02. Use of npm

Usage scenarios for npm
- Allows users to download third-party packages written by others from the NPM server for local use.
- Allows users to download and install command-line programs written by others from the NPM server for local use.
- Allows users to upload their own packages or command-line programs to the NPM server for others to use.
npm is installed with nodejs, you can use "npm -v" to view the version number and test whether the installation is successful.
Upgrade npm using the following command
```
npm install npm -g
```
Use the npm command to install and uninstall the module

installation syntax format such as npm install <Module Name>
Use npm to locally install the Node.js web framework module express, such as
```
npm install express
```
After installation, the express package is placed in the node_modules directory under the project directory, which can be directly referenced such as
```
var express = require('express'); //Use require to import directly
```
Uninstall syntax format such as npm uninstall <Module Name>
uninstall express such as
```
npm uninstall express
```
After uninstalling, the package no longer exists in the /node_modules/ directory, you can use the following command to view
```
above sea level ls
```
Global installation and local installation

Local installation Enter the following command in the terminal
```
npm install express
```
The directory of the local installation
is the path/node_modules/ where the npm command is run. You
can import the local installation package through require() and install it

globally. Enter the following command in the terminal
```
npm install express -g
```
全局安装目录为
C:\Users\[当前用户名]\AppData\Roaming\npm 或者 node的安装目录
可直接在命令行中使用

如果你希望具备两者功能，则需要在两个地方安装它或使用 npm link。

可以通过npm list 或 npm ls查看安装信息,如
例子1,查看所有的全局安全模块
```
npm list -g
```
例子2,查看单个本地安装模块
```
npm list express
```

package.json的定义

package.json 位于模块的目录下，用于定义包的属性。如node_modules/express/package.json

package.json属性说明

name	包名
version	版本
description	包描述
homepage	包官网url
author	包作者姓名
contributors	包的其他贡献者姓名
dependencies	依赖包列表,如果依赖包没有安装,npm会自动将包安装在node_modules目录下
repository	包代码存放的地方类型,可以是git或svn,git可以放在github上
main	main字段指定了程序的主入口文件,require('moduleName')就会加载这个文件.这个字段的默认值是模块根目录下面的index.js
keywords	关键字

npm更新与搜索模块

npm search <Module Name>
npm update <Module Name>

创建模块
首先,创建模块需先生成package.json,可使用以下命令并且根据提示要求输入信息生成
```
npm init
```
然后,在npm资源库中注册用户
```
npm adduser
Username: yonghuming
Password: mima
Email: (this IS public) [email protected]
```
之后,发布模块
```
npm publish
```
发布后就可以用npm命令安装使用本模块了

模块版本号为X.Y.Z三位

Z	如果只是修复bug，需要更新Z位。
Y	如果是新增了功能，但是向下兼容，需要更新Y位。
X	如果有大变动，向下不兼容，需要更新X位。

"argv": "0.0.x"表示依赖于0.0.x系列的最新版argv

npm其它常用命令
- npm help查看所有命令,npm help <command>查看指定命令
- 在package.json所在目录使用npm install . -g 可先在本地安装当前命令行程序,可用于发布前的本地测试
- npm update <package> 可以更新本地安装模块,npm update <package> -g 可以更新全局安装模块(命令行程序)
- npm cache clear 清空本地缓存,可以解决用相同版本号发布新版本的冲突.
- npm unpublish <package>@<version>可以撤销发布自己发布过的某个版本的代码

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