In-depth analysis of node.js exports, module.exports and ES6 export, export default

This article mainly introduces to you the relevant information about exports and module.exports in node.js and what export and export default are in ES6. The article introduces it in detail through sample code, which is very useful for everyone. Studying or working has certain reference learning value. Friends who need it can come and study together.

Preface

#I have rarely had time recently, so I decided to start learning node programming again in a standardized way. But when introducing modules, I saw the require method, which reminded me of our various ES6 exports and export defaults.

Axi, it’s all over the place...

It’s all over, let’s sit down and sort out their scope of use first.

• require: Import supported by both node and es6

• export / import: Only export supported by es6

• module.exports / exports: Only exports supported by node

From this moment on, I think it’s time to clarify the relationship between them, otherwise I have to die in chaos. Not much to say, let’s get started! !

node module

The module system in Node follows the CommonJS specification.

Then the question comes again, what is the CommonJS specification?

Because js was relatively confusing in the past, everyone wrote their own code, and there was no concept of a module, and this specification is actually a definition of a module.

The modules defined by CommonJS are divided into: module identification (module), module definition (exports), module reference (require)

First explain exports and module.exports

When a node executes a file, an exports and module object will be generated in the file,

and module has an exports attribute. The relationship between them is as shown below, they all point to a {} memory area.

exports = module.exports = {};

Let’s take a look at the code.

//utils.js
let a = 100;

console.log(module.exports); //能打印出结果为：{}
console.log(exports); //能打印出结果为：{}

exports.a = 200; //这里辛苦劳作帮 module.exports 的内容给改成 {a : 200}

exports = '指向其他内存区'; //这里把exports的指向指走

//test.js

var a = require('/utils');
console.log(a) // 打印为 {a : 200}

As can be seen from the above, in fact, the content exported by require is the content of the memory block pointed to by module.exports, not exports.

In short, the difference between them is that exports is just a reference to module.exports, which is used to assist the latter in adding content.

To put it in plain language, exports only assists module.exports in operating the data in the memory. After all the hard work of operating the data, I am exhausted. In the end, the content that is actually required is still the module. .exports is really hard work.

In fact, if you use the concept of memory block to understand it, it will be very clear.

Then, in order to avoid confusion, try to use module.exports to export, and then use require to import.

Module export and import in ES

To be honest, the modules in es are very clear. However, there are some details that need to be clarified.

For example, export and export default, and when importing, import a from .., import {a} from .., in short, it is a bit confusing, so let’s start to sort them out. .

export and export default

First we talk about these two exports, and now we talk about their differences

• Export and export default can both be used to export constants, functions, files, modules, etc.

• In a file or module, there can be multiple exports and imports, but there is only one export default.

• Export through export, add { } when importing, export default is not required

• export can directly export variable expressions. export default does not work.

Let’s take a look at the code to verify it

testEs6Export.js

'use strict'
//导出变量
export const a = '100'; 

 //导出方法
export const dogSay = function(){ 
 console.log('wang wang');
}

 //导出方法第二种
function catSay(){
 console.log('miao miao'); 
}
export { catSay };

//export default导出
const m = 100;
export default m; 
//export defult const m = 100;// 这里不能写这种格式。

index.js

//index.js
'use strict'
var express = require('express');
var router = express.Router();

import { dogSay, catSay } from './testEs6Export'; //导出了 export 方法 
import m from './testEs6Export'; //导出了 export default 

import * as testModule from './testEs6Export'; //as 集合成对象导出



/* GET home page. */
router.get('/', function(req, res, next) {
 dogSay();
 catSay();
 console.log(m);
 testModule.dogSay();
 console.log(testModule.m); // undefined , 因为 as 导出是 把 零散的 export 聚集在一起作为一个对象，而export default 是导出为 default属性。
 console.log(testModule.default); // 100
 res.send('恭喜你，成功验证');
});

module.exports = router;

You can see it from above Yes, I do feel that the ES6 module system is very flexible.

Code address

How to use the JavaScript project construction tool gulp based on Node.js

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