angular-docs-cn/aio/content/guide/upgrade-lite.md

# Upgrading from AngularJS (in a more flexible way)

<div class="alert is-helpful">

  _Angular_ is the name for the Angular of today and tomorrow.<br />
  _AngularJS_ is the name for all 1.x versions of Angular.

</div>

This guide describes some of the built-in tools for efficiently migrating AngularJS projects over
to the Angular platform, one piece at a time. It is very similar to the
[main upgrade guide](guide/upgrade) with the exception that this one uses the {@link downgradeModule
downgradeModule()} helper function instead of the {@link UpgradeModule UpgradeModule} class. This
affects how the application is bootstrapped and how change detection is propagated between the two
frameworks (more on that later).


## Preparation

Before we start discussing how you can use `downgradeModule()` to create hybrid applications, there
are things that you can do to ease the upgrade process even before you begin upgrading. Although not
strictly necessary, preparation goes a long way! The steps are the same regardless how you upgrade,
so go ahead and read the [Preparation](guide/upgrade#preparation) section of the main upgrade guide.


## Upgrading with ngUpgrade

With the ngUpgrade library in Angular you can upgrade an existing AngularJS application
incrementally, by building a hybrid application where you can run both frameworks side-by-side. In
these hybrid applications you can mix and match AngularJS and Angular components and services and
have them interoperate seamlessly. That means you don't have to do the upgrade work all at once,
since there is a natural coexistence between the two frameworks during the transition period.


### How ngUpgrade Works

Regardless of whether you choose `downgradeModule()` or `UpgradeModule`, the basic principles of
upgrading, the mental model behind hybrid applications and how you use the {@link upgrade/static
upgrade/static} utilities remain the same. You can read about all that in the
[How ngUpgrade Works](guide/upgrade#how-ngupgrade-works) section of the main upgrade guide.

<div class="alert is-helpful">

  The [Change Detection](guide/upgrade#change-detection) sub-section only applies to applications
  that use `UpgradeModule`. Change detection is handled differently with `downgradeModule()`.<br />
  We still recommend reading the sub-section in order to better understand the differences and their
  implications.

</div>


#### Change Detection with `downgradeModule()`

As mentioned before, one of the key differences between `downgradeModule()` and `UpgradeModule` has
to do with change detection and how it is propagated between the two frameworks.

With `UpgradeModule`, the two change detection systems are tied together more tightly. Whenever
something happens in the AngularJS part of the application, change detection is automatically
triggered on the Angular part and vice versa. This is convenient as it ensures that no important
change is missed by either framework. Most of the time, though, these extra change detection runs
are unnecesary.

`downgradeModule()`, on the other side, avoids explicitly triggering change detection, unless it
knows the other part of the application is interested in the changes. One way to know, for example,
is when a value is bound to the {@link Input input} of a downgraded component. If the component
defines an `Input`, chances are it needs to be change-detected when that value changes. Thus,
`downgradeComponent()` _will_ automatically trigger change detection on that component.

In most cases, though, the changes made locally in a particular component are of no interest to the
rest of the application. For example, if the user clicks a button that submits a form the component
will usually handle the result of this action. That being said, there _are_ cases, where you want to
propagate changes to some other part of the application, that may be controlled by the other
framework. In such cases, you are responsible for notifying the interested parties, by manually
triggering change detection.

If you want a particular piece of code to trigger change detection in the AngularJS part of the
application, you need to wrap it in
[scope.$apply(...)](https://docs.angularjs.org/api/ng/type/$rootScope.Scope#$apply). Similarly, for
triggering change detection in Angular you would use {@link NgZone#run ngZone.run(...)}.

In many cases, a few extra change detection runs may not matter much. On larger or
change-detection-heavy applications, though, they can have a noticeable impact. By giving you more
fine-grained control over the change detection propagation, `downgradeModule()` allows you to
achieve better performance for your hybrid applications.


### Using `downgradeModule()`

Both AngularJS and Angular have their own concept of modules to help organize an application into
cohesive blocks of functionality.

Their details are quite different in architecture and implementation. In AngularJS, you create a
module by specifying its name and dependencies with
[angular.module()](https://docs.angularjs.org/api/ng/function/angular.module). Then you can add
assets using its various methods. In Angular, you create a class adorned with an {@link NgModule
NgModule} decorator that describes assets in metadata. The differences blossom from there.

In a hybrid application you run both frameworks at the same time. This means that you need at least
one module each from both AngularJS and Angular.

For the most part, you specify the modules in the same way as you would for a regular application.
Then, you use the `upgrade/static` helpers to let the two frameworks know about assets they can use
from each other. This is known as "upgrading" and "downgrading" (more on how this is done later).

<div class="alert is-helpful">

  <b>Definitions:</b>

  - _Upgrading_: The act of making an AngularJS asset (e.g. component or service) available to the
    Angular part of the application.
  - _Downgrading_: The act of making an Angular asset (e.g. component or service) available to the
    AngularJS part of the application.

</div>

An important part of inter-linking dependencies, is linking the two main modules together. This is
where `downgradeModule()` comes in. It is used to create an AngularJS module &mdash; one that you
can use as a dependency in your main AngularJS module &mdash; that will bootstrap your main Angular
module and kick off the Angular part of the hybrid application. In a sense, it takes an Angular
module and "downgrades" it to an AngularJS module.

There are a few things to note, though:

1. You don't pass the Angular module directly to `downgradeModule()`. All `downgradeModule()` needs
   is a "recipe" (e.g. a factory function) for creating an instance for your module.

2. The Angular module is not instantiated until it is actually needed.

We will expand on these two points below. For now, let's see how we can use `downgradeModule()` to
link the two modules.

```ts
// Import `downgradeModule()`.
import { downgradeModule } from '@angular/upgrade/static';

// Use it to "downgrade" the Angular module to an AngularJS module.
const downgradedModule = downgradeModule(MainAngularModuleFactory);

// Use the downgraded module as a dependency to the main AngularJS module.
angular.module('mainAngularJsModule', [
  downgradedModule
]);
```


#### Specifying a factory for the Angular module

As mentioned before, `downgradeModule()` needs to know how to instantiate the Angular module. It
needs a "recipe". You define that recipe, by providing a factory function that can create an
instance of the Angular module. `downgradeModule()` accepts two types of factory functions:

1. {@link NgModuleFactory NgModuleFactory}
2. (extraProviders: {@link StaticProvider StaticProvider}[]) => Promise<{@link NgModuleRef NgModuleRef}>

If you pass an `NgModuleFactory`, it will be used to instantiate the module using
{@link platformBrowser platformBrowser}'s {@link PlatformRef#bootstrapModuleFactory
bootstrapModuleFactory()}. This is great, because it is compatible with Ahead-of-Time (AoT)
compilation. You can read more about AoT compilation and how to create an `NgModuleFactory` in the
[AoT Compilation](guide/aot-compiler) guide.

Alternatively, you can pass a plain function, which is expected to return a promise resolving to an
{@link NgModuleRef NgModuleRef} (i.e. an instance of your Angular module). The function is called
with an array of extra {@link StaticProvider Providers} that are expected to be available on the returned
`NgModuleRef`'s {@link Injector Injector}. For example, if you are using {@link platformBrowser
platformBrowser} or {@link platformBrowserDynamic platformBrowserDynamic}, you can pass the
`extraProviders` array to them:

```ts
const bootstrapFn = (extraProviders: StaticProvider[]) => {
  const platformRef = platformBrowserDynamic(extraProviders);
  return platformRef.bootstrapModule(MainAngularModule);
};
// or
const bootstrapFn = (extraProviders: StaticProvider[]) => {
  const platformRef = platformBrowser(extraProviders);
  return platformRef.bootstrapModuleFactory(MainAngularModuleFactory);
};
```

Using an `NgModuleFactory` requires less boilerplate and is a good default option as it supports
AoT out-of-the-box. Using a custom function requires slightly more code, but gives you greater
flexibility.


#### Instantiating the Angular module "on-demand"

Another key difference between `downgradeModule()` and `UpgradeModule` is that the latter requires
you to instantiate both the AngularJS and Angular modules up-front. This means that you have to pay
the cost of instantiating the Angular part of the application, even if you don't use any Angular
assets until later. `downgradeModule()` is again less aggressive: It will only instantiate the
Angular part when it is required for the first time; i.e. as soon as a downgraded component needs to
be created.

You could go a step further and not even download the code for the Angular part of the application
to the user's browser, until it is needed. This is especially useful, when you use Angular on parts
of the hybrid application that are not necessary for the initial rendering or are not often reached
by the user (or not reached by all types of users).

A few examples:

- You use Angular on specific routes only and you don't need it until/if such a route is visited by
  the user.
- You use Angular for features that are only visible to specific types of users (e.g. logged-in
  users or administrators or VIP members). You don't need to load Angular until a user is
  authenticated.
- You use Angular for a feature that is not critical for the initial rendering of the application
  and you can afford a small delay in favor of better initial load performance.


### Bootstrapping with `downgradeModule()`

As you may have guessed, you don't need to change anything in the way you bootstrap your existing
AngularJS application. Unlike `UpgradeModule` &mdash; which requires some extra steps &mdash;
`downgradeModule()` is able to take care of bootstrapping the Angular module (as long as you provide
the recipe).

In order to start using any `upgrade/static` APIs, you still need to load the Angular framework (as
you would in a normal Angular application). You can see how this can be done with SystemJS by
following the instructions in the [Setup](guide/setup) guide, selectively copying code from the
[QuickStart github repository](https://github.com/angular/quickstart).

You also need to install the `@angular/upgrade` package via `npm install @angular/upgrade --save`
and add a mapping for the `@angular/upgrade/static` package:

<code-example title="system.config.js">
'@angular/upgrade/static': 'npm:@angular/upgrade/bundles/upgrade-static.umd.js',
</code-example>

Next, create an `app.module.ts` file and add the following `NgModule` class:

<code-example title="app.module.ts">
import { NgModule } from '@angular/core';
import { BrowserModule } from '@angular/platform-browser';

@NgModule({
  imports: [
    BrowserModule
  ]
})
export class MainAngularModule {
  // Empty placeholder method to prevent the `Compiler` from complaining.
  ngDoBootstrap() {}
}
</code-example>

This bare minimum `NgModule` imports `BrowserModule`, the module every Angular browser-based app
must have. It also defines an empty `ngDoBootstrap()` method, to prevent the {@link Compiler
Compiler} from complaining. This is necessary, because the module will not have a `bootstrap`
declaration on its `NgModule` decorator.

<div class="alert is-important">

  You do not add a `bootstrap` declaration to the `NgModule` decorator, since AngularJS will own the
  root template of the application and ngUpgrade will be bootstrapping the necessary components.

</div>

You can now link the AngularJS and Angular modules together using `downgradeModule()`.

<code-example title="app.module.ts">
import { platformBrowserDynamic } from '@angular/platform-browser-dynamic';
import { downgradeModule } from '@angular/upgrade/static';

const bootstrapFn = (extraProviders: StaticProvider[]) => {
  const platformRef = platformBrowserDynamic(extraProviders);
  return platformRef.bootstrapModule(MainAngularModule);
};
const downgradedModule = downgradeModule(bootstrapFn);

angular.module('mainAngularJsModule', [
  downgradedModule
]);
</code-example>

Congratulations! You are running a hybrid application! The existing AngularJS code works as before
_and_ you are ready to start adding Angular code.


### Using Components and Injectables

The differences between `downgradeModule()` and `UpgradeModule` end here. The rest of the
`upgrade/static` APIs and concepts work in the exact same way for both types of hybrid applications.
Head over to the [main upgrade guide](guide/upgrade) to learn about:

- [Using Angular Components from AngularJS Code](guide/upgrade#using-angular-components-from-angularjs-code)
- [Using AngularJS Component Directives from Angular Code](guide/upgrade#using-angularjs-component-directives-from-angular-code)
- [Projecting AngularJS Content into Angular Components](guide/upgrade#projecting-angularjs-content-into-angular-components)
- [Transcluding Angular Content into AngularJS Component Directives](guide/upgrade#transcluding-angular-content-into-angularjs-component-directives)
- [Making AngularJS Dependencies Injectable to Angular](guide/upgrade#making-angularjs-dependencies-injectable-to-angular)
- [Making Angular Dependencies Injectable to AngularJS](guide/upgrade#making-angular-dependencies-injectable-to-angularjs)

<div class="alert is-important">

  While it is possible to downgrade injectables, the downgraded injectables will _not_ be available
  until the Angular module is instantiated too. In order to be safe, you need to ensure that the
  downgraded injectables are not used anywhere _outside_ the part of the application that is
  controlled by Angular.

  For example, it is _OK_ to use a downgraded service in an upgraded component that is only used
  from Angular components, but it is _not OK_ to use it in an AngularJS component that may be used
  independently of Angular.

</div>


## Using Ahead-of-Time compilation with hybrid applications

You can take advantage of Ahead-of-Time (AoT) compilation on hybrid applications just like on any
other Angular application. The setup for a hybrid application is mostly the same as described in the
[AoT Compilation](guide/aot-compiler) guide save for differences in `index.html` and `main-aot.ts`.

The `index.html` will likely have script tags loading AngularJS files, so the `index.html` for AoT
must also load those files. An easy way to copy them is by adding each to the `copy-dist-files.js`
file.

You will also need to pass the generated `MainAngularModuleFactory` to `downgradeModule()`, instead of
the custom bootstrap function:

<code-example title="app/main-aot.ts">
import { downgradeModule } from '@angular/upgrade/static';
import { MainAngularModuleNgFactory } from '../aot/app/app.module.ngfactory';

const downgradedModule = downgradeModule(MainAngularModuleNgFactory);

angular.module('mainAngularJsModule', [
  downgradedModule
]);
</code-example>

And that is all you need to do to get the full benefit of AoT for Angular applications!


## Wrap up

You have learned how to use the {@link upgrade/static upgrade/static} package to incrementally
upgrade existing AngularJS applications at your own pace and without impeding further development of
the application for the duration of the upgrade process.

More specifically, you have seen how you can achieve better performance and greater flexibility in
your hybrid applications, by using {@link downgradeModule downgradeModule()} (instead of
{@link UpgradeModule UpgradeModule}).

To summarize, the key differentiating factors of `downgradeModule()` are:

1. It allows instantiating (or even loading) the Angular part lazily, which improves the initial
   loading time (and is some cases may waive the cost of running a second framework altogether).
2. It improves performance by avoiding unnecessary change detection runs, instead putting more
   responsibility on the developer.
3. It does not require you to change how you bootstrap your AngularJS app.

Based on that, `downgradeModule()` is a good option for hybrid applications that want keep the
AngularJS and Angular parts less coupled. You can still mix and match components and services from
both frameworks, but you might need to manually propagate change detection. In return,
`downgradeModule()` offers more control and better performance characteristics.

There is merit in both approaches, so you should always weight the pros and cons before deciding
which one better meets the upgrading needs of each project.