default 关键字可以在 JavaScript 的两种情况下使用:在 export
中。
语法
在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;] }
在export
中使用:
export default nameN
描述
更多细节,参见
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
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
The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out
https://github.com/mdn/browser-compat-data and send us a pull request.
Update compatibility data on GitHub
Desktop | Mobile | Server | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
default keyword in switch |
Chrome Full support 1 | Edge Full support 12 | Firefox Full support 1 | IE Full support 4 | Opera Full support Yes | Safari Full support Yes | WebView Android Full support 1 | Chrome Android Full support 18 | Firefox Android Full support 4 | Opera Android Full support Yes | Safari iOS Full support Yes | Samsung Internet Android Full support 1.0 | nodejs Full support Yes |
default keyword with export |
Chrome Full support 61 | Edge Full support 16
|
Firefox Full support 60
|
IE No support No | Opera Full support 47 | Safari Full support 10.1 | WebView Android No support No | Chrome Android Full support 61 | Firefox Android Full support 60
|
Opera Android Full support 44 | Safari iOS Full support 10.3 | Samsung Internet Android Full support 8.0 | nodejs ? |
Legend
- Full support
- Full support
- No support
- No support
- Compatibility unknown
- Compatibility unknown
- User must explicitly enable this feature.
- User must explicitly enable this feature.