Assignment Operators

Syntax

x = y, x += y, x -= y, x *= y, x /= y, x ??= y

Examples

let total = 10;
total += 5;  // total = total + 5 -> 15
total -= 3;  // total = total - 3 -> 12
total *= 2;  // total = total * 2 -> 24
total /= 4;  // total = total / 4 -> 6

let config = { theme: null };

// Only assigns if the current value is null/undefined
config.theme ??= "dark";
console.log(config.theme); // "dark"

config.theme ??= "light"; // no-op, theme is already set
console.log(config.theme); // still "dark"

let isActive = true;
isActive &&= false; // only assigns if isActive was truthy
console.log(isActive); // false

Best practices

• Use ??= specifically for "assign a default if null or undefined", since it will not overwrite legitimate falsy values like 0 or ""
• Prefer compound operators (+=, -=) over spelling out x = x + y for cleaner, more readable code
• Reach for &&= and ||= sparingly - they can make control flow less obvious to readers unfamiliar with them
• Avoid chaining too many assignment operators in a single line, as it hurts readability

At a glance

Purpose

Application logic and interaction

File extension

.js · .mjs

Runs in

Browsers and JavaScript runtimes

Usually used with

HTML, CSS and Web APIs

Specifications & further reading

• ECMAScript specification

Related JavaScript documentation