Increment & Decrement Operators in JavaScript

In programming, increasing the value of a variable by 1 is so common, there is an operator designed to do that.

That operator is the increment operator (++):

let number = 10;
number++; // short for number += 1

console.log(number); // Output: 11

There is also one such operator for decreasing the value of a variable by 1.

That operator is the decrement operator (--):

let number = 10;
number--; // short for number -= 1

console.log(number); // Output: 9

Postfix:

In the examples above, we have used these two operators in their postfix form, with operator (++) after operand (number) such as number++.

If used postfix, the increment operator increments and returns the value before incrementing:

let number = 10;

let oldNumber = number++;

console.log(oldNumber); // Output: 10
console.log(number); // Output: 11

This code first assigns the value of number (which is 10) to oldNumber and then increments number (making it 11). It's equivalent to this:

let oldNumber = number;
number++;

Prefix:

However, there is also the prefix form, with operator before operand (for example, ++number).

When used prefix, the increment operator increments and returns the value after incrementing:

let number = 10;

let newNumber = ++number;

console.log(newNumber); // Output: 11
console.log(number); // Output: 11

This code first increments number (making it 11) and then assigns the new value of number to oldNumber (making it 11). It's equivalent to this:

number++;
let newNumber = number;

Here's an example with the decrement operator:

let number = 10;

let newNumber = --number;

console.log(newNumber); // Output: 9
console.log(number); // Output: 9

Assignment
Follow the Coding Tutorial and let's practice with these operators!

Hint
Look at the examples above if you get stuck.