node environment and CommonJS specification

1. Environment

First, let's take a look at what nodejs mainly needs to execute

• CommonJS

commonjs is the module specification used by nodejs

• global object

We know that JavaScript running in the browser contains BOM and DOM, of which window is a global object, but nodejs does not have BOM and DOM, and there is no such global object as window. Instead, it is global, which also has some attributes and method for us to use.

• process

Represents the currently executing process, hanging under global

1.1 CommonJS

• Each file is a module and has its own scope
• Inside a module the module variable represents the module itself
• The module.exports attribute represents the external interface of the module

To understand the above, let's write a piece of code like this

Then run it on the command line and enter the following line of code to enter debug mode (--inspect enters debug mode, -brk stops at the beginning)

Then we open chrome to enter chrome://inspect , press Enter, you can see the following interface

Click inspect to enter the debugging interface

As we can see from the figure, when the code we actually execute is running in nodejs, it is wrapped by a parenthesis and a semicolon, and inside is a function, which is done by nodejs for us (the scope of the module itself). ), this function passes in 5 variables, which are

• exports

exports is an object that represents the output of the module, and provides a module interface to the outside world, pointing to module.exports.

Note that you can't point the exports variable directly to a value, because doing so would cut exports from module.exports.

The following is invalid, because exports no longer points to module.exports.

exports = function(x) {console.log(x)};
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The following writing hello function cannot be exported, because module.exports has been reassigned.

exports.hello = function() {
  return 'hello';
};
module.exports = 'Hello world';`
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This means that if the external interface of a module is a single value, it cannot be exported using exports, but can only be exported using module.exports.

• require

require is a function, indicating the function that needs to be called when calling other modules

• module

module is an object with an exports property in it. represents the module itself

• __filename

file path

• __dirname

The path to the folder where the file is located

2. require

2.1 require rules

• / means absolute path, ./ means relative to the current file
• Support js, json, node extensions, try not to write in turn
• If the path is not written, it is considered to be a build-in module or a third-party module in node_modules at all levels

2.2 require feature

• Executed when the module is loaded, cached after loading

For example, let's write a module first

This module exports a variable num and prints a 'hello' at the beginning

Next, we import this module in another file

run this file

As you can see, we loaded the demo module twice, and hello was only printed once

So, the module will only be executed when it is loaded for the first time

• Once a module is loaded cyclically, only the executed part will be output, and the unexecuted part will not be output

This definition is a bit difficult to understand, but that's okay, let's understand it by example

Let's write three js files first

Then we execute the main.js file, the steps are as follows

1. First introduced the a.js file, let's load the a.js file

2. In the first line of a.js, x is assigned 'a1', and then b.js is introduced, and we go to load the b.js file

3. In the first line of b.js, assign x to 'b1', and then import the a.js file, which constitutes a circular reference, so only part of the content of a.js can be referenced, that is exports.x='a1', assign x to ' a1', so first print outb.js a1

4. Then, a.js is printed out a.js b2, because b.js has been executed, so the referenced x is b2

5. In the main.js file, since both a.js and b.js have been executed, print out main.js a2,main.js b2

Control point final print

Reprinted at: https://juejin.im/post/5d00a0336fb9a07ec27b9c3a