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

Syntax

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

Description

A function's last parameter can be prefixed with ... which will cause all remaining (user supplied) arguments to be placed within a "standard" javascript array. Only the last parameter can be a "rest parameter".

function myFun(a, b, ...manyMoreArgs) {
  console.log("a", a); 
  console.log("b", b);
  console.log("manyMoreArgs", manyMoreArgs); 
}

myFun("one", "two", "three", "four", "five", "six");

// Console Output:
// a, one
// b, two
// manyMoreArgs, [three, four, five, six]

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 (i.e. formally defined in function expression), 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, "arguments" could be converted to a normal array using:

function f(a, b) {

  var normalArray = Array.prototype.slice.call(arguments);
  // -- or --
  var normalArray = [].slice.call(arguments);
  // -- or --
  var normalArray = Array.from(arguments);

  var first = normalArray.shift(); // OK, gives the first argument
  var first = arguments.shift(); // ERROR (arguments is not a normal array)

}

// Now we can easily gain access to a normal array using a rest parameter

function f(...args) {
  var normalArray = args;
  var first = normalArray.shift(); // OK, gives the first argument
}

Destructuring rest parameters

Rest parameters can be destructured (arrays only), that means that their data can be unpacked into distinct variables. See Destructuring assignment.

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

In this example, the first argument is mapped to "a" and the second to "b",  so these named arguments are used like normal. However the third argument  "manyMoreArgs" will be an array that contains the 3rd, 4th, 5th, 6th ... nth -- as many arguments that the user includes.

function myFun(a, b, ...manyMoreArgs) {
  console.log("a", a); 
  console.log("b", b);
  console.log("manyMoreArgs", manyMoreArgs); 
}

myFun("one", "two", "three", "four", "five", "six");

// a, one
// b, two
// manyMoreArgs, [three, four, five, six]

Below... even though there is just one value, the last argument still gets put into an array.

// using the same function definition from example above

myFun("one", "two", "three");

// a, one
// b, two
// manyMoreArgs, [three]

Below... the third argument wasn't provided, yet "manyMoreArgs" is still an array (although empty).

// using the same function definition from example above

myFun("one", "two");

// a, one
// b, two
// manyMoreArgs, []

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 all arguments after the first one in an array. Each one of them is then multiplied by the first parameter and the array is 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]

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)); // 1, 3, 5, 7

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


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

To use Array methods on the arguments object, it must be converted to a real array first.

function sortArguments() {
  var args = Array.from(arguments);
  var sortedArgs = args.sort();
  return sortedArgs;
}
console.log(sortArguments(5, 3, 7, 1)); // 1, 3, 5, 7

 

Specifications

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

Browser compatibility

Update compatibility data on GitHub
DesktopMobileServer
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung InternetNode.js
Basic supportChrome Full support 47Edge Full support 12Firefox Full support 15IE No support NoOpera Full support 34Safari Full support 10WebView Android Full support 47Chrome Android Full support 47Edge Mobile Full support 12Firefox Android Full support 15Opera Android Full support 34Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0
Full support 6.0.0
Full support 4.0.0
Disabled
Disabled From version 4.0.0: this feature is behind the --harmony runtime flag.
Destructuring rest parametersChrome Full support 49Edge No support NoFirefox Full support 52IE No support NoOpera Full support 36Safari ? WebView Android Full support 49Chrome Android Full support 49Edge Mobile No support NoFirefox Android Full support 52Opera Android Full support 36Safari iOS ? Samsung Internet Android Full support 5.0nodejs Full support Yes

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
User must explicitly enable this feature.
User must explicitly enable this feature.

See also