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Default 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.

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 multiple 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); // 5

With default parameters, 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); // 5

Examples

Passing undefined

In the second call here, even if the second argument is set explicitly to undefined when calling, the value of the color argument is the default one.

function setBackgroundColor(element, color = 'rosybrown') {
  element.style.backgroundColor = color;
}

setBackgroundColor(someDiv);            // color set to 'rosybrown'
setBackgroundColor(someDiv, undefined); // color set to 'rosybrown' too
setBackgroundColor(someDiv, 'blue');    // color set to 'blue' 

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 case 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", window, arguments, "=^_^="]


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

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

f(); // [1, 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.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support Not supported 15.0 (15.0) Not supported Not supported Not supported
Parameters without defaults after default parameters ? 26.0 (26.0) ? ? ?
Destructured parameter with default value assignment ? 41.0 (41.0) ? ? ?
Feature Android Chrome for Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) (Yes) 15.0 (15.0) Not supported Not supported Not supported
Parameters without defaults after default parameters ? ? 26.0 (26.0) ? ? ?
Destructured parameter with default value assignment ? ? 41.0 (41.0) ? ? ?

See also

Document Tags and Contributors

Last updated by: Zander,
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