Detailed explanation of exports in js

This article mainly shares with you the detailed explanation of exports in js, mainly in the form of code. I hope it can help everyone.

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Writing 1

exports.hello = function(){
    console.log(‘world’);
}

Writing 2

var f = {
    hello : function(){
        console.log(‘world’);
    }
}
module.exports = f;

Suppose we The file name of this module written is hello.js. Execute the following code

var h = require(‘hello’);
h.hello();

For the above two writing methods, the result obtained after executing this code is the same.

module.exports:

For example:

1,

//a.js
module.exports = ['aaa',18]
//b.js
var a= require('a')console.log(a[1]) //输出18

2,

//a.js
module.exports =function(){
this.show=function(){
console.log('hahah~');
}
}
//b.js
var a= require('a');
var obj = new a();obj .show();//输出hahah~
module.exports

My understanding is: you assign something to module.exports, and you will get something after require

exports ： 

//a.js
exports.show =function(){
console.log('hahah~');
}
//b.js
var a= require('a');
a.show();//输出hahah~

exports is already an object, you can add attributes to this object , what you get after require is this exports object.

But you cannot assign a new object to exports, such as exports={}

It should also be noted that if module.exports already has content, then all operations of exports will be invalid. Remember

Let’s talk about prototype again. What is prototype used for? It is to add attributes to the prototype. The prototype is like the parent class in C++. Let me give you another example
1,

//a.js
module.exports =function(){
}
module.exports.prototype.show = function(){
console.log('hahah~');
}
//b.js
var a= require('a');
var obj = new a()
obj.show()//输出hahah~

Finally, let’s talk about class methods. When it comes to classes, module.exports must be used.

Chestnuts are here

1,

//a.js
module.exports =function(){
}
module.exports.show = function(){
console.log('hahah~');
}
//b.js
var a= require('a');
a.show()//输出hahah~
##module.exports与exports的区别

Each node.js execution file automatically creates a module object. At the same time, the module object will create a module called The attribute of exports, the initialized value is {}

module.exports = {};
Node.js为了方便地导出功能函数，node.js会自动地实现以下这个语句
foo.js
exports.a = function(){
console.log('a')
}
exports.a = 1 
test.js
var x = require('./foo');
console.log(x.a)

Seeing this, I believe everyone has seen the answer, exports refers to the value of module.exports.

When module.exports is changed, exports will not be changed, and when the module is exported, the actual export execution is module.exports, not exports

Look at the following example

foo.js
exports.a = function(){
 console.log('a')
}
module.exports = {a: 2}
exports.a = 1 
test.js
var x = require('./foo');
console.log(x.a)
result:
2

exports becomes invalid after module.exports is changed.
Are you starting to feel a little enlightened? Here are some commonly seen usage methods in open source modules.

##module.exports = View
function View(name, options) { 
  options = options || {};
  this.name = name;
  this.root = options.root;
  var engines = options.engines;
  this.defaultEngine = options.defaultEngine;
  var ext = this.ext = extname(name);
  if (!ext && !this.defaultEngine) throw new Error('No default engine was specified and no         extension was provided.');
  if (!ext) name += (ext = this.ext = ('.' != this.defaultEngine[0] ? '.' : '') +     this.defaultEngine);
  this.engine = engines[ext] || (engines[ext] = require(ext.slice(1)).__express);
  this.path = this.lookup(name);
}
module.exports = View;

There is a saying in javascript that function is an object, View is an object, module.export =View, which is equivalent to exporting the entire view object.

When an external module calls it, it can call all methods of View. However, it should be noted that only static methods of View can be called. Methods created by prototype are private methods of View.

foo.js
function View(){
}
View.prototype.test = function(){
 console.log('test')
}
View.test1 = function(){
 console.log('test1')
}
module.exports = View
test.js
var x = require('./foo');
console.log(x) //{ [Function: View] test1: [Function] }
console.log(x.test) //undefined
console.log(x.test1) //[Function]
x.test1() //test1
##var app = exports = module.exports = {};

In fact, when we understand the principle, it is not difficult to understand that this way of writing is a bit redundant. In fact, it is to ensure that the initialization environment of the module is clean. At the same time, it is also convenient for us. Even after changing the object pointed to by module.exports, we can still use the characteristics of exports

exports = module.exports = createApplication;
/**
 * Expose mime.
 */
exports.mime = connect.mime;

Example, in which module.exports = createApplication changes module.exports, making exports invalid , let it restore its original characteristics through the exports = module.exports method.

##exports.init= function(){}

This is the simplest and directly exported module init method.

##var mongoose = module.exports = exports = new Mongoose;

It has multiple functions, but based on the above description, you should not be able to get the answer.

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