default

default

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: //Statements executed when the result of expression matches value1 [break;] default: //Statements executed when none of the values match the value of the expression [break;] }

export中使用:

export default nameN

描述

更多细节,参见

  • switch 语句和

  • export 语句页面。

示例

在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 + '.' }

在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

SpecificationStatusComment
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'switch statement' in that specification.Standard
ECMAScript 2015 (6th Edition, ECMA-262)The definition of 'Exports' in that specification.Standard
ECMAScript Latest Draft (ECMA-262)The definition of 'switch statement' in that specification.Draft
ECMAScript Latest Draft (ECMA-262)The definition of 'Exports' in that specification.Draft

浏览器兼容性

FeatureChromeEdgeFirefox (Gecko)Internet ExplorerOperaSafari
Switch default(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)
Export defaultNo support?No supportNo supportNo supportNo support

FeatureAndroidChrome for AndroidEdgeFirefox Mobile (Gecko)IE MobileOpera MobileSafari Mobile
Switch default(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)(Yes)
Export defaultNo supportNo support?No supportNo supportNo supportNo support