rest parameters

The rest parameter syntax allows us 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 (inclusive) to theArgs.length (exclusive) 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 f(a, b, ...args) {
  
}

Destructuring rest parameters

Rest parameters can be destructured, that means that their data can be extracted into distinct variables.

function f(...[a, b, c]) {
  return a + b + c;
}

f(1)          // NaN (b and c are undefined)
f(1, 2, 3)    // 6
f(1, 2, 3, 4) // 6 (the fourth parameter is not destructured)

Examples

Since theArgs is an array, a count of its elements is given by the length property:

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

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

In the next example, a rest parameter is used to collect arguments from the second one to the end in an array. Each is then multipled them by the first and the array returned:

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 Array methods can be used on rest parameters, 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, it must be converted 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
ECMAScript 2017 Draft (ECMA-262) The definition of 'Function Definitions' in that specification.	Draft

Browser compatibility

See also

• Arguments object
• Array
• Functions
• Spread Operator
• Original proposal at ecmascript.org
• JavaScript arguments object and beyond
• Destructuring assignment