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HomeWeb Front-endJS TutorialExamples of export and import in js

export

export is used to output the external interface of the module. The export command can be used as long as it is at the top level of the module. That is to say, if it is within a certain function scope, judgment statement, or conditional statement, an error will be reported. The export command has several output forms.

PS: export命令可用于输出任何数据类型,可以是Number, String, Boolean, Null, Undefined, Array, Object, Function .... 以输出函数与对象举例

1. export var obj = { name: 'keith' } // 直接输出

2. var obj = { name: 'keith' }
   export { obj }  // 使用该种形式输出时需要添加大括号
   export obj   // 不添加大括号时会报错,因为我们要输出的是该对象的引用。注意是对该对象的引用,而不是拷贝。这也意味着在该模块改变name属性,会导致另一个模块下name属性的变化,这点与CommonJS不同,CommonJS只是对某个对象的拷贝
   var num = function () { return 123 }
   export { num }  // 正确
   export num // 报错,输出对num的引用,而不是直接输出函数num

3. var obj = { name: 'keith' }
   export { obj as person }  // export命令支持接口的重命名

4  var obj = { name: 'keith' }
   export default obj
   // 输出默认值时不需要添加大括号, export default在一个模块中只能使用一次,但export命令可以使用多次

   // export default的本质:就是将某个变量命名为default
   // export default num
   // 等同于 export { num as default }
   // import Num from './module.js'
   // 等同于 import { defualt as Num } from './module.js'

5  export { num, obj }
   // export命令处于模块顶层的任何位置,都可以将变量输出
   // 不管是用var声明的变量,还是let声明的变量
   // 在执行过程中export命令会被默认放置在整个模块的最底层。
   var num = function () { return 123 };
   let obj = { name: 'keith' }
   // 相当于
   var num = function () { return 123 };
   var obj = { name: 'keith' }
   // ..函数、对象等其他数据类型
   // 被放置在模块最底层
   export { name, obj }

import

The export command is used to output the external interface of the module, and the import command is used to introduce functional interfaces provided by other modules. Like the export command, the import command can only be used at the top level of a module and cannot be used in function scopes, conditional statements, and judgment statements. The import command has the following forms.

1. import { num, obj } from './export.js'  // 模块名,可以不添加.js后缀,但需要写配置文件,与Node知识相关
   // import入的接口名字,要与export出的名字对应
   console.log(num()) // 123
   console.log(obj.name) // 'keith'

2. import { obj as person } from './export.js'
   // 可以修改import进来的变量名
   console.log(person.name) // 'keith'

3. import * as $ from './export.js'
   // 使用*用于模块的整体加载,并重命名为$对象.此时可以通过$对象取到export的对外接口
   console.log($.num()) // 123
   console.log($.obj.name) // 'keith'

4. import 'normalize.css'
   // 用于加载整个模块,此时不需要添加变量名

5. let obj = { name: 'keith' }
   export default obj

   import person from './module.js'
   import boy from './module.js'
   // 对应于export default 命令
   // 此时import进来的接口不需要添加大括号
   // 且支持import时的任意命名
   console.log(person.name) // 'keith'
   console.log(boy.name) // 'keith'

6. console.log(obj.name)  // 'keith'
   import { obj } from './export.js'
   // 与export相反,import默认会被提升到模块最顶层
   // 即
   import { obj } from './export.js'
   console.log(obj)

Composite writing method of export and import

If you introduce other modules into a module and then export the module, you can use the composite writing method of export and import

1. 使用{}导出模块
export { Hello, World } from './modules'
// 相当于
import { Hello, World } from './moudles'
export { Hello, World }

2. 改写模块的名字
export { Hello as Person } from './modules'
// 相当于
import { Hello as Person } from './modules'
export { Person }

3. 改写默认export default模块的名字
export { default as Person } from './modules'
// 相当于
import Person from './modoules'  // ./modules里的模块是通过export default导出的
export { Person }

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