JavaScript is one of the most commonly used programming languages, especially by software engineers who are front-end developers or full-stack developers. If you are preparing for a coding interview, reviewing some of the basics of JavaScript will help you employ it’s features in the best way possible.

In this article, we’re focusing on the var and let keywords. Before ES6, you could initialize a variable in Javascript using var keyword. However, after the launch of ES6 in 2015, JavaScript also allowed use of the let keyword to initialize any variable. So, both keywords essentially help in declaring a variable in JavaScript.

var a = 1;
let b = 2;

However, there are many differences between both the keywords in terms of functionality and behavior. We will cover each of these differences one by one along with the help of examples:

• Scoping
• Hoisting 
• Global Properties
• Variable redeclaration

Scoping

Scope is the availability of variables, functions, and objects in some particular part of your code during runtime. A variable defined inside a scope is accessible only within that scope, but inaccessible outside.

Variables declared by var keyword are function scoped, i.e., accessible to the immediate function body, while let variables are block scoped, i.e., scoped to the immediate enclosing block denoted by { }.

function foo () {
  var a = 1
  if(a>0) {
    let b = 2;
    console.log(b); // 2
  }
  console.log(a); // 1
  console.log(b); // ReferenceError
}

foo();
‍

In the above example, variable b is initialized with let and since it is block scoped, it can only be accessible in the if block and trying to access it outside the block will give ReferenceError. However, variable a can be accessed inside the entire function foo.

Due to such scoping rules, let variables are easy to use in loops and closures. Consider following example:

for (let i = 0; i < 5; i++) {
  setTimeout(() => console.log(`i: ${i}`), 500);
}

Output:

i: 0
i: 1
i: 2
i: 3
i: 4

Here, since the scope of variable i is the block in which it is initialized, each iteration can be considered as a separate block and thus the value of i in the output will be different in each loop. However, if you replace let with var in the above example, output will be different. In the latter case, i will have global scope and the same variable i will get reinitialized and the latest value will be visible in the output.

for (var i = 0; i < 5; i++) {
setTimeout(_ => console.log(`i: ${i}`), 500);
}

Output:

i: 5
i: 5
i: 5
i: 5
i: 5

Hoisting 

Variables declared with var are hoisted, while let variables are not initialized until their definition is evaluated. So, accessing var variables before their initialization in code returns undefined as all the hoisted variables have value undefined before initialization. Accessing let variables before their initialization gives ReferenceError. Such a piece of block where you cannot access variables as their initialization is not completed is known as “temporal dead zone.”

var a;
console.log(a); // undefined
a = 2;
console.log(a); // 2
console.log(b); // ReferenceError
let b;
console.log(b); // ReferenceError
b = 4;
console.log(b); // 4

Global Properties

var variables declared in global scope are also created as members of the global object, while the same is not true for the let variables. JS code running in a web browser has window as their global object, while code running in the node.js environment has global as global object.

var a = 1; // globally scoped
let b = 2; // not allowed to be globally scoped
console.log(window.a); // 1
console.log(window.b); // undefined

Note: var variables are added as global object property only when code runs in a web browser. Code running in node.js environment does not allow this.

Variable Redeclaration

Variables declared with var keyword can be redeclared in the same scope while redeclaring let variables lead to SyntaxError. Consider the following example:

var a = 1;
var a = 2; // 1 is replaced with 2.
let b = 3;
let b = 4; // SyntaxError: Identifier 'b' has already been declared

Run code snippet

Here, you will find SyntaxError while trying to initialize variable b again.

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Article contributed Prerak Dholakiya