How to declare variable in JavaScript

In JavaScript, you can declare variables using the var, let, or const keywords. Here’s how you can declare variables with each of these keywords:

var: This keyword was traditionally used for variable declaration in JavaScript, but it has some scoping quirks. Variables declared with var are function-scoped or globally-scoped.

var myVariable = 10;

let: Introduced in ES6 (ECMAScript 2015), let allows block-scoping of variables. It is generally recommended to use let instead of var for most cases.

let myVariable = 10;

const: Also introduced in ES6, const is used to declare constants, which are variables that cannot be reassigned. Like let, const is block-scoped.

const myConstant = 10;

Note: variables declared with var and let can be reassigned, while variables declared with const cannot be reassigned once they are assigned a value.

Additionally, it’s good practice to always use let or const unless you have a specific reason to use var due to its scoping behavior.

• Hoisting: Variables declared with var are hoisted to the top of their scope, meaning they can be used before they are declared. This behavior can sometimes lead to unexpected results. Variables declared with let and const are not hoisted and must be declared before they are used.
• Initialization: Variables declared with var can be left uninitialized, meaning they will have a default value of undefined. Variables declared with let and const must be initialized with a value before they can be used.

Declare variable in JavaScript example

Simple example code.

// using 'var'
var age;
age = 25;

// using 'let'
let name = "John";

// using 'const'
const PI = 3.14159;

console.log(age);  // Output: 25
console.log(name); // Output: John
console.log(PI);   // Output: 3.14159

Output:

Here’s an example to illustrate the differences:

function example() {
  var x = 10;
  let y = 20;
  const z = 30;

  if (true) {
    var x = 50; // This reassigns the existing 'x' variable
    let y = 60; // This creates a new 'y' variable in the block scope
    const z = 70; // This creates a new 'z' variable in the block scope

    console.log(x); // Output: 50
    console.log(y); // Output: 60
    console.log(z); // Output: 70
  }

  console.log(x); // Output: 50
  console.log(y); // Output: 20 (original value outside the block remains unchanged)
  console.log(z); // Output: 30 (original value outside the block remains unchanged)
}

It’s important to note that the variable name follows certain rules:

• It must start with a letter, underscore (_), or dollar sign ($).
• It can contain letters, digits, underscores, or dollar signs.
• It is case-sensitive.

Comment if you have any doubts or suggestions on this JS variable topic.

Note: The All JS Examples codes are tested on the Firefox browser and the Chrome browser.

OS: Windows 10

Code: HTML 5 Version