This translation is incomplete. Please help translate this article from English.

Default function parameters allow formal parameters to be initialized with default values if no value or undefined is passed.

Syntax

function [name]([param1[ = defaultValue1 ][, ..., paramN[ = defaultValueN ]]]) {
   statements
}

Description

In JavaScript, parameters of functions default to undefined. However, in some situations it might be useful to set a different default value. This is where default parameters can help.

In the past, the general strategy for setting defaults was to test parameter values in the body of the function and assign a value if they are undefined. If in the following example, no value is provided for b in the call, its value would be undefined  when evaluating a*b and the call to multiply would have returned NaN. However, this is caught with the second line in this example:

function multiply(a, b) {
  b = (typeof b !== 'undefined') ?  b : 1;
  return a * b;
}

multiply(5, 2); // 10
multiply(5, 1); // 5
multiply(5);    // 5

With default parameters in ES2015, the check in the function body is no longer necessary. Now, you can simply put 1 as the default value for b in the function head:

function multiply(a, b = 1) {
  return a * b;
}

multiply(5, 2); // 10
multiply(5, 1); // 5
multiply(5);    // 5

Examples

Passing undefined vs. other falsy values

In the second call here, even if the first argument is set explicitly to undefined (though not null or other falsy values) when calling, the value of the num argument is the default one.

function test(num = 1) {
  console.log(typeof num);
}

test();          // 'number' (num is set to 1)
test(undefined); // 'number' (num is set to 1 too)

// test with other falsy values:
test('');        // 'string' (num is set to '')
test(null);      // 'object' (num is set to null)

Evaluated at call time

The default argument gets evaluated at call time, so unlike e.g. in Python, a new object is created each time the function is called.

function append(value, array = []) {
  array.push(value);
  return array;
}

append(1); //[1]
append(2); //[2], not [1, 2]

This even applies to functions and variables:

function callSomething(thing = something()) {
 return thing;
}

function something() {
  return 'sth';
}

callSomething();  //sth

Default parameters are available to later default parameters

Parameters already encountered are available to later default parameters:

function singularAutoPlural(singular, plural = singular + 's', 
                            rallyingCry = plural + ' ATTACK!!!') {
  return [singular, plural, rallyingCry]; 
}

//["Gecko","Geckos", "Geckos ATTACK!!!"]
singularAutoPlural('Gecko');

//["Fox","Foxes", "Foxes ATTACK!!!"]
singularAutoPlural('Fox', 'Foxes');

//["Deer", "Deer", "Deer ... change."]
singularAutoPlural('Deer', 'Deer', 'Deer peaceably and respectfully \
   petition the government for positive change.')

This functionality is approximated in a straight forward fashion and demonstrates how many edge cases are handled.

function go() {
  return ':P';
}

function withDefaults(a, b = 5, c = b, d = go(), e = this, 
                      f = arguments, g = this.value) {
  return [a, b, c, d, e, f, g];
}

function withoutDefaults(a, b, c, d, e, f, g) {
  switch (arguments.length) {
    case 0:
      a;
    case 1:
      b = 5;
    case 2:
      c = b;
    case 3:
      d = go();
    case 4:
      e = this;
    case 5:
      f = arguments;
    case 6:
      g = this.value;
    default:
  }
  return [a, b, c, d, e, f, g];
}

withDefaults.call({value: '=^_^='});
// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]


withoutDefaults.call({value: '=^_^='});
// [undefined, 5, 5, ":P", {value:"=^_^="}, arguments, "=^_^="]

Functions defined inside function body

Introduced in Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30). Functions declared in the function body cannot be referred inside default parameters and throw a ReferenceError (currently a TypeError in SpiderMonkey, see bug 1022967). Default parameters are always executed first, function declarations inside the function body evaluate afterwards.

// Doesn't work! Throws ReferenceError.
function f(a = go()) {
  function go() { return ':P'; }
}

Parameters without defaults after default parameters

Prior to Gecko 26 (Firefox 26 / Thunderbird 26 / SeaMonkey 2.23 / Firefox OS 1.2), the following code resulted in a SyntaxError. This has been fixed in bug 777060 and works as expected in later versions. Parameters are still set left-to-right, overwriting default parameters even if there are later parameters without defaults.

function f(x = 1, y) { 
  return [x, y]; 
}

f(); // [1, undefined]
f(2); // [2, undefined]

Destructured parameter with default value assignment

You can use default value assignment with the destructuring assignment notation:

function f([x, y] = [1, 2], {z: z} = {z: 3}) { 
  return x + y + z; 
}

f(); // 6

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 AndroidSafari on iOSSamsung InternetNode.js
Basic supportChrome Full support 49Edge Full support 14Firefox Full support 15IE No support NoOpera Full support 36Safari Full support 10WebView Android Full support 49Chrome Android Full support 49Edge Mobile Full support 14Firefox Android Full support 15Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support 6.0.0
Parameters without defaults after default parametersChrome Full support 49Edge Full support 14Firefox Full support 26IE No support NoOpera Full support 36Safari Full support 10WebView Android Full support 49Chrome Android Full support 49Edge Mobile Full support 14Firefox Android Full support 26Opera Android Full support 36Safari iOS Full support 10Samsung Internet Android Full support 5.0nodejs Full support Yes
Destructured parameter with default value assignmentChrome Full support 49Edge ? Firefox Full support 41IE No support NoOpera ? Safari ? WebView Android Full support 49Chrome Android Full support 49Edge Mobile ? Firefox Android Full support 41Opera Android ? Safari iOS ? Samsung Internet Android Full support 5.0nodejs Full support Yes

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown

See also

Document Tags and Contributors

Contributors to this page: brandon1780
Last updated by: brandon1780,