使用npm更新已有模块

安装npm-check-updates 模块升级插件

```
npm install -g npm-check-updates
或者
cnpm install -g npm-check-updates

查看安装版本：

```
ncu -v   #ncu是npm-check-updates的缩写命令
2.14.2

查看ncu的帮助

ncu  --help

  Usage: ncu [options] [filter]

  [filter] is a list or regex of package names to check (all others will be ignored).

  Options:

    -d, --dev                    check only devDependencies
    -e, --error-level         set the error-level. 1: exits with error code 0 if no errors occur. 2: exits with error code 0 if no packages need updating (useful for continuous integration). Default is 1. (default: 1)
    -f, --filter        include only package names matching the given string, comma-delimited list, or regex
    -g, --global                 check global packages instead of in the current project
    -j, --jsonAll                output new package file instead of human-readable message
    --jsonUpgraded               output upgraded dependencies in json
    -l, --loglevel            what level of logs to report: silent, error, minimal, warn, info, verbose, silly (default: warn) (default: warn)
    -m, --packageManager   npm (default) or bower (default: npm)
    -n, --newest                 find the newest versions available instead of the latest stable versions
    -o, --optional               check only optionalDependencies
    --packageData                include stringified package file (use stdin instead)
    --packageFile      package file location (default: ./package.json)
    --packageFileDir             use same directory as packageFile to compare against installed modules. See #201.
    -p, --prod                   check only dependencies (not devDependencies)
    --peer                       check only peerDependencies
    -r, --registry          specify third-party npm registry
    --configFilePath       rc config file path (default: ./)
    --configFileName       rc config file name (default: .ncurc.{json,yml,js})
    -s, --silent                 don't output anything (--loglevel silent)
    -t, --greatest               find the highest versions available instead of the latest stable versions
    --timeout                a global timeout in ms
    -u, --upgrade                overwrite package file
    -x, --reject        exclude packages matching the given string, comma-delimited list, or regex
    -a, --upgradeAll             include even those dependencies whose latest version satisfies the declared semver dependency
    --semverLevel         find the highest version within "major" or "minor"
    --removeRange                remove version ranges from the final package version
    -v, --version                2.14.2
    -V                           
    -h, --help                   output usage information

ncu # 查看更新
 redux          ^3.7.2  →  ^4.0.0 
 autoprefixer   ^6.5.3  →  ^8.6.5 
 babel-eslint   ^7.1.1  →  ^8.2.6 
 eslint        ^4.11.0  →  ^5.1.0 

ncu -a # 更新

npm update，只能按照package.json中标注的版本号进行更新，升级后不会修改package.json中的版本号，需要自己手动修改，比较麻烦。
npm-check-updates 升级插件升级后会自动修改package.json里的版本号，简单方便。