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