默认参数值

函数默认参数允许在没有值或undefined被传入时使用默认形参。

语法

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

描述

JavaScript 中函数的参数默认是undefined。然而,在某些情况下可能需要设置一个不同的默认值。这是默认参数可以帮助的地方。

以前,一般设置默认参数的方法是在函数体测试参数是否为undefined,如果是的话就设置为默认的值。

下面的例子中,如果在调用multiply时,参数b的值没有提供,那么它的值就为undefined。如果直接执行a * b,函数会返回 NaN

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

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

为了防止这种情况,第二行代码解决了这个问题,其中如果只使用一个参数调用multiply,则b设置为1

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

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

有了默认参数,我们不需要再在函数体内做不必要的检查。现在你可以在函数头将b的默认值置为1

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

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

示例

传入 undefined vs 其他假值

在第二次调用中,即使第一个参数在调用时显式设置为undefined(虽然不是null或其他falsy值),但是num参数的值是默认值。

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)

调用时解析

在函数被调用时,参数默认值会被解析,所以不像Python中的例子,每次函数调用时都会创建一个新的参数对象。

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

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

这个规则对于函数和变量也是适用的。

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

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

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

默认参数可用于后面的默认参数

已经遇到的参数可用于以后的默认参数:

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!"]

以下这个例子近似模拟了一些比较简单的情况,并说明了特殊情况是怎么被处理的。

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, "=^_^="]

函数嵌套定义

在 Gecko 33 (Firefox 33 / Thunderbird 33 / SeaMonkey 2.30) 中引入。在函数体内的函数声明不能引用内部的默认参数,并且会在 SpiderMonkey 抛出一个bug 1022967。默认参数总是会被首先执行,而在函数体内部的函数声明会在之后生效。

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

位于默认参数之后非默认参数

在Gecko 26 (Firefox 26 / Thunderbird 26 / SeaMonkey 2.23 / Firefox OS 1.2)之前,以下代码会造成bug 1022967中修复,并在以后的版本中按预期方式工作。参数仍然设置为从左到右,覆盖默认参数,即使后面的参数没有默认值。

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

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

有默认值的解构参数

你可以通过解构赋值为参数赋值:

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

f(); // 6

规范

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
Function Definitions
Standard Initial definition.
ECMAScript Latest Draft (ECMA-262)
Function Definitions
Draft  

浏览器兼容

Update compatibility data on GitHub
Desktop Mobile Server
Chrome Edge Firefox Internet Explorer Opera Safari Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet Node.js
Default parameters Chrome Full support 49 Edge Full support 14 Firefox Full support 15 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Firefox Android Full support 15 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support 6.0.0
Destructured parameter with default value assignment Chrome Full support 49 Edge Full support 14 Firefox Full support 41 IE No support No Opera Full support 36 Safari ? WebView Android Full support 49 Chrome Android Full support 49 Firefox Android Full support 41 Opera Android ? Safari iOS ? Samsung Internet Android Full support 5.0 nodejs Full support Yes
Parameters without defaults after default parameters Chrome Full support 49 Edge Full support 14 Firefox Full support 26 IE No support No Opera Full support 36 Safari Full support 10 WebView Android Full support 49 Chrome Android Full support 49 Firefox Android Full support 26 Opera Android Full support 36 Safari iOS Full support 10 Samsung Internet Android Full support 5.0 nodejs Full support Yes

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
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