Function Parameters: Default & Rest

Syntax

function name(param = defaultValue, ...rest) { }

Examples

function greet(name = "Guest", greeting = "Hello") {
  console.log(`${greeting}, ${name}!`);
}

greet();                    // "Hello, Guest!"
greet("Alice");             // "Hello, Alice!"
greet("Bob", "Hi");         // "Hi, Bob!"

function sum(...numbers) {
  return numbers.reduce((total, n) => total + n, 0);
}

console.log(sum(1, 2, 3));       // 6
console.log(sum(1, 2, 3, 4, 5)); // 15

function logDetails(name, ...tags) {
  console.log(name, "->", tags);
}
logDetails("Article", "js", "tutorial", "web"); // "Article -> [js, tutorial, web]"

Best practices

• Place rest parameters last in the parameter list - they must be, since they collect everything remaining
• Prefer default parameters over manually checking `if (param === undefined)` inside the function body
• Use rest parameters instead of the old `arguments` object for a real array with all standard array methods available
• Keep the number of parameters manageable - beyond 3-4, consider accepting a single options object instead

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