What is the Difference Between let, const, and var in Javascript
In Javascript, variables are used to store data values. The three most commonly used keywords to declare variables are let, const, and var. Although they are similar, they have some key differences that are important to understand when working with Javascript.
In this tutorial, we will go over the differences between let, const, and var in Javascript.
var Keyword
The var keyword is the oldest way to declare variables in Javascript. It is also the most lenient, as it has no restrictions on redeclaration, assignment, or scope. Here is an example of how to use the var keyword to declare a variable:
var x = 10;
The variable x is now declared and assigned the value 10. If we were to declare the same variable again using var, it would not throw any errors:
var x = 20;
This can lead to potential bugs and is considered bad practice. In addition, the scope of a variable declared using var is the entire function in which it is declared. If it is declared outside of any function, its scope is the global object.
let Keyword
The let keyword was introduced in ES6 (ECMAScript 2015) and is now the recommended way to declare variables in Javascript. Unlike var, let has a block scope, which means that it is only accessible within the block in which it is declared. Here is an example of how to use the let keyword:
let x = 10;
If we were to declare the same variable again using let, it would throw a SyntaxError:
let x = 20; // throws SyntaxError
This is because redeclaration of a variable with let is not allowed within the same block scope.
const Keyword
The const keyword is also introduced in ES6 and is used to declare constants. Constants are like variables, but their values cannot be re-assigned. Here is an example of how to use the const keyword:
const PI = 3.14;
If we were to try to re-assign the value of PI, it would throw a TypeError:
const PI = 3.14; PI = 4.13; // throws TypeError
This makes const useful for values that should not change, like mathematical constants or configuration settings. Similar to let, const also has a block scope.
In summary, the let keyword should be used for variables that may be reassigned, while const should be used for constants that should not be reassigned. The var keyword should be avoided due to its potential for bugs and the fact that it has no block scope. By understanding the differences between these three keywords, you can write more effective and bug-free code in Javascript.
