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Rest Parameters

Rest Parameters

Introduction

Rest parameters (...) allow a function to accept an indefinite number of arguments as an array. They replaced the older arguments object in modern JavaScript.

Syntax

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

Example

function logAll(...args) {
  args.forEach(arg => console.log(arg));
}

logAll(1, 2, 3, 4); // logs 1, 2, 3, 4

With Named Parameters

function greet(greeting, ...names) {
  return names.map(name => `${greeting}, ${name}!`);
}

console.log(greet('Hello', 'Alice', 'Bob', 'Charlie'));
// ['Hello, Alice!', 'Hello, Bob!', 'Hello, Charlie!']

Rest vs arguments

// arguments — array-like, not a real array
function oldWay() {
  const args = Array.from(arguments); // must convert
}

// rest — real array
function newWay(...args) {
  // args is a real array
  args.map, args.filter, args.reduce all work
}

Summary

...params collects all remaining arguments into an array
Must be the last parameter
It’s a real Array (unlike arguments)
Cleaner, modern alternative to arguments

Further Reading

MDN: Rest Parameters