On the import and export of webpack

disturbwe 2021-05-04 11:05:29
import export webpack


Kwai , The interviewer asked esmodule and commonjs In two ways webpack Import and export parsing strategy for , And then not surprisingly , I was confused when asked . So we did some simple experiments , Later, we may go deep into the source code level to analyze these phenomena .

1.ES6 Import and export methods

// module.js
export let a = 1 // Named export 
let b = 2
export default b // The default is derived 
// index.js
import b, { a } from './module'
console.log(a + b)
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Let's take a look at the whole module

import * as module from './module'
/* {a: 1
default: 2
Symbol(Symbol.toStringTag): "Module"
__esModule: true
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You can see , This way of importing the namespace will import the whole module , If there is The default is derived Will take default In the way of , And to prevent naming conflicts ,default It's set as a keyword .

By the way Symbol(Symbol.toStringTag) This attribute

This is a built-in symbol, It is usually used as an object's property key , The corresponding property value should be of string type , This string is used to represent the custom type label of the object , Usually only built-in Object.prototype.toString() Method will read the tag and include it in its return value

class ValidatorClass {
[Symbol.toStringTag] = 'Validator'
get [Symbol.toStringTag]() {
return "Validator";
console.log(Object.prototype.toString.call(new ValidatorClass())); // [object Validator]
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When the interviewer asks about the type of candidate, there's something new to blow ( By mistake ).


Use commonjs Of require Back to a js object

// module.js
exports.someValue = 42;
// index.js
const module = require('./module')
/*{someValue: 42}*/
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exports (CommonJS)

The default value of this variable is module.exports( I.e. an object ). If module.exports If it's rewritten , exports No longer exported .

3. A mixture of es Grammar and commonjs grammar

es introduce common export

// module.js
module.exports = {
someValue: 42
// index.js
import * as module from './module'
// perhaps import module from './module' The result is the same 
/*{someValue: 42}*/
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It can be seen that , If the export is module.export Words , No matter how you import , It's all an ordinary js object .

If you use the following named import method , It will be extra take default Value is also initialized to this object . in other words , use exports When exporting in the same way , In named import parsing , It's going to fit two es Import the way .

// module.js
exports.someValue = 42;
// index.js
import * as module from './module'
/* {default: {someValue: 42}
someValue: (...)
Symbol(Symbol.toStringTag): "Module"
__esModule: true
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common introduce es export

In this situation ,const ...= require(...) Can be equated with import * as ... from '...', That is to say, it will be introduced into parsing as a name .

// module.js
export let someValue = 42;
// index.js
const module = require('./module')
/* {someValue: 42
Symbol(Symbol.toStringTag): "Module"
__esModule: true
 Copy code 

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