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Set the minimum typescript version to 1.5.0 

fixes #172

^1.5.0-beta means >= 1.5.0 and < 2.0.0 and is technically valid. however
it's probably nicer not to refer to beta versions in the doc.
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Victor Berchet 2015-08-12 13:59:54 -07:00 committed by YuCheng Hu
parent e38fc6b3a9
commit ac2bfed40d
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include _util-fns
.callout.is-helpful
header Angular is in developer preview
p.
This quickstart does not reflect the final development process for writing apps with Angular.
The following setup is for those who want to try out Angular while it is in developer preview.
:marked
Let's start from zero and build a super simple Angular application in JavaScript.
// STEP 1 - Create a project ##########################
.l-main-section
h2#section-create-project 1. Create a project
p.
This quickstart shows how to write your Angular components in TypeScript. You could instead choose
another language such as <a href="/docs/dart/latest/quickstart.html">Dart</a>, ES5, or ES6.
p.
The goal of this quickstart is to write a component in TypeScript that prints a string.
We assume you have already installed <a href="https://docs.npmjs.com/getting-started/installing-node">Node and npm</a>.
p.
To get started, create a new empty project directory. All the following commands should be run
from this directory.
p.
To get the benefits of TypeScript, we want to have the type definitions available for the compiler and the editor.
TypeScript type definitions are typically published in a repo called <a href="http://definitelytyped.org/">DefinitelyTyped</a>.
To fetch one of the type definitions to the local directory, we use the <a href="https://www.npmjs.com/package/tsd">tsd package manager</a>.
code-example.
$ npm install -g tsd@^0.6.0
$ tsd install angular2 es6-promise rx rx-lite
p.
Next, create two empty files, <code>index.html</code> and <code>app.ts</code>, both at the root of the project:
code-example.
$ touch app.ts index.html
// STEP 2 - Start the TypeScript compiler ##########################
.l-main-section
h2#start-tsc 2. Run the TypeScript compiler
p.
Since the browser doesn't understand TypeScript code, we need to run a compiler to translate
your code to browser-compliant JavaScript as you work. This quickstart uses the TypeScript
compiler in <code>--watch</code> mode, but it is also possible to do the translation in the browser as files
are loaded, or configure your editor or IDE to do it.
code-example.
$ npm install -g typescript@^1.5.0
$ tsc --watch -m commonjs -t es5 --emitDecoratorMetadata app.ts
.callout.is-helpful
header Don't want JavaScript?
:marked
Although we're getting started in JavaScript, you can also write Angular apps
in TypeScript by selecting it from the combo-box in the banner.
p.
Windows users: if you get an error that an option is unknown, you are probably running
an older version of TypeScript.
See <a href="http://stackoverflow.com/questions/23267858/how-do-i-install-typescript">
Stack Overflow: How do I install Typescript</a>
// STEP 3 - Import Angular ##########################
.l-main-section
:marked
## See It Run!
h2#section-transpile 3. Import Angular
Running the <live-example></live-example>
is the quickest way to see an Angular app come to life.
Clicking that link fires up a browser, loads the sample in [plunker](http://plnkr.co/ "Plunker"),
and displays a simple message:
p Inside of <code>app.ts</code>, import the type definitions from Angular:
code-example.
/// &lt;reference path="typings/angular2/angular2.d.ts" /&gt;
figure.image-display
img(src='/resources/images/devguide/quickstart/hello-angular.png' alt="Output of quickstart app")
:marked
Here is the file structure:
.filetree
.file angular-quickstart
.children
.file src
.children
.file app
.children
.file app.component.js
.file app.module.js
.file main.js
.file index.html
.file styles.css
:marked
Functionally, it's an `index.html`, `styles.css` and three JavaScript files in an `app/` folder.
We can handle that!
Of course we won't build many apps that only run in plunker.
Let's follow a process that's closer to what we'd do in real life.
1. Set up our development environment
1. Write the Angular root component for our app
1. Add an Angular Module
1. Bootstrap it to take control of the main web page
1. Write the main page (`index.html`)
1. Add some CSS (`styles.css`)
.l-sub-section
:marked
We really can build the QuickStart from scratch in five minutes
if we follow the instructions and ignore the commentary.
Most of us will be interested in the "why" as well as the "how" and that will take longer.
p Now your editor should be able to complete the available imports:
code-example.
import {Component, View, bootstrap} from 'angular2/angular2';
p.
The above import statement uses ES6 module syntax to import three symbols from the Angular module.
The module will load at runtime.
// STEP 4 - Create a component ##########################
.l-main-section
:marked
## Development Environment
We'll need a place to stand (the application project folder), some libraries
and your editor of choice.
### Create a new project folder
code-example(format="").
mkdir angular-quickstart
cd angular-quickstart
:marked
### Add the libraries we need
We recommend the **npm** package manager for acquiring and managing our development libraries.
h2#section-angular-create-account 4. Define a component
.l-sub-section
:marked
Don't have npm?
[Get it now](https://docs.npmjs.com/getting-started/installing-node "Installing Node.js and updating npm")
because we're going to use it now and repeatedly throughout this documentation.
:marked
Add a **package.json** file to the project folder and copy/paste the following:
+makeJson('quickstart/js/package.1.json', null, 'package.json')(format=".")
p.
Components structure and represent the UI. This quickstart demonstrates the process of creating a component
that has an HTML tag named <strong><code>&lt;my-app&gt;</code></strong>.
.l-sub-section
:marked
Itching to know the details? We explain in the [appendix below](#package-json)
:marked
Install these packages by opening a terminal window (command window in Windows) and
running this npm command.
code-example(format="").
npm install
:marked
.l-sub-section
:marked
Scary <span style="color:red; font-weight: bold">error messages in red</span> may appear **during** install.
Ignore them. The install will succeed. See the [appendix below](#npm-errors) for more information.
p.
A component consists of two parts, the <strong>component controller</strong>
which is an ES6 class, and the <strong>decorators</strong> which tell Angular
how to place the component into the page.
code-example(language="javascript" format="linenums").
// Annotation section
@Component({
selector: 'my-app'
})
@View({
template: '&lt;h1&gt;Hello {{ name }}&lt;/h1&gt;'
})
// Component controller
class MyAppComponent {
name: string;
constructor() {
this.name = 'Alice';
}
}
.l-sub-section
h3 @Component and @View annotations
p.
A component annotation describes details about the component. An annotation can be identified by its at-sign (<code>@</code>).
p.
The <code>@Component</code> annotation defines the HTML tag for the component by specifying the component's CSS selector.
p.
The <code>@View</code> annotation defines the HTML that represents the component. The component you wrote uses an inline template, but you can also have an external template. To use an external template, specify a <code>templateUrl</code> property and give it the path to the HTML file.
code-example(language="javascript" format="linenums").
@Component({
selector: 'my-app' // Defines the &lt;my-app&gt;&lt;/my-app&gt; tag
})
@View({
template: '&lt;h1&gt;Hello {{ name }}&lt;/h1&gt;' // Defines the inline template for the component
})
p.
The annotations above specify an HTML tag of <code>&lt;my-app&gt;</code>
and a template of <code ng-non-bindable>&lt;h1&gt;Hello &#123;&#123; name }}&lt;/h1&gt;</code>.
.l-sub-section
h3 The template and the component controller
p.
The component controller is the backing of the component's template. This component
controller uses TypeScript <code>class</code> syntax.
code-example(language="javascript" format="linenums").
class MyAppComponent {
name: string;
constructor() {
this.name = 'Alice';
}
}
p.
Templates read from their component controllers. Templates have access to any properties
or functions placed on the component controller.
p.
The template above binds to a <code>name</code> property through
the double-mustache syntax (<code ng-non-bindable>{{ ... }}</code>).
The body of the constructor assigns "Alice" to the name property. When the
template renders, "Hello Alice" appears instead of
<span ng-non-bindable>"Hello {{ name }}"</span>.
// STEP 5 - Bootstrap ##########################
.l-main-section
:marked
## Our First Angular Component
The *Component* is the most fundamental of Angular concepts.
A component manages a view - a piece of the web page where we display information
to the user and respond to user feedback.
Technically, a component is a class that controls a view template.
We'll write a lot of them as we build Angular apps. This is our first attempt
so we'll keep it ridiculously simple.
### Create an application source sub-folder
We like to keep our application code in a sub-folder off the root called `src/app/`.
Execute the following command in the console window.
code-example(format="").
mkdir app
cd app
:marked
### Add the component file
Now add a file named **app.component.js** and paste the following lines:
+makeExample('quickstart/js/src/app/app.component.js', null, 'src/app/app.component.js')(format=".")
:marked
We're creating a visual component named **`AppComponent`** by chaining the
`Component` and `Class` methods that belong to the **global Angular core namespace, `ng.core`**.
+makeExample('quickstart/js/src/app/app.component.js', 'ng-namespace-funcs', 'src/app/app.component.js ' +
'(component schema)')(format=".")
h2#section-transpile 5. Bootstrap
:marked
The **`Component`** method takes a configuration object with three
properties. The **`Class`** method is where we implement the component itself,
giving it properties and methods that bind to the view and whatever
behavior is appropriate for this part of the UI.
Let's review this file in detail.
p.
At the bottom of <code>app.ts</code>, call the <code>bootstrap()</code> function
to load your new component into its page:
### Modules
Angular apps are modular. They consist of many files each dedicated to a purpose.
ES5 JavaScript doesn't have a native module system.
There are several popular 3rd party module systems we could use.
Instead, for simplicity and to avoid picking favorites,
we'll create a single global namespace for our application.
We'll call it `app` and we'll add all of our code artifacts to this one global object.
We don't want to pollute the global namespace with anything else.
So within each file we surround the code in an IIFE ("Immediately Invoked Function Expression").
+makeExample('quickstart/js/src/app/app.component.js', 'iife', 'src/app/app.component.js (IIFE)')(format=".")
:marked
We pass the global `app` namespace object into the IIFE,
taking care to initialize it with an empty object if it doesn't yet exist.
Most application files *export* one thing by adding that thing to the `app` namespace.
Our `app.component.js` file exports the `AppComponent`.
+makeExample('quickstart/js/src/app/app.component.js', 'export', 'src/app/app.component.js (export)')(format=".")
:marked
A more sophisticated application would have child components that descended from
`AppComponent` in a visual tree.
A more sophisticated app would have more files and modules, at least as many as it had components.
Quickstart isn't sophisticated; one component is all we need.
Yet modules play a fundamental organizational role in even this small app.
Modules rely on other modules. In JavaScript Angular apps, when we need something
provided by another module, we get it from the `app` object.
When another module needs to refer to `AppComponent`, it gets it from the `app.AppComponent` like this:
+makeExample('quickstart/js/src/app/app.module.js', 'import', 'src/app/app.module.js (import)')(format=".")
code-example(language="javaScript").
bootstrap(MyAppComponent);
:marked
Angular is also modular. It is a collection of library modules.
Each library is itself a module made up of several, related feature modules.
When we need something from Angular, we use the `ng` object.
:marked
### The Class definition object
At the bottom of the file is an empty, do-nothing class definition object for
our `AppComponent` class.
When we're ready to build a substantive application,
we can expand this object with properties and application logic.
Our `AppComponent` class has nothing but an empty constructor because we
don't need it to do anything in this QuickStart.
+makeExample('quickstart/js/src/app/app.component.js', 'class','app.component.js (class)')(format=".")
:marked
### The Component definition object
`ng.core.Component()` tells Angular that this class definition object
*is an Angular component*.
The configuration object passed to the `ng.core.Component()` method has two
fields, a `selector` and a `template`.
+makeExample('quickstart/js/src/app/app.component.js', 'component','app.component.js (component)')(format=".")
:marked
The `selector` specifies a simple CSS selector for a host HTML element named `my-app`.
Angular creates and displays an instance of our `AppComponent`
wherever it encounters a `my-app` element in the host HTML.
.alert.is-helpful
:marked
Remember the `my-app` selector! We'll need that information when we write our `index.html`
:marked
The `template` property holds the component's companion template.
A template is a form of HTML that tells Angular how to render a view.
Our template is a single line of HTML announcing "Hello Angular".
Now we need something to tell Angular to load this component.
p.
The <code>bootstrap()</code> function takes a
component as a parameter, enabling the component
(as well as any child components it contains) to render.
### Add an NgModule
Angular apps are composed of [Angular Modules](guide/ngmodule.html) that
snuggly contain all our components and everything else we need for our app.
Create the `src/app/app.module.js` file with the following content:
+makeExample('quickstart/js/src/app/app.module.js', null, 'src/app/app.module.js')
.l-sub-section
:marked
Read more about the `NgModule` configuration in the [appendix below](#ngmodule).
:marked
### Bootstrap it!
Add a new file , `main.js`, to the `src/app/` folder as follows:
+makeExample('quickstart/js/src/main.js', null, 'src/main.js')(format=".")
:marked
We need two things to launch the application:
1. Angular's `platformBrowserDynamic().bootstrapModule` function
1. The application root module that we just wrote.
We have them both in our 'namespaces'.
Then we call `bootstrapModule`, passing in the **root app module**, `AppModule`.
.l-sub-section
:marked
Learn why we need `bootstrapModule` from `ng.platformBrowserDynamic`
and why we create a separate js files in the [appendix below](#main).
:marked
We've asked Angular to launch the app in a browser with our component at the root.
Where will Angular put it?
// STEP 6 - Declare the HTML ##########################
.l-main-section
:marked
## Add the `index.html`
Angular displays our application in a specific location on our `index.html`.
It's time to create that file.
We won't put our `index.html` in the `src/app/` folder.
We'll locate it **up one level, in the project root folder**.
code-example(format="").
cd ..
:marked
Now create the`index.html` file and paste the following lines:
+makeExample('quickstart/js/src/index.html', null, 'src/index.html')(format=".")
:marked
There are three noteworthy sections of HTML:
h2#section-angular-create-account 6. Declare the HTML
1. We load the JavaScript libraries we need; learn about them [below](#libraries).
p.
Inside the <code>head</code> tag of <code>index.html</code>,
include the traceur-runtime and the Angular bundle.
Instantiate the <code>my-app</code> component in the <code>body</code>.
2. We load our JavaScript files.
code-example(language="html" format="linenums").
&lt;!-- index.html --&gt;
&lt;html&gt;
&lt;head&gt;
&lt;title&gt;Angular 2 Quickstart&lt;/title&gt;
&lt;script src="https://github.jspm.io/jmcriffey/bower-traceur-runtime@0.0.87/traceur-runtime.js"&gt;&lt;/script&gt;
&lt;script src="https://code.angularjs.org/2.0.0-alpha.28/angular2.dev.js"&gt;&lt;/script&gt;
&lt;/head&gt;
&lt;body&gt;
3. We add the `<my-app>` tag in the `<body>`. **This is where our app lives!**
When Angular calls the `bootstrapModule` function in `main.js`,
it reads the `AppModule` metadata, sees that `AppComponent` is the bootstrap component,
finds the `my-app` selector, locates an element tag named `my-app`,
and renders our application's view between those tags.
&lt;!-- The app component created in app.ts --&gt;
&lt;my-app&gt;&lt;/my-app&gt;
:marked
### Add some style
Styles aren't essential but they're nice, and `index.html` assumes we have
a stylesheet called `styles.css`.
Create a `styles.css` file in the *#{_indexHtmlDir}* folder and start styling, perhaps with the minimal
styles shown below. For the full set of master styles used by the documentation samples,
see [styles.css](https://github.com/angular/angular.io/blob/master/public/docs/_examples/_boilerplate/styles.css).
+makeExample('quickstart/js/src/styles.1.css', null, 'src/styles.css')(format=".")
:marked
### Configure the server
We're going to use a static server called **lite-server** that loads `index.html` in a browser
and refreshes the browser when application files change.
The static server will use the `bs-config.json` file as configuration.
This files tells the server that `src/` is the base directory to serve, and that any
requests to `node_modules/` should be routed to `node_modules/` in the root directory
instead of `src/node_modules/`
&lt;/body&gt;
&lt;/html&gt;
// STEP 7 - Declare the HTML ##########################
.l-main-section
:marked
## Run!
Open a terminal window and enter this command:
code-example(format="").
npm start
:marked
That command runs **lite-server**.
In a few moments, a browser tab should open and display
figure.image-display
img(src='/resources/images/devguide/quickstart/hello-angular.png' alt="Output of quickstart app")
h2#section-load-component-module 7. Load the component
:marked
Congratulations! We are in business.
p.
The last step is to load the module for the <code>my-app</code> component.
To do this, we'll use the System library.
.alert.is-helpful
:marked
If you see `Loading...` displayed instead, see the
[Browser ES2015 support appendix](#es6support).
:marked
### Make some changes
Try changing the message to "Hello Angular!".
`lite-server` is watching, so it should detect the change,
refresh the browser, and display the revised message.
.l-sub-section
h3 System.js
It's a nifty way to develop an application!
We close the terminal window when we're done to terminate the server.
p.
<a href="https://github.com/systemjs/systemjs">System</a> is a third-party open-source library that
adds ES6 module loading functionality to browsers.
p.
Add the System.js dependency in the <code>&lt;head&gt;</code> tag, so that
it looks like:
code-example(language="html" format="linenums").
&lt;head&gt;
&lt;title&gt;Angular 2 Quickstart&lt;/title&gt;
&lt;script src="https://github.jspm.io/jmcriffey/bower-traceur-runtime@0.0.87/traceur-runtime.js"&gt;&lt;/script&gt;
&lt;script src="https://jspm.io/system@0.16.js"&gt;&lt;/script&gt;
&lt;script src="https://code.angularjs.org/2.0.0-alpha.28/angular2.dev.js"&gt;&lt;/script&gt;
&lt;/head&gt;
p.
Add the following module-loading code:
code-example(language="html" format="linenums").
&lt;my-app&gt;&lt;/my-app&gt;
&lt;script&gt;System.import('app');&lt;/script&gt;
// STEP 8 - Run a local server ##########################
.l-main-section
:marked
## Final structure
Our final project folder structure looks like this:
.filetree
.file angular-quickstart
.children
.file node_modules
.file src
.children
.file app
.children
.file app.component.js
.file app.module.js
.file main.js
.file index.html
.file styles.css
.file bs-config.json
.file package.json
:marked
And here are the files:
+makeTabs(`
quickstart/js/src/app/app.component.js,
quickstart/js/src/app/app.module.js,
quickstart/js/src/main.js,
quickstart/js/src/index.html,
quickstart/js/src/styles.1.css,
quickstart/js/bs-config.1.json,
quickstart/js/package.1.json
`,null,
`src/app/app.component.js,
src/app/app.module.js,
src/main.js,
src/index.html,
src/styles.css,
bs-config.json,
package.json
`)
h2#section-load-component-module 8. Run a local server
p Run a local HTTP server, and view <code>index.html</code>.
p.
If you don't already have an HTTP server,
you can install one using <code>npm install -g http-server</code>.
(If that results in an access error, then you might need to use
<code><b>sudo</b> npm ...</code>)
For example:
code-example.
# From the directory that contains index.html:
npm install -g http-server # Or sudo npm install -g http-server
http-server # Creates a server at localhost:8080
# In a browser, visit localhost:8080/index.html
// WHAT'S NEXT... ##########################
.l-main-section
:marked
## Wrap Up
Our first application doesn't do much. It's basically "Hello, World" for Angular.
We kept it simple in our first pass: we wrote a little Angular component,
we added some JavaScript libraries to `index.html`, and launched with a
static file server. That's about all we'd expect to do for a "Hello, World" app.
**We have greater ambitions.**
The good news is that the overhead of setup is (mostly) behind us.
We'll probably only touch the `package.json` to update libraries.
Besides adding in the script files for our app 'modules',
we'll likely open `index.html` only if we need to add a library or some css stylesheets.
We're about to take the next step and build a small application that
demonstrates the great things we can build with Angular.
Join us on the [Tour of Heroes Tutorial](./tutorial)!
.l-main-section
:marked
## Appendices
The balance of this chapter is a set of appendices that
elaborate some of the points we covered quickly above.
There is no essential material here. Continued reading is for the curious.
<a id="ie-polyfills"></a>
<a id="es6support"></a>
<a id="libraries"></a>
.l-main-section
:marked
### Appendix: Libraries
We loaded the following scripts
+makeExample('quickstart/js/src/index.html', 'libraries', 'src/index.html')(format=".")
:marked
We began with an Internet Explorer polyfill.
IE requires a polyfill to run
an application that relies on ES2015 promises and dynamic module loading.
Most applications need those capabilities and most applications
should run in Internet Explorer.
Next are the polyfills for Angular, `zone.js` and `Reflect.js`,
followed by the Reactive Extensions RxJS library.
.l-sub-section
:marked
Our QuickStart doesn't use the Reactive Extensions
but any substantial application will want them
when working with observables.
We added the library here in QuickStart so we don't forget later.
:marked
Finally, we loaded the web development version of Angular itself.
We'll make different choices as we gain experience and
become more concerned about production qualities such as
load times and memory footprint.
.l-main-section
:marked
<a id="package-json"></a>
### Appendix: package.json
[npm](https://docs.npmjs.com/) is a popular package manager and Angular application developers rely on it
to acquire and manage the libraries their apps require.
We specify the packages we need in an npm [package.json](https://docs.npmjs.com/files/package.json) file.
The Angular team suggests the packages listed in the `dependencies` and `devDependencies`
sections listed in this file:
+makeJson('quickstart/js/package.1.json',{ paths: 'dependencies, devDependencies'}, 'package.json (dependencies)')(format=".")
:marked
.l-sub-section
:marked
There are other possible package choices.
We're recommending this particular set that we know work well together.
Play along with us for now.
Feel free to make substitutions later to suit your tastes and experience.
:marked
A `package.json` has an optional **scripts** section where we can define helpful
commands to perform development and build tasks.
We've included a number of such scripts in our suggested `package.json`:
+makeJson('quickstart/js/package.1.json',{ paths: 'scripts'}, 'package.json (scripts)')(format=".")
:marked
We've seen how we can run the server with this command:
code-example(format="").
npm start
:marked
We are using the special `npm start` command, but all it does is run `npm run lite`.
We execute npm scripts in that manner: `npm run` + *script-name*. Here's what these scripts do:
* `npm run lite` - run the [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.
.l-main-section
:marked
<a id="npm-errors"></a>
### Appendix: npm errors and warnings
All is well if there are no console messages starting with `npm ERR!` *at the end* of **npm install**.
There might be a few `npm WARN` messages along the way &mdash; and that is perfectly fine.
We often see an `npm WARN` message after a series of `gyp ERR!` messages.
Ignore them. A package may try to re-compile itself using `node-gyp`.
If the re-compile fails, the package recovers (typically with a pre-built version)
and everything works.
Just make sure there are no `npm ERR!` messages at the very end of `npm install`.
.l-main-section
:marked
<a id="ngmodule"></a>
### Appendix: ***NgModule***
The `NgModule` decorator is listing:
1. What other Angular Modules ours uses
1. Which components and directives we declare in our components
1. The component to bootstrap at the start
We import our lone `app.AppComponent` and add it to both `declarations` and `bootstrap` array.
Notice that we also add `ng.platformBrowser.BrowserModule` to the `imports` array.
This is the Angular Module that contains all the needed Angular bits and pieces to run our app in the browser.
Angular itself is split into separate Angular Modules so we only need to import the ones we really use.
One of the most common ones is `FormsModule`, and soon we'll also see `RouterModule` and `HttpModule`.
.l-main-section
:marked
<a id="main"></a>
### Appendix: ***main.js***
#### Bootstrapping is platform-specific
We use the `platformBrowserDynamic().bootstrapModule` function from `ng.platformBrowserDynamic`,
not `ng.core`. There's a good reason.
We only call "core" those capabilities that are the same across all platform targets.
True, most Angular applications run only in a browser and we'll call the bootstrap function from
this library most of the time. It's pretty "core" if we're always writing for a browser.
But it is possible to load a component in a different environment.
We might load it on a mobile device with [Apache Cordova](https://cordova.apache.org/) or [NativeScript](https://www.nativescript.org/).
We might wish to render the first page of our application on the server
to improve launch performance or facilitate
[SEO](http://www.google.com/webmasters/docs/search-engine-optimization-starter-guide.pdf).
These targets require a different kind of bootstrap function that we'd import from a different library.
#### Why do we create a separate ***main.js***, ***app.module.js*** and ***app.component.js*** files?
The *main.js* file is tiny. This is just a QuickStart.
We could have folded its few lines into the `app.module.js` file, and that one into
`app.component.js` and spared ourselves some complexity.
We didn't for what we believe to be good reasons:
1. Doing it right is easy
1. Testability
1. Reusability
1. Separation of concerns
1. We learned about import and export
#### It's easy
Sure it's an extra step and an extra file. How hard is that in the scheme of things?
We'll see that a separate `main.js` and `app.module.js` is beneficial for *most* apps
even if it isn't critical for the QuickStart.
Let's develop good habits now while the cost is low.
#### Testability
We should be thinking about testability from the beginning
even if we know we'll never test the QuickStart.
It is difficult to unit test a component when there is a call to `bootstrapModule` in the same file.
As soon as we load the component file to test the component,
the `bootstrapModule` function tries to load the application in the browser.
It throws an error because we're not expecting to run the entire application,
just test the component.
Relocating the `bootstrapModule` function to `main.js` eliminates this spurious error
and leaves us with a clean component module file.
#### Reusability
We refactor, rename, and relocate files as our application evolves.
We can't do any of those things while the file calls `bootstrapModule`.
we can't move it.
We can't reuse the component in another application.
We can't pre-render the component on the server for better performance.
#### Separation of concerns
A component's responsibility is to present and manage a view, and a NgModule's
reponsibility is to define the application context.
Launching the application has nothing to do with any of these.
That's a separate concern. The friction we're encountering in testing and reuse
stems from this unnecessary mix of responsibilities.
#### Import/Export
While writing a separate `main.js` and `app.module.js` files we learned an essential Angular skill:
how to 'export' from one 'module' and 'import' into another via our simple
namespace abstraction.
We'll do a lot of that as we learn more Angular.
h2#section-transpile Great job! We'll have the next steps out soon.