In JavaScript, a variable is a container that stores data values. Variables are declared using the var, let, or const keyword followed by a variable name. Here’s a brief overview of each type of variable declaration in JavaScript:
1. var :
var is the oldest way to declare variables in JavaScript. Variables declared with var are function-scoped, which means that their scope is limited to the function in which they are defined, or the global scope if they are defined outside of a function.
Example:
var x = 5;
function myFunction() {
var y = 10;
console.log(x); // 5
console.log(y); // 10
}
2. let :
let was introduced in ECMAScript 6 (ES6) as a replacement for var. Variables declared with let are block-scoped, which means that their scope is limited to the block in which they are defined.
Example:
let x = 5;
if (true) {
let y = 10;
console.log(x); // 5
console.log(y); // 10
}
console.log(y);
// ReferenceError: y is not defined
3. const :
const is also introduced in ECMAScript 6 (ES6) and is used to declare constants in JavaScript. Variables declared with const cannot be reassigned to a new value, but their properties can be modified if they are objects or arrays. const variables are also block-scoped.
Example:
const x = 5;
x = 10;
// TypeError: Assignment to constant
variable.
const myObj = { name: ‘John’, age: 30 };
myObj.name = ‘Jane’; // OK
myObj = {};
// TypeError: Assignment to constant
variable.
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