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New variable declaration keywords in JavaScript

In this article, we will learn the new variable declaration keywords in JavaScript and show examples how to use them.
Submitted by Abhishek Pathak, on October 17, 2017

JavaScript - New Variable Declaration Keywords

The latest standardized version of JavaScript, which is ES6 released in 2015, has a lot of improvements and features to offer. Also, it has some new variable declaration keywords that are required to be understood in order to build interactive websites with modern JavaScript code.

var Keyword

Traditionally, there was only one way of declaring variables before ES6, which was using var keyword. The keyword declared had only 2 scope rules. If it was declared outside all the functions, then the variable will have global scope, otherwise, its scope was limited to the function it was defined in. Consider this example,

Example

var age = 10;
//This varible will have global scope.

function life() {
	var life = 3;
	//This variable will have function scope	

	console.log(age); //Global variable accessible here
	console.log(life); //Local variable accessible here
}

console.log(age); //Global variable still accessible
console.log(life); //Local variable will give reference error.

As it is clear from the above code, the function level scope variable is not accessible outside the function and will give reference error. Reference error is when JavaScript is not able to find the variable which has been referenced in the code.

let and const Keywords

This was all about var, but with ES6, we have two new keywords called let and const.

The const keyword is just like var, but only difference being the value once assigned to it can't be changed. It is like the constant keyword in C/C++. If you try to change the value of this constant variable, it will show error.

Example

const age = 10;
console.log(age); //Output: 10
age = 20; //Error, as it constant variable.

The const keyword should be used for variables which won't change their value.

Declaration Inside a Block

The let keyword however, is just like var, but with a little difference, that it has a block scope. That is, if it is declared inside a block, then it won't be able to accessible outside. Here's an example,

Example

function life() {
	for (var i=0; i<3; i++) {
		var age = 10;
		let life = 3;
		console.log(age) // Output: 10
		console.log(life) //Output: 3
	}
	console.log(age); //Output: 3
	console.log(life); //Error, reference error to variable life
}

The var as already mentioned has only function scope and global scope and not block scope, like let. So, it even accessible outside the for loop, but not the let declared variable.

Hope this makes you clear with the three different types of variable declaration keywords in JavaScript. Share your thoughts in the comments below.

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