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
在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;]
}
在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
Specification | Status | Comment |
---|---|---|
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
Feature | Chrome | Edge | Firefox (Gecko) | Internet Explorer | Opera | Safari |
---|---|---|---|---|---|---|
Switch default | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Export default | No support | ? | No support | No support | No support | No support |
Feature | Android | Chrome for Android | Edge | Firefox Mobile (Gecko) | IE Mobile | Opera Mobile | Safari Mobile |
---|---|---|---|---|---|---|---|
Switch default | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) | (Yes) |
Export default | No support | No support | ? | No support | No support | No support | No support |