JavaScriptFunctionsBeginner

Rest parameters (...args)

Collects remaining arguments into a real array.

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function sum(...nums) { return nums.reduce((a,n)=>a+n,0); }
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What it does

Overview

Collects remaining arguments into a real array.

The rest parameter syntax allows a function to accept an indefinite number of arguments as an array. Unlike the 'arguments' object, which is an array-like object but lacks built-in methods such as map, filter, or sort, rest parameters are actual Array instances. This leads to cleaner code and better performance by eliminating the need for manual conversions like Array.prototype.slice.call(arguments). A critical edge case is that rest parameters do not count toward the function's length property, which only reflects parameters preceding the '...'. In modern engines, rest parameters are optimized and preferred over 'arguments'. However, they must be the last parameter in the function signature, and a function can only contain one rest parameter. IE is not supported.

Quick reference

Syntax

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

Inputs

Parameters

...argsany[] · All remaining arguments.

See it in practice

Examples

1

Basic Summation of Indefinite Arguments

function sumAll(...numbers) {
  return numbers.reduce(function(acc, curr) {
    return acc + curr;
  }, 0);
}

console.log(sumAll(1, 2, 3, 4));
Output:
10

Captures all passed arguments into a 'numbers' array and uses the reduce method to calculate the sum.

2

Combining Fixed and Rest Parameters

function multiplyAndMap(multiplier, ...values) {
  return values.map(function(val) {
    return val * multiplier;
  });
}

var result = multiplyAndMap(2, 10, 20, 30);
console.log(result);
Output:
[20, 40, 60]

The first argument is assigned to 'multiplier', and all subsequent arguments are gathered into the 'values' array.

3

Using Rest Parameters in Arrow Functions

var logger = (prefix, ...messages) => {
  console.log(prefix + ': ' + messages.join(', '));
};

logger('DEBUG', 'User logged in', 'Session ID: 456');
Output:
DEBUG: User logged in, Session ID: 456

Arrow functions do not have their own 'arguments' object, making rest parameters the standard way to handle variable arguments in them.

Debug faster

Common Errors

1

SyntaxError

Cause: Placing the rest parameter before other parameters in the function definition.

Fix: Always place the rest parameter as the very last item in the parameter list.

function wrong(first, ...rest, last) { /* logic */ }
2

SyntaxError

Cause: Attempting to use more than one rest parameter in a single function signature.

Fix: Use a single rest parameter and destructure the resulting array if specific groupings are needed.

function multiRest(...group1, ...group2) { /* logic */ }

Runtime support

Compatibility

Node.js, BrowserAll modern browsers, IE not supportedES6 (2015)

Source: MDN Web Docs

Common questions

Frequently Asked Questions

Collects remaining arguments into a real array.

...args: All remaining arguments.

SyntaxError: Always place the rest parameter as the very last item in the parameter list. SyntaxError: Use a single rest parameter and destructure the resulting array if specific groupings are needed.