基本上我们写

npm install package_name

但特别为了测试目的，我们不需要在应用程序运行在正常状态下运行一些包，因此Node引入了很好的方法来解决这个问题。每当我们写作时

npm install package_name --save-dev

当时，该包仅用于开发目的而安装。

基本上我们写

npm install package_name

但特别为了测试目的，我们不需要在应用程序运行在正常状态下运行一些包，因此Node引入了很好的方法来解决这个问题。每当我们写作时

npm install package_name --save-dev

当时，该包仅用于开发目的而安装。

——save-dev用于保存用于开发目的的包。 例如:单元测试、简化… ——save用于保存 应用程序运行所需的包。

一个完美的例子是:

$ npm install typescript --save-dev

在这种情况下，你会希望使用Typescript(一种可解析javascript的编码语言)进行开发，但一旦应用被部署，就不再需要Typescript了，因为所有的代码都已转译为javascript。因此，将它包含在已发布的应用程序中毫无意义。事实上，它只会占用空间并增加下载时间。

让我给你们举个例子，

你是一个非常严肃的npm库的开发者，它使用不同的测试库来测试包。 用户下载您的库，并希望在他们的代码中使用它。他们也需要下载您的测试库吗?也许你用笑话来测试，而他们用摩卡。你想让他们也安装笑话吗?只是为了管理你的图书馆?

不。对吧?这就是为什么它们在devDependencies中。

当有人这样做时，npm i yourPackage只会安装运行你的库所需的库。你用来捆绑代码或测试和模拟的其他库将不会被安装，因为你把它们放在了devDependencies中。很简洁，对吧?

那么，为什么开发人员需要公开devdependencies呢?

Let's say your package is an open-source package and 100s of people are sending pull requests to your package. Then how they will test the package? They will git clone your repo and when they would do an npm i the dependencies as well as devDependencies. Because they are not using your package. They are developing the package further, thus, in order to test your package they need to pass the existing test cases as well write new. So, they need to use your devDependencies which contain all the testing/building/mocking libraries that YOU used.

阅读完成并忘记——保存开发头痛

最简单的答案是——当你为其他开发人员创建包，并希望将包托管在NPM Registry(如lodash、mongoose、express等)时，save-dev非常有用。当你构建或编写Node Server时，——save和——save-dev之间没有区别，因为你的Node Server实现对你来说是私有的，你永远不会在NPM上发布它。

NPM安装如何工作

Whenever we install a new package using npm like npm install express then NPM installs that package to our system and put it into node_modules folder, now NPM will analyze the package.json file of newly installed package i.e express in this case, after analyzing NPM will install all those packages which were mentioned in dependencies section of package.json file of express package. After installing those packages on which express was dependent NPM again analyze the package.json file of all newly installed packages and again install the packages for them, this cycle goes on until all packages are available into node_modules folder to function properly. You can check package dependencies by running npm list in terminal where terminal should point location of your project directory.

——save-dev如何与上述解释的内容相关

Suppose you want to create a new package like express, now while development of this new package you probably want to write some unit testing code and test the package with any other available testing package let's assume mocha in this case. Now you know mocha is only required to test the package not required to use the package. In this case you should install mocha using --save-dev flag, otherwise NPM will install it whenever a developer install your package using NPM. So if we want a dependency not installed when someone install our package from NPM we must install that package using --save-dev in development phase.

最后一件事

不要把——save-dev和协作开发混在一起，如果有人从像github这样的源版本控制系统中克隆了你的包代码，那么NPM肯定会安装所有的devDependencies，也就是使用——save-dev安装的包。