angular-cn/aio/content/guide/npm-packages.md

@title Npm Packages

@intro Recommended npm packages, and how to specify package dependencies.

@description Angular applications and Angular itself depend upon features and functionality provided by a variety of third-party packages. These packages are maintained and installed with the Node Package Manager (npm).

Node.js and npm are essential to Angular development.

<a href="https://docs.npmjs.com/getting-started/installing-node" target="_blank" title="Installing Node.js and updating npm">
Get them now</a> if they're not already installed on your machine.

**Verify that you are running node `v4.x.x` or higher and npm `3.x.x` or higher**
by running the commands `node -v` and `npm -v` in a terminal/console window.
Older versions produce errors.

Consider using [nvm](https://github.com/creationix/nvm) for managing multiple
versions of node and npm. You may need [nvm](https://github.com/creationix/nvm) if
you already have projects running on your machine that use other versions of node and npm.

During Setup, a package.json file is installed with a comprehensive starter set of packages as specified in the dependencies and devDependencies sections.

You can use other packages but the packages in this particular set work well together and include everything you need to build and run the sample applications in this series.

Note: A cookbook or guide page may require an additional library such as *jQuery*.

You'll install more than you need for the QuickStart guide. No worries! You only serve to the client those packages that the application actually requests.

This page explains what each package does. You can make substitutions later to suit your tastes and experience.

dependencies and devDependencies

The package.json includes two sets of packages, dependencies and devDependencies.

The dependencies are essential to running the application. The devDependencies are only necessary to develop the application. You can exclude them from production installations by adding --production to the install command, as follows: npm install my-application --production

{@a dependencies}

dependencies

The dependencies section of package.json contains:

• Features: Feature packages give the application framework and utility capabilities.

• Polyfills: Polyfills plug gaps in the browser's JavaScript implementation.

• Other: Other libraries that support the application such as bootstrap for HTML widgets and styling.

Feature Packages

@angular/core: Critical runtime parts of the framework needed by every application. Includes all metadata decorators, Component, Directive, dependency injection, and the component lifecycle hooks.

@angular/common: The commonly needed services, pipes, and directives provided by the Angular team.

@angular/compiler: Angular's Template Compiler. It understands templates and can convert them to code that makes the application run and render. Typically you don’t interact with the compiler directly; rather, you use it indirectly via platform-browser-dynamic or the offline template compiler.

@angular/platform-browser: Everything DOM and browser related, especially the pieces that help render into the DOM. This package also includes the bootstrapStatic() method for bootstrapping applications for production builds that pre-compile templates offline.

@angular/platform-browser-dynamic: Includes Providers and a bootstrap method for applications that compile templates on the client. Don’t use offline compilation. Use this package for bootstrapping during development and for bootstrapping plunker samples.

@angular/http: Angular's HTTP client.

@angular/router: Component router.

@angular/upgrade: Set of utilities for upgrading AngularJS applications to Angular.

system.js: A dynamic module loader compatible with the ES2015 module specification. Other viable choices include the well-regarded webpack.

Your future applications are likely to require additional packages that provide HTML controls, themes, data access, and various utilities.

{@a polyfills}

Polyfill packages

Angular requires certain polyfills in the application environment. Install these polyfills using the npm packages that Angular lists in the peerDependencies section of its package.json.

You must list these packages in the dependencies section of your own package.json.

For background on this requirement, see [Why peerDependencies?](guide/npm-packages#why-peer-dependencies).

core-js: Patches the global context (window) with essential features of ES2015 (ES6). You may substitute an alternative polyfill that provides the same core APIs. When these APIs are implemented by the major browsers, this dependency will become unnecessary.

rxjs: A polyfill for the Observables specification currently before the TC39 committee that determines standards for the JavaScript language. You can pick a preferred version of rxjs (within a compatible version range) without waiting for Angular updates.

zone.js: A polyfill for the Zone specification currently before the TC39 committee that determines standards for the JavaScript language. You can pick a preferred version of zone.js to use (within a compatible version range) without waiting for Angular updates.

{@a other}

Other helper libraries

angular-in-memory-web-api: An Angular-supported library that simulates a remote server's web api without requiring an actual server or real HTTP calls. Good for demos, samples, and early stage development (before you even have a server). Read about it in the HTTP Client page.

bootstrap: Bootstrap is a popular HTML and CSS framework for designing responsive web apps. Some of the samples improve their appearance with bootstrap.

{@a dev-dependencies}

devDependencies

The packages listed in the devDependencies section of the package.json help you develop the application. You don't have to deploy them with the production application although there is no harm in doing so.

concurrently: A utility to run multiple npm commands concurrently on OS/X, Windows, and Linux operating systems.

lite-server: A light-weight, static file server, by John Papa with excellent support for Angular apps that use routing.

typescript: the TypeScript language server, including the tsc TypeScript compiler.

@types/*: TypeScript definition files. Learn more about it in the TypeScript Configuration guide.

{@a why-peer-dependencies}

Why peerDependencies?

There isn't a peerDependencies section in the QuickStart package.json. But Angular has a peerDependencies section in its package.json, which has important consequences for your application.

This section explains why you load the polyfill dependency packages in the QuickStart application's package.json, and why you'll need those packages in your own applications.

Packages depend on other packages. For example, your application depends on the Angular package.

Two packages, "A" and "B", could depend on the same third package "C". "A" and "B" might both list "C" among their dependencies.

What if "A" and "B" depend on different versions of "C" ("C1" and "C2"). The npm package system supports that. It installs "C1" in the node_modules folder for "A" and "C2" in the node_modules folder for "B". Now "A" and "B" have their own copies of "C" and they run without interferring with one another.

But there is a problem. Package "A" may require the presence of "C1" without actually calling upon it directly. "A" may only work if everyone is using "C1". It falls down if any part of the application relies on "C2".

The solution is for "A" to declare that "C1" is a peer dependency.

The difference between a dependency and a peerDependency is roughly this:

A dependency says, "I need this thing directly available to me."

A peerDependency says, "If you want to use me, you need this thing available to you."

The Angular package.json specifies several peer dependency packages, each pinned to a particular version of a third-party package.

You must install Angular's peerDependencies yourself.

When npm installs packages listed in your dependencies section, it also installs the packages listed within their packages dependencies sections. The process is recursive.

However, as of version 3, npm does not install packages listed in peerDependencies sections.

This means that when your application installs Angular, npm doesn't automatically install the packages listed in Angular's peerDependencies section.

Fortunately, npm issues a warning (a) When any peer dependencies are missing, or (b) When the application or any of its other dependencies installs a different version of a peer dependency.

These warnings guard against accidental failures due to version mismatches. They leave you in control of package and version resolution.

It is your responsibility to list all peer dependency packages among your own devDependencies.

#### The future of *peerDependencies*

The Angular polyfill dependencies are hard requirements. Currently, there is no way to make them optional.

However, there is an npm feature request for "optional peerDependencies," which would allow you to model this relationship better.
When this feature request is implemented, Angular will switch from *peerDependencies* to *optionalPeerDependencies* for all polyfills.