Nodejs2day (modularization of nodejs, NPM download package, module loading mechanism)

Node.js Module classification ：

• Built-in module （ The built-in module is made up of Node.js Official , for example fs、path、http etc. ）
• Custom module （ Each user created .js file , Are custom modules ）
• Third-party module （ Modules developed by third parties , Not an official built-in module , Nor is it a user created custom module , You need to download it before using it ）

Load module

Each module has its own Scope , Prevent global variables from being contaminated .

Loading user-defined modules can also be omitted js suffix .

module object

console.log(module)

module.exports object

In the custom module , have access to module.exports object , Share the members in the module , For external use .(export It means export , adopt export Method to expose the objects in the module for external use )
For the outside world require() Method to import a custom module , What you get is module.exports The object that is pointed to .

exports object

because module.exports Words are complicated to write , To simplify the code for sharing members out ,Node Provides exports object . By default ,exports and module.exports Point to the same object . The final shared result , Or to module.exports The object pointed to shall prevail .

exports and module.exports The use of mistakes

require() When the module , What you get is always module.exports Object to point to ：

Be careful ： To prevent chaos , Don't use... In the same module at the same time exports and module.exports

Node.js Followed CommonJS Modular specification

CommonJS Regulations ：

1. Inside each module ,module Variable Represents the current module .
2. module Variable is an object , its exports attribute （ namely module.exports） It's an external interface .
3. Load a module , In fact, the module is loaded module.exports attribute .require() Method for loading modules .

npm With bag

• The world's largest package sharing platform : https://www.npmjs.com/ , You can search this website for any package you need , As long as you have enough patience ！
• Share the platform through this package , Over time , You can also publish and share packages .
• npm, Inc. company Provided an address of https://registry.npmjs.org/ Server for , To share all the packages , We can download the packages we need from this server .( Direct access is useless )

Use moment Package example

Command line download script （ route ： Project folder ）

npm i moment

Code

// 1.  Import required packages 
const moment = require('moment')
const dt = moment().format('YYYY-MM-DD HH:mm:ss')
console.log(dt)

Command line run ：

moment Official documents ：
https://momentjs.com/docs/#/use-it/

What documents are added after the initial packaging

1. node_modules Folder ： Used to store all packages installed in the project .require() When importing third party packages , Is to find and load packages from this directory .
2. package-lock.json The configuration file ： Used to record node_modules Download information of each package in the directory , For example, the name of the bag 、 Version number 、 Download address, etc .
   Be careful ： Programmers should not manually modify node_modules or package-lock.json Any code in the file ,npm Package management tools maintain them automatically .
3. package.json( Current version npm Automatically created ): It is used to record some configuration information related to the project
   package.json The contents are as follows ：

{
    
  "dependencies": {
    
    "moment": "^2.29.4"
  }
}

dependencies node

You can see ：package.json In file , There is one dependencies node , Specifically used to record your use of npm install Command which packages are installed .

npm install One time download dependecies All packages in

devDependencies node

If some packages are only used in the project development phase , stay It will not be used after the project goes online , It is recommended that these packages be recorded in devDependencies In nodes .
Corresponding , If Some packages need to be used after development and project launch , It is recommended that these packages be recorded in dependencies In nodes .

You can use the following command , Record the package to devDependencies In nodes ：

Package management profile

npm Regulations , stay Project root in , A must be provided called package.json Package management profile for . It is used to record some configuration information related to the project . for example ：
Project name 、 Version number 、 describe etc.
What packages are used in the project
What kind of package Only during development use
Those bags are in Development and deployment You need to use

• In the project root directory , Create a package.json Configuration file for , It can be used to record which packages are installed in the project . thus Convenient elimination node_modules After the catalog , Share the source code of the project among team members .
  Be careful ： In the future, in the project development , Must put node_modules Folder , Add to .gitignore Ignore files .

Switch npm The next package image source

nrm

In order to switch the image source of the next package more conveniently , We can install nrm This gadget , utilize nrm Terminal commands provided , You can quickly view and switch the image source of the next package .

Module loading mechanism

Modules are cached after the first load . This also means multiple calls require() It will not cause the code of the module to be executed multiple times .
Be careful ： Whether it's a built-in module 、 User defined module 、 Third party modules , They are all preferentially loaded from the cache , thus Improve the loading efficiency of the module .

• Built-in module By Node.js Official module , Built in modules have the highest priority to load .

Custom module loading mechanism

Use require() When loading a custom module , Must be specified to ./ or …/ The path identifier at the beginning .
When loading custom modules , If not specified ./ or …/ Such a path identifier , be node It will be loaded as a built-in module or a third-party module .
meanwhile , In the use of require() When importing custom modules , If the file extension is omitted , be Node.js The following files will be tried to load in order ：
1. Load according to the exact file name
2. completion .js Extension to load
3. completion .json Extension to load
4. completion .node Extension to load
5. Loading failed , The terminal reported an error

Loading mechanism of third-party modules

If passed to require() The module identifier of is not a built-in module , And it didn't ‘./’ or ‘…/’ start , be Node.js Will start from the parent directory of the current module , Try from /node_modules Load third party modules in the folder .
If the corresponding third-party module is not found , Then move to the next parent directory , Loading , Up to the root of the file system .
for example , Suppose that 'C:\Users\itheima\project\foo.js' The file calls require(‘tools’), be Node.js It will be searched in the following order ：

• C:\Users\ithei\project\node_modules\tools
• C:\Users\ithei\node_modules\tools
• C:\Users\node_modules\tools
• C:\node_modules\tools

Directory as module identifier

When the directory is used as the module identifier , Pass to require() When loading , There are three ways to load ：

• Find a directory called... Under the loaded directory package.json The file of , And look for main attribute , As require() Loaded entry
  main Property example ：

{
    
"main":"./a.js"    // Need to have a.js This document and package.json In the peer Directory 
}

• If it's not in the catalog package.json file , perhaps main The entry does not exist or cannot be resolved , be Node.js Will try to load... In the directory index.js file .
• If both steps fail , be Node.js Error messages will be printed on the terminal , Missing report module ：Error: Cannot find module 'xxx'

require('./testm')   // Access the same level directory testm This folder example 

There is no package.json, But there are index.js The running results of the file are as follows