Logical Operators

Syntax

a && b, a || b, !a, a ?? b

Examples

console.log(true && false);  // false
console.log(true || false);  // true
console.log(!true);          // false

// Short-circuit evaluation for default values
const name = "" || "Guest";
console.log(name); // "Guest" - "" is falsy

// && for conditional execution
const isLoggedIn = true;
isLoggedIn && console.log("Welcome back!");

const count = 0;
console.log(count || 10); // 10 - 0 is falsy, so || moves on
console.log(count ?? 10); // 0 - 0 is not null/undefined, so ?? keeps it

const config = { retries: 0 };
const retries = config.retries ?? 3; // correctly keeps 0, not 3

Best practices

• Use ?? instead of || when 0, "", or false should be treated as valid values rather than replaced with a default
• Take advantage of && for concise conditional execution, but avoid it when the intent would be clearer as a full if statement
• Remember that && and || return one of their operand values, not necessarily a boolean
• Combine logical operators with optional chaining (?.) for safe, readable conditional access

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