was declared in, as well as any nested functions. However, it is not accessible outside of the function.

function foo() {
  var x = 10;
  console.log(x); // 10
}

foo();
console.log(x); // ReferenceError: x is not defined

In the example above, x is declared within the foo function using the var keyword. This means that x is only accessible within the foo function, and not outside of it. If we try to access x outside of the function, we get a ReferenceError because x is not defined.

Hoisting

Another difference between var and the newer keywords is hoisting. In JavaScript, variable declarations are hoisted to the top of their scope, regardless of where they are declared. This means that a variable declared with var can be accessed before it is declared in the code.

function foo() {
  console.log(x); // undefined
  var x = 10;
}

foo();

In the example above, x is declared within the foo function using the var keyword. However, we try to access x before it is declared. In this case, x is hoisted to the top of the function, but its value is undefined until it is assigned a value later in the function.

Redeclaration

Variables declared with var can be redeclared within the same scope without raising an error.

var x = 10;
var x = 20;
console.log(x); // 20

In the example above, x is declared twice using the var keyword. This is allowed in JavaScript, and the value of x is ultimately 20.

When to Use let const and var in JavaScript

The choice between let const and var depends on the specific use case and the desired behavior of the variable. Here are some guidelines to follow when deciding which keyword to use:

Use var When…

  • The variable needs to be accessible within nested functions
  • The variable needs to be redeclared within the same scope
  • The codebase already uses var and consistency is important

Use let When…

  • The variable should only be accessible within a specific block
  • The variable value may need to change within the block
  • The variable value will not be reassigned outside of the block

Use const When…

  • The variable should only be accessible within a specific block
  • The variable value will not change within the block
  • The variable value should not be reassigned outside of the block

By following these guidelines, you can write code that is more readable, maintainable, and less error-prone.

FAQs

Can you reassign a variable declared with const?

No, you cannot reassign a variable declared with const. Once a const variable has been assigned a value, it cannot be changed.

What happens if you try to redeclare a variable declared with let or const?

If you try to redeclare a variable declared with let or const, you will get a SyntaxError:

let x = 10;
let x = 20; // SyntaxError: Identifier ‘x’ has already been declared

const y = 10;
const y = 20; // SyntaxError: Identifier ‘y’ has already been declared

What is the difference between let and const

The main difference between let and const is that let allows the variable value to be reassigned, whereas const does not. Once a const variable has been assigned a value, it cannot be changed.

Conclusion

In conclusion, understanding the differences between let, const, and var in JavaScript is important for writing clean, efficient code. While var still has its uses, let and const are more modern alternatives that provide block scope and prevent accidental reassignment, respectively.

By using the appropriate keyword for each variable, you can write code that is easier to read, maintain, and debug. Remember to choose the keyword that best fits the variable’s specific use case and desired behavior.

Resources for Learning More

There are many resources available for learning JavaScript, from online courses to books and tutorials. Some popular resources include CodecademyW3Schools, and the Mozilla Developer Network.

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