An in-depth analysis of packages and NPM in Node.js

This article will take you through the package management tools of Node.js: packages and NPM. I hope it will be helpful to you!

Abstract: Packages and NPM Node organize their own core modules, and also enable third-party file modules to be written and used in an orderly manner.

Package and NPM

Node organizes its own core modules and also enables third-party file modules to be written and used in an orderly manner. [Recommended learning: "nodejs Tutorial"]

However, in third-party modules, modules are still hashed in various places, and cannot directly reference each other.

So outside the module, packages and NPM are the mechanisms that connect modules.

• Schematic diagram of package organization module

The package specification definition of CommonJS is actually very simple, consisting of two parts: package structure and package description file composition.

Package structure

is used to organize various files in the package. It is an archive file, that is, a directory is directly packaged into a file in .zip or tar.gz format. .

Compliant package directory:

• package.json: package description file

• bin: used to store executables Directory for binary files

• lib: Directory for storing JavaScript code

• doc: Directory for storing documents

• test: Code used to store unit test cases

package.json package description file

All behaviors of NPM They are closely related to the fields of the package description file

Some fields:

• name: package name. Specification definitions need to be composed of lowercase letters and numbers, and no spaces are allowed. The package name must be unique to avoid duplicate name conflicts when it is announced to the public

• description: package introduction

• version: version number, about it The introduction is also mentioned in "Node.js Learning (1) - Introduction"

• keywords: Keyword array, mainly used for classification search in NPM.

• maintainers: List of package maintainers. Each maintainer consists of three attributes: name, email and web. NPM uses this attribute to authenticate permissions.

Format:

"maintainers":[{ "name":"kongchengji", "email":"111@.com", "web":"[http:](https://blog.csdn.net/qq_36171287)" }]

• contributors: list of contributors, the format is the same as the list of maintainers

• bugs: A web page address or email address where you can report bugs

• licenses: A list of licenses used by the current package, indicating under which licenses the package is used

Format:

 "licenses":[{ "type": "GPLv2", "url":"" }]
 // 或者
 "license": "ISC"

• repositories: A list of locations where the source code is hosted, indicating how and where the package source code can be accessed.

Format:

"repository": {
"type": "git",
"url": "git+https://github.com/kongchengji/UiSelfMade.git"
},

• dependencies: List of packages that are required to use the current package. This attribute is very important

• homepage: The website address of the current package

• os: Operating system support list, if the list is empty, no operation will be performed The system makes no assumptions

• cpi: CPU architecture support list

• engine: Supported JavaScript engine list

• directories: Package directory description

• implements: List of implementation specifications. Marks which CommonJS specifications the current package implements

• #scripts: Script description object. Mainly used by package managers to install, compile, test and uninstall packages

Format:

"scripts": {
        "dev": "webpack-dev-server --inline --progress --config build/webpack.dev.conf.js",
        "start": "npm run dev",
        "lint": "eslint --ext .js,.vue src",
        "build": "node build/build.js"
    },

The difference between NPM and package specifications is that there are four more fields:

• author: Package author:ok_man:

• bin: Some package authors want the package to be available as a command line tool.

• main: When a module is imported into a package, this field will be checked to a limited extent and used as the entry module for other modules in the package. If it does not exist, require will look for index.js, index.node, index.json in the package directory as the default entry

• devDependencies: Some modules only need dependencies during development.

devDependencies: development environment uses dependencies: production environment uses

front-end and back-end shared modules

After JavaScript appears in Node, There is an advantage--> Some modules can be shared between the front and back ends.

But there are still some differences between the front and back ends: sweat_drops:

Focus of the front and rear end modules

The front and back end JavaScript are respectively placed in HTPP The two ends play different roles.

Browser-side JavaScript needs to be distributed from the same server to multiple clients for execution. The bottleneck is bandwidth. When loading code from the network, server-side JavaScript requires the same code to be executed multiple times. The bottleneck is CPU and memory. Resources, loaded from disk

In front-end JavaScript, the main application is AMD specification.

CommonJS并不完全适用于前端JavaScript，比如Node的模块引入基本是同步的，但是前端引入如果使用同步引入，UI在初始化过程中需要花费很多时间等待脚本加载完成。

AMD规范

AMD规范是CommonJS规范的一个延伸，全称：Asynchronous Module Definition。

是异步模块定义

模块定义：define(id?,dependencies?, factory);

id 是模块的名字，它是可选的参数。

dependencies 指定了所要依赖的模块列表，它是一个数组，也是可选的参数

AMD需要在声明模块时指定所有的依赖，通过形参传递依赖到模块内容中：

define(['./a', './b'], function (dep1, dep2) {
    a.doSomethimg()
    b.doSomething()
});

CMD规范

与AMD规范相对的还有CMD规范，全称：CommonModule Definition。

是公共模块定义

这是由国内的玉伯（也是一位大佬）提出的

模块定义：define(factory)

CMD支持动态引入：

define(function(require, exports, module) {
    var a=require('./a')
    a.doSomethimg()
    var b=require('./b')
    b.doSomething()
})

在需要依赖模块时，随时调用require()引入即可

• CMD 推崇依赖就近; AMD 推崇依赖前置

• CMD 是延迟执行; AMD 是提前执行

• CMD性能好,因为只有用户需要的时候才执行; AMD用户体验好,因为没有延迟,依赖模块提前执行了

AMD和CMD最大的区别是对依赖模块的执行时机处理不同

兼容多种模块规范

创建一个hello方法，让hello方法能在不同运行环境中运行，兼容Node、AMD、CMD和常见浏览器

• 匿名函数前加一个;是个好习惯 name是方法名，definition是方法体

• 通过typeof检测环境是否为AMD或CMD还是Node环境

• 可以将模块执行结果挂载在window变量中，这样可以直接调用

原文地址：https://bbs.huaweicloud.com/blogs/307034?utm_source=juejin&utm_medium=bbs-ex&utm_campaign=other&utm_content=content

作者：空城机

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