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default 关键字可以在 JavaScript 的两种情况下使用:在 switch ,或 export 中。

语法

switch 语句中使用:

switch (expression) {
  case value1:
    //当表达式的值和value1匹配执行这里的语句
    [break;]
  default:
    //当表达式的值没有匹配,执行这里的语句
    [break;]
}

export 中使用:

export default nameN 

描述

更多细节,参见

示例

switch语句中使用default

在以下示例中,如果expr为“Oranges”或“Apples”,程序将匹配“Oranges”或“Apples”的值并执行相应的声明。在任何其它情况下,default关键字将执行关联的语句。

switch (expr) {
  case "Oranges":
    console.log("Oranges are $0.59 a pound.");
    break;
  case "Apples":
    console.log("Apples are $0.32 a pound.");
    break;
  default:
    console.log("Sorry, we are out of " + expr + ".");
}

export语句中使用default

如果要导出单个值或需要模块的回掉值,则可以使用默认导出: 

// module "my-module.js"
let cube = function cube(x) {
  return x * x * x;
}
export default cube;

然后,在另一个脚本中,默认导出将直接被导入:

// module "my-module.js"
import myFunction from 'my-module';
console.log(myFunction(3)); // 27

规范

Specification Status Comment
ECMAScript 2015 (6th Edition, ECMA-262)
switch statement
Standard  
ECMAScript 2015 (6th Edition, ECMA-262)
Exports
Standard  
ECMAScript Latest Draft (ECMA-262)
switch statement
Draft  
ECMAScript Latest Draft (ECMA-262)
Exports
Draft  

浏览器兼容

FeatureChromeEdgeFirefoxInternet ExplorerOperaSafari
default keyword in switch Yes Yes1 Yes Yes Yes
default keyword with export61

16

151

60

54 — 602

No4710.1
FeatureAndroid webviewChrome for AndroidEdge mobileFirefox for AndroidOpera AndroidiOS SafariSamsung Internet
default keyword in switch Yes Yes Yes4 Yes Yes Yes
default keyword with export No61 Yes

60

54 — 602

4710.1 No

1. From version 15: this feature is behind the Experimental JavaScript Features preference.

2. From version 54 until version 60 (exclusive): this feature is behind the dom.moduleScripts.enabled preference. To change preferences in Firefox, visit about:config.

See also

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此页面的贡献者: zhangchen, binguanghe, Lukas-LiuYi, fscholz
最后编辑者: zhangchen,