Rest parameters

This is a new technology, part of the ECMAScript 2015 (ES6) standard .
This technology's specification has been finalized, but check the compatibility table for usage and implementation status in various browsers.

The rest parameter syntax allows to represent an indefinite number of arguments as an array.

Syntax

function(a, b, ...theArgs) {
  // ...
}

Description

If the last named argument of a function is prefixed with ..., it becomes an array whose elements from 0 to theArgs.length are supplied by the actual arguments passed to the function.

In the above example, theArgs would collect the third argument of the function (because the first one is mapped to a, and the second to b) and all the consecutive arguments.

Difference between rest parameters and the arguments object

There are three main differences between rest parameters and the arguments object:

  • rest parameters are only the ones that haven't been given a separate name, while the arguments object contains all arguments passed to the function;
  • the arguments object is not a real array, while rest parameters are Array instances, meaning methods like sort, map, forEach or pop can be applied on it directly;
  • the arguments object has additional functionality specific to itself (like the callee property).

From arguments to an array

Rest parameters have been introduced to reduce the boilerplate code that was induced by the arguments

// Before rest parameters, the following could be found:
function f(a, b){
  var args = Array.prototype.slice.call(arguments, f.length);

  // ...
}

// to be equivalent of

function(a, b, ...args) {
  
}

Examples

Since theArgs is an array, you can get the count of its elements by using the length property:

function fun1(...theArgs) {
  console.log(theArgs.length);
}

fun1();  // 0
fun1(5); // 1
fun1(5, 6, 7); // 3

In the next example, we use the rest parameters to collect arguments from the second one to the end. We then multiply them by the first one:

function multiply(multiplier, ...theArgs) {
  return theArgs.map(function (element) {
    return multiplier * element;
  });
}

var arr = multiply(2, 1, 2, 3); 
console.log(arr); // [2, 4, 6]

The following example shows that you can use Array methods on rest params, but not on the arguments object:

function sortRestArgs(...theArgs) {
  var sortedArgs = theArgs.sort();
  return sortedArgs;
}

console.log(sortRestArgs(5,3,7,1)); // shows 1,3,5,7

function sortArguments() {
  var sortedArgs = arguments.sort(); 
  return sortedArgs; // this will never happen
}

// throws a TypeError: arguments.sort is not a function
console.log(sortArguments(5,3,7,1));

In order to use Array methods on the arguments object, you would need to convert it to a real array first.

Specifications

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
The definition of 'Function Definitions' in that specification.
Standard Initial definition.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 45 15.0 (15.0) Not supported Not supported Not supported
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support Not supported 45 15.0 (15.0) Not supported Not supported Not supported 45

See also

Document Tags and Contributors

Last updated by: jpmedley,
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