Default function parameters allow named 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, function parameters default to undefined. However, it's often 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 function body and assign a value if they are undefined.

In the following example, if no value is provided for b when multiply is called, b’s value would be undefined  when evaluating a * b and multiply would return NaN.

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

multiply(5, 2); // 10
multiply(5);    // NaN !

To guard against this, something like the second line would be used, where b is set to 1 if multiply is called with only one argument:

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

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

With default parameters in ES2015, checks in the function body are no longer necessary. Now, you can assign 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);    // 5

Examples

Passing undefined vs. other falsy values

In the second call in this example, even if the first argument is set explicitly to undefined (though not null or other falsy values), the value of the num argument is still the default.

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 is evaluated at call time, so unlike e.g. 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;
}

let numberOfTimesCalled = 0;
function something() {
  numberOfTimesCalled += 1;
  return numberOfTimesCalled;
}

callSomething(); // 1
callSomething(); // 2

Default parameters are available to later default parameters

Parameters defined beforehand (to the left) are available to later default parameters:

function greet(name, greeting, message = greeting + ' ' + name) {
    return [name, greeting, message];
}

greet('David', 'Hi');  // ["David", "Hi", "Hi David"]
greet('David', 'Hi', 'Happy Birthday!');  // ["David", "Hi", "Happy Birthday!"]

This functionality can be approximated like this, which 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 to inside the outer function]s default parameters. If attempted, a ReferenceError is thrown. Default parameters are always executed first, so 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 was fixed in bug 777060. 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