yarn包管理器在添加、更新、删除模块时，在项目中是如何体现的

技术很久不用，就变得生疏起来。对npm深受其害，决定对yarn再整理一遍。

yarn包管理器

介绍
安装yarn
帮助信息
最常用命令

介绍

yarn官网：https://yarn.bootcss.com，学任何技术的最新知识，都可以通过其对应的网站了解。无法直接访问，那就只能科学上网了。

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安装yarn

–global 或 -g选项很重要，表示全局安装，后期本机各个项目都得用这个包管理器

# 安装命令
npm install --global yarn

帮助信息

不管在哪个操作系统（Linux、Windows、Mac还是其它），一般都是可以应用命令 -h或者命令 --help获得命令的帮助信息。

F:\yarn>yarn -hUsage: yarn [command] [flags]Displays help information.Options:--cache-folder <path>               specify a custom folder that must be used to store the yarn cache--check-files                       install will verify file tree of packages for consistency--cwd <cwd>                         working directory to use (default: F:\yarn)--disable-pnp                       disable the Plug'n'Play installation--emoji [bool]                      enable emoji in output (default: false)--enable-pnp, --pnp                 enable the Plug'n'Play installation--flat                              only allow one version of a package--focus                             Focus on a single workspace by installing remote copies of its sibling workspaces.--force                             install and build packages even if they were built before, overwrite lockfile--frozen-lockfile                   don't generate a lockfile and fail if an update is needed--global-folder <path>              specify a custom folder to store global packages--har                               save HAR output of network traffic--https-proxy <host>--ignore-engines                    ignore engines check--ignore-optional                   ignore optional dependencies--ignore-platform                   ignore platform checks--ignore-scripts                    don't run lifecycle scripts--json                              format Yarn log messages as lines of JSON (see jsonlines.org)--link-duplicates                   create hardlinks to the repeated modules in node_modules--link-folder <path>                specify a custom folder to store global links--modules-folder <path>             rather than installing modules into the node_modules folder relative to the cwd, output them here--mutex <type>[:specifier]          use a mutex to ensure only one yarn instance is executing--network-concurrency <number>      maximum number of concurrent network requests--network-timeout <milliseconds>    TCP timeout for network requests--no-bin-links                      don't generate bin links when setting up packages--no-default-rc                     prevent Yarn from automatically detecting yarnrc and npmrc files--no-lockfile                       don't read or generate a lockfile--non-interactive                   do not show interactive prompts--no-node-version-check             do not warn when using a potentially unsupported Node version--no-progress                       disable progress bar--offline                           trigger an error if any required dependencies are not available in local cache--otp <otpcode>                     one-time password for two factor authentication--prefer-offline                    use network only if dependencies are not available in local cache--preferred-cache-folder <path>     specify a custom folder to store the yarn cache if possible--prod, --production [prod]--proxy <host>--pure-lockfile                     don't generate a lockfile--registry <url>                    override configuration registry-s, --silent                        skip Yarn console logs, other types of logs (script output) will be printed--scripts-prepend-node-path [bool]  prepend the node executable dir to the PATH in scripts--skip-integrity-check              run install without checking if node_modules is installed--strict-semver--update-checksums                  update package checksums from current repository--use-yarnrc <path>                 specifies a yarnrc file that Yarn should use (.yarnrc only, not .npmrc) (default: )-v, --version                       output the version number--verbose                           output verbose messages on internal operations-h, --help                          output usage informationCommands:- access- add- audit- autoclean- bin- cache- check- config- create- exec- generate-lock-entry / generateLockEntry- global- help- import- info- init- install- licenses- link- list- login- logout- node- outdated- owner- pack- policies- publish- remove- run- tag- team- unlink- unplug- upgrade- upgrade-interactive / upgradeInteractive- version- versions- why- workspace- workspacesRun `yarn help COMMAND` for more information on specific commands.Visit https://yarnpkg.com/en/docs/cli/ to learn more about Yarn.

通过帮助信息，一般可以了解该工具有哪些命令，哪些选项了。

最常用命令

1、初始化一个新项目

yarn init

一般会生成一个package.json文件，默认情况下，文件如下：

{"name": "yarn","version": "1.0.0","main": "index.js","license": "MIT"
}

yarn使用package.json文件来标识每个包，其含义看配置文档。
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2、安装所有依赖项

yarn
yarn install

会多出来一个node_modules目录（用来存在后期安装的依赖文件）和yarn.lock文件。yarn.lock文件是用来固化依赖，提交代码时，也应该一同提交。
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3、添加依赖项
一次性也可以添加多个依赖项

yarn add [package]
yarn add [package1] [package2] [package3]
yarn add [package]@[version]
yarn add [package]@[tag]

不指定版本，一般安装的是最新版本，以安装jquery模块为例
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安装好以后，node_modules和package.json都有体现

可以去https://www.npmjs.com/搜索查询jquery，查看可用历史版本

注意：一个项目相同模块，只会存在一个。例如jquery3.7.1和jquery3.5.0在一个项目不可能同时存在。

添加不同类型依赖项

dependencies，普通依赖项，运行项目时需要用到的依赖。不管是在开发还是生产都需要用得到，比如jquery
```
# 普通依赖项用--save
yarn add jquery --save
```
devDependencies，开发依赖项。开发时使用到的依赖，生产不需要，如Babel（ES6转ES5）
```
# 开发依赖项，--save-dev
yarn add jquery --save-dev
```
peerDependencies，对等依赖，发布依赖包时使用。
optionalDependencies，可选依赖。可选的依赖包，如果此包安装失败，Yarn依然会提示安装进程成功。
bundledDependencies，要打包的依赖/捆绑依赖。