angular-cn/public/docs/ts/latest/guide/npm-packages.jade

include ../_util-fns

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  Angular applications and Angular itself depend upon features and functionality provided by a variety of third-party packages
  maintained and installed with the Node Package Manager (<a href="https://docs.npmjs.com/" target="_blank">npm</a>).
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  :marked
    Don't have npm?
    <a href="https://docs.npmjs.com/getting-started/installing-node" target="_blank" title="Installing Node.js and updating npm">Get it now</a>
    because we're going to use it now and repeatedly throughout this documentation.
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  The Angular team recommends the starter-set of packages specified in the `dependencies` and `devDependencies`
  sections of the QuickStart
  <a href="https://docs.npmjs.com/files/package.json" target="_blank">package.json</a> file:
+makeJson('quickstart/ts/package.1.json',{ paths: 'dependencies, devDependencies'}, 'package.json (dependencies)')(format=".") 
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  There are other possible package choices. 
  We're recommending this particular set that we know work well together. 
  
  In this chapter we explain what each package does and why we include it.
  Feel free to make substitutions later to suit your tastes and experience.
  
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  ## *dependencies* and *devDependencies*
  The `package.json` distinguishes between two sets of packages, 
  [dependencies](#dependencies) and [devDependencies](#dev-dependencies).
  
  The packages listed under *dependencies* are essential to *running* the application. 
  The *devDependencies* are only necessary to *develop* the application. 
  They can be excluded from production installations as in this example:
code-example(format="." language="bash").
  npm install my-application --production
  
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  ## *dependencies*
  There are two package categories in the `dependencies` section of the application `package.json`:
  
  * ***Features*** - Feature packages provide our application with framework and utility capabilites.
  
  * ***Polyfills*** - Polyfills plug gaps in the browser's JavaScript implementation.

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  ### Feature Packages
  
  ***angular2*** - The framework we know and love; the reason we're all here.
  
  ***[system.js](https://github.com/systemjs/systemjs)*** -  A dynamic module loader compatible with the 
  [ES2015 module](http://www.2ality.com/2014/09/es6-modules-final.html) specification.
  There are other viable choices including the well-regarded [webpack](https://webpack.github.io/).
  SystemJS happens to be the one we use in the documentation samples. It works.
  
  Our applications are likely to require additional packages that provide
  HTML controls, themes, data access, and various utilities.
  

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  ### Polyfill Packages
  
  Angular requires certain [polyfills](https://en.wikipedia.org/wiki/Polyfill) in the application environment.
  We install these polyfills with very specific npm packages that Angular lists in the *peerDependencies* section of its `package.json`.
  
  We must list these packages in the `dependencies` section of our own `package.json`.
  
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    See "[Why peerDependencies?](#why-peer-dependencies)" below for background on this requirement.
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  ***es6-promise*** - Angular applications require ES2015 promise support.
  Although some browsers provide a native implementation of ES2015 promises many do not.
  We still need this library even for the browsers that do support promises 
  in order to integrate with the current version of *zone.js* (see below).
  This integration dependency goes away in the next version of *zone.js* 
  The need for *es6-promise* should disappear entirely once promises are implemented by all supported ever-green browsers.
  
  ***es6-shim*** - monkey patches the global context (window) with essential features of ES2016 (ES6).
   Developers may substitute an alternative polyfill that provides the same core APIs. 
   This dependency should go away once these APIs are implemented by all supported ever-green browsers.
  
  ***reflect-metadata*** - a dependency shared between Angular and the ***TypeScript compiler***. 
  Developers should be able to update a TypeScript package without upgrading Angular, 
  which is why this is a dependency of the application and not a dependency of Angular.
  
  ***rxjs*** - a polyfill for the [Observables specification](https://github.com/zenparsing/es-observable) currently before the 
  [TC39](http://www.ecma-international.org/memento/TC39.htm) committee that determines standards for the JavaScript language.
  Developers should be able to pick a preferred version of *rxjs* (within a compatible version range) 
  without waiting for Angular updates.
  
  ***zone.js*** - a polyfill for the [Zone specification](https://gist.github.com/mhevery/63fdcdf7c65886051d55) currently before the 
  [TC39](http://www.ecma-international.org/memento/TC39.htm) committee that determines standards for the JavaScript language.
  Developers should be able to pick a preferred version of *zone.js* to use (within a compatible version range) 
  without waiting for Angular updates.

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  ## *devDependencies*
  The packages listed in the *devDependencies* section of the `package.json` help us develop the application.
  They do not have to be deployed with the production application although there is rarely harm in doing so.

  ***[concurrently](https://www.npmjs.com/package/concurrently)*** - 
  a utility to run multiple *npm* commands concurrently on OS/X, Windows, and Linux operating systems.
  
  ***[lite-server](https://www.npmjs.com/package/lite-server)*** - 
  a light-weight, static file server, written and maintained by [John Papa](http://johnpapa.net/) 
  with excellent support for Angular apps that use routing.
  
  ***[typescript](https://www.npmjs.com/package/typescript)*** - 
  the TypeScript language server including the *tsc* TypeScript compiler.
  
  ***[typings](https://www.npmjs.com/package/typings)*** - a manager for TypeScript definition files.
  Learn more about it in the [TypeScript Configuration](typescript-configuration.html#typings) chapter.
  
  This section likely grows as we add more tools, testing, and build support. 
  The QuickStart set is sufficient for developing the documentation sample applications.

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  ## Why *peerDependencies*?
  
  We don't have a *peerDependencies* section in the QuickStart `package.json`. 
  But Angular itself has a *peerDependencies* section in 
  [*its* package.json](https://github.com/angular/angular/blob/master/modules/angular2/package.json) 
  and that has important consequences for our application. 
  
  It explains why we load the [polyfill](#polyfills) *dependency* packages in the QuickStart `package.json`,
  and why we'll need those packages in our own applications.
  
  Let's briefly explain what [peer dependencies](https://nodejs.org/en/blog/npm/peer-dependencies/) are about.
  
  As we know, packages depend on other packages. For example, our application depends upon 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. This is great.
  
  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*."
   
  Angular finds itself in this situation. 
  Accordingly, the Angular `package.json` specifies several *peer dependency* packages, 
  each pinned to a particular version of a third-party package.

  ### We must install Angular's *peerDependencies* ourselves
  
  When *npm* installs packages listed in *our* `dependencies` section,
  it also installs the packages listed within *their* packages `dependencies` sections.
  The process is recursive.
  
  But as of version 3, *npm* does *not* install packages listed in *peerDependencies* sections.
  
  That means when our application installs Angular, ***npm* will not automatically install
  the packages listed in Angular's *peerDependencies* section**.
  
  Fortunately, *npm* warns us (a) when any *peer dependencies* are missing or (b)
  when the application or any its other dependencies
  installs a different version of a *peer dependency*. 
  
  These warnings are a critical guard against accidental failures due to version mismatches.
  They leave us in control of package and version resolution.
  
  It is our responsibility to list all *peer dependency* packages **among our own *devDependencies***.
  
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    #### The future of *peerDependencies*
    
    The Angular polyfill dependencies should be just a suggestion or a hint to developers so that they know what Angular expects.
    They should not be hard requirements as they are today. We don't have a way to make them optional today.
    
    There is a npm feature request for "optional peerDependencies" which would allow us to model this relationship better. 
    Once implemented, Angular will switch from *peerDependencies* to *optionalPeerDependencies* for all polyfills.