Detailed explanation of nodejs interview questions

A classic interview question for nodejs

this.d = 4;
exports.c = 3;
module.exports = {
    
    
     a: 1,
     b: 2
 }
//module.exports.a = 1
//module.exports.b = 2

const a = require('./a.js')
console.log(a);

The result of execution here is:

。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。。

this.d = 4;
exports.c = 3;
//module.exports = {
    
    
//     a: 1,
//     b: 2
// }
module.exports.a = 1
module.exports.b = 2

const a = require('./a.js')
console.log(a);

The result of the execution here is:

here only the two output methods are different, which leads to different results. The reason is related to the underlying code of node:
because require will go through the following steps when it is executed:
Note that the following codes are all The theoretical code cannot be executed, it is only used for analysis
1. The resolve method is called and spliced ​​into an absolute path

function require(modulePath){
    
    //模块路径
	
}

2. In the cache section, determine whether the module has a cache. cache is the cache area of ​​the require function

	if(require.cache["C:\\Users\\admin\\Desktop\\0315nodejs\\04.js"]){
    
    
		return require.cache["C:\\Users\\admin\\Desktop\\0315nodejs\\04.js"]
	}	

3. Read the content of the target file
4. Wrap it into a function

function require(modulePath){
    
    //模块路径
	if(require.cache["C:\\Users\\admin\\Desktop\\0315nodejs\\04.js"]){
    
    
		return require.cache["C:\\Users\\admin\\Desktop\\0315nodejs\\04.js"]
	}	
	function _temp(module,exports,require,__dirname,__filename){
    
    
	//在require目标对象的所有代码都在这个内部函数执行
		this.d = 4;
		exports.c = 3;
		module.exports = {
    
    
			a : 1,
			b : 2
		}
	}
}
	

5. Create a module.exports object (emphasis)

function require(modulePath){
    
    //模块路径
	if(require.cache["C:\\Users\\admin\\Desktop\\0315nodejs\\04.js"]){
    
    
		return require.cache["C:\\Users\\admin\\Desktop\\0315nodejs\\04.js"]
	}	
	function _temp(module,exports,require,__dirname,__filename){
    
    
	//在require目标对象的所有代码都在这个内部函数执行
		this.d = 4;
		exports.c = 3;
		module.exports = {
    
    
			a : 1,
			b : 2
		}
	}
	// 5.创建module.exports对象
		module.exports = {
    
    }
		const exports = module.exports
		_temp.call(module.exports,exports,require,__dirname,__filename)
	
		return module.exports;
}

When executing the internal function _temp, it will change the point of this through call, point it to module.exports and create a module.exports object, then create exports and assign module.exports to exports, that is, now this, module.exports, exports is actually a thing:

console.log(module.exports === exports)//true
console.log(module.exports === this)//true

Finally return to module.exports;

so

this.d = 4;
exports.c = 3;
module.exports.a = 1
module.exports.b = 2

The output {d:4, c:3, a:1, b:2} is because they are all adding things to module.exports;

but

this.d = 4;
exports.c = 3;
module.exports = {
    
    
     a: 1,
     b: 2
 }

There is a problem here. At the beginning, an empty object of module.exports was created, but now a new object is assigned. This leads to module.exports pointing to a new address.
Here I draw a picture to show the
first case: the

second case:

and the require function returns module.exports, which naturally outputs {a:1, b:2)