# The Ahead-of-Time (AOT) compiler

An Angular application consists mainly of components and their HTML templates. Because the components and templates provided by Angular cannot be understood by the browser directly, Angular applications require a compilation process before they can run in a browser.

The Angular Ahead-of-Time (AOT) compiler converts your Angular HTML and TypeScript code into efficient JavaScript code during the build phase _before_ the browser downloads and runs that code. Compiling your application during the build process provides a faster rendering in the browser.

This guide explains how to specify metadata and apply available compiler options to compile your applications efficiently using the AOT compiler.

<div class="alert is-helpful"

  <a href="https://www.youtube.com/watch?v=kW9cJsvcsGo">Watch compiler author Tobias Bosch explain the Angular compiler</a> at AngularConnect 2016.

</div>

{@a why-aot}

Here are some reasons you might want to use AOT.

* *Faster rendering*
   With AOT, the browser downloads a pre-compiled version of the application.
   The browser loads executable code so it can render the application immediately, without waiting to compile the app first.

* *Fewer asynchronous requests*
   The compiler _inlines_ external HTML templates and CSS style sheets within the application JavaScript,
   eliminating separate ajax requests for those source files.

* *Smaller Angular framework download size*
   There's no need to download the Angular compiler if the app is already compiled.
   The compiler is roughly half of Angular itself, so omitting it dramatically reduces the application payload.

* *Detect template errors earlier*
   The AOT compiler detects and reports template binding errors during the build step
   before users can see them.

* *Better security*
   AOT compiles HTML templates and components into JavaScript files long before they are served to the client.
   With no templates to read and no risky client-side HTML or JavaScript evaluation,
   there are fewer opportunities for injection attacks.

{@a overview}

## Choosing a compiler

Angular offers two ways to compile your application:

* **_Just-in-Time_ (JIT)**, which compiles your app in the browser at runtime.
* **_Ahead-of-Time_ (AOT)**, which compiles your app at build time.

JIT compilation is the default when you run the [`ng build`](cli/build) (build only) or [`ng serve`](cli/serve)  (build and serve locally) CLI commands:

<code-example language="sh" class="code-shell">
  ng build
  ng serve
</code-example>

{@a compile}

For AOT compilation, include the `--aot` option with the `ng build` or `ng serve` command:

<code-example language="sh" class="code-shell">
  ng build --aot
  ng serve --aot
</code-example>

<div class="alert is-helpful">

The `ng build` command with the `--prod` meta-flag (`ng build --prod`) compiles with AOT by default.

See the [CLI command reference](cli) and [Building and serving Angular apps](guide/build) for more information.

</div>

## How AOT works

The Angular AOT compiler extracts **metadata** to interpret the parts of the application that Angular is supposed to manage.
You can specify the metadata explicitly in **decorators** such as `@Component()` and `@Input()`, or implicitly in the constructor declarations of the decorated classes.
The metadata tells Angular how to construct instances of your application classes and interact with them at runtime.

In the following example, the `@Component()` metadata object and the class constructor tell Angular how to create and display an instance of `TypicalComponent`.

```typescript
@Component({
  selector: 'app-typical',
  template: '<div>A typical component for {{data.name}}</div>'
)}
export class TypicalComponent {
  @Input() data: TypicalData;
  constructor(private someService: SomeService) { ... }
}
```

The Angular compiler extracts the metadata _once_ and generates a _factory_ for `TypicalComponent`.
When it needs to create a `TypicalComponent` instance, Angular calls the factory, which produces a new visual element, bound to a new instance of the component class with its injected dependency.

### Compilation phases

There are three phases of AOT compilation.
* Phase 1 is *code analysis*.
   In this phase, the TypeScript compiler and  *AOT collector* create a representation of the source. The collector does not attempt to interpret the metadata it collects. It represents the metadata as best it can and records errors when it detects a metadata syntax violation.

* Phase 2 is *code generation*.
    In this phase, the compiler's `StaticReflector` interprets the metadata collected in phase 1, performs additional validation of the metadata, and throws an error if it detects a metadata restriction violation.

* Phase 3 is *template type checking*.
   In this optional phase, the Angular *template compiler* uses the TypeScript compiler to validate the binding expressions in templates. You can enable this phase explicitly by setting the `fullTemplateTypeCheck` configuration option; see [Angular compiler options](guide/angular-compiler-options).


### Metadata restrictions

You write metadata in a _subset_ of TypeScript that must conform to the following general constraints:

* Limit [expression syntax](#expression-syntax) to the supported subset of JavaScript.
* Only reference exported symbols after [code folding](#code-folding).
* Only call [functions supported](#supported-functions) by the compiler.
* Decorated and data-bound class members must be public.

For additional guidelines and instructions on preparing an application for AOT compilation, see [Angular: Writing AOT-friendly applications](https://medium.com/sparkles-blog/angular-writing-aot-friendly-applications-7b64c8afbe3f).

<div class="alert is-helpful">

Errors in AOT compilation commonly occur because of metadata that does not conform to the compiler's requirements (as described more fully below).
For help in understanding and resolving these problems, see [AOT Metadata Errors](guide/aot-metadata-errors).

</div>

### Configuring AOT compilation

You can provide options in the `tsconfig.json` [TypeScript configuration file](guide/typescript-configuration) that control the compilation process. See [Angular compiler options](guide/angular-compiler-options) for a complete list of available options.

## Phase 1: Code analysis

The TypeScript compiler does some of the analytic work of the first phase. It emits the `.d.ts` _type definition files_ with type information that the AOT compiler needs to generate application code.
At the same time, the AOT **collector** analyzes the metadata recorded in the Angular decorators and outputs metadata information in **`.metadata.json`** files, one per `.d.ts` file.

You can think of `.metadata.json` as a diagram of the overall structure of a decorator's metadata, represented as an [abstract syntax tree (AST)](https://en.wikipedia.org/wiki/Abstract_syntax_tree).

<div class="alert is-helpful">

Angular's [schema.ts](https://github.com/angular/angular/blob/master/packages/compiler-cli/src/metadata/schema.ts)
describes the JSON format as a collection of TypeScript interfaces.

</div>

{@a expression-syntax}
### Expression syntax limitations

The  AOT collector only understands a subset of JavaScript.
Define metadata objects with the following limited syntax:

<style>
  td, th {vertical-align: top}
</style>

<table>
  <tr>
    <th>Syntax</th>
    <th>Example</th>
  </tr>
  <tr>
    <td>Literal object </td>
    <td><code>{cherry: true, apple: true, mincemeat: false}</code></td>
  </tr>
  <tr>
    <td>Literal array  </td>
    <td><code>['cherries', 'flour', 'sugar']</code></td>
  </tr>
  <tr>
    <td>Spread in literal array</td>
    <td><code>['apples', 'flour', ...the_rest]</code></td>
  </tr>
   <tr>
    <td>Calls</td>
    <td><code>bake(ingredients)</code></td>
  </tr>
   <tr>
    <td>New</td>
    <td><code>new Oven()</code></td>
  </tr>
   <tr>
    <td>Property access</td>
    <td><code>pie.slice</code></td>
  </tr>
   <tr>
    <td>Array index</td>
    <td><code>ingredients[0]</code></td>
  </tr>
   <tr>
    <td>Identity reference</td>
    <td><code>Component</code></td>
  </tr>
   <tr>
    <td>A template string</td>
    <td><code>`pie is ${multiplier} times better than cake`</code></td>
   <tr>
    <td>Literal string</td>
    <td><code>pi</code></td>
  </tr>
   <tr>
    <td>Literal number</td>
    <td><code>3.14153265</code></td>
  </tr>
   <tr>
    <td>Literal boolean</td>
    <td><code>true</code></td>
  </tr>
   <tr>
    <td>Literal null</td>
    <td><code>null</code></td>
  </tr>
   <tr>
    <td>Supported prefix operator </td>
    <td><code>!cake</code></td>
  </tr>
   <tr>
    <td>Supported binary operator </td>
    <td><code>a+b</code></td>
  </tr>
   <tr>
    <td>Conditional operator</td>
    <td><code>a ? b : c</code></td>
  </tr>
   <tr>
    <td>Parentheses</td>
    <td><code>(a+b)</code></td>
  </tr>
</table>


If an expression uses unsupported syntax, the collector writes an error node to the `.metadata.json` file.
The compiler later reports the error if it needs that piece of metadata to generate the application code.

<div class="alert is-helpful">

 If you want `ngc` to report syntax errors immediately rather than produce a `.metadata.json` file with errors, set the `strictMetadataEmit` option in the TypeScript configuration file, `tsconfig.json`.

```
  "angularCompilerOptions": {
   ...
   "strictMetadataEmit" : true
 }
 ```

Angular libraries have this option to ensure that all Angular `.metadata.json` files are clean and it is a best practice to do the same when building your own libraries.

</div>

{@a function-expression}
{@a arrow-functions}
### No arrow functions

The AOT compiler does not support [function expressions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/function)
and [arrow functions](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions), also called _lambda_ functions.

Consider the following component decorator:

```typescript
@Component({
  ...
  providers: [{provide: server, useFactory: () => new Server()}]
})
```

The AOT collector does not support the arrow function, `() => new Server()`, in a metadata expression.
It generates an error node in place of the function.
When the compiler later interprets this node, it reports an error that invites you to turn the arrow function into an _exported function_.

You can fix the error by converting to this:

```typescript
export function serverFactory() {
  return new Server();
}

@Component({
  ...
  providers: [{provide: server, useFactory: serverFactory}]
})
```

In version 5 and later, the compiler automatically performs this rewriting while emitting the `.js` file.

{@a exported-symbols}
{@a code-folding}
### Code folding

The compiler can only resolve references to **_exported_** symbols.
The collector, however, can evaluate an expression during collection and record the result in the `.metadata.json`, rather than the original expression.
This allows you to make limited use of non-exported symbols within expressions.

For example, the collector can evaluate the expression `1 + 2 + 3 + 4` and replace it with the result, `10`.
This process is called _folding_. An expression that can be reduced in this manner is _foldable_.

{@a var-declaration}
The collector can evaluate references to module-local `const` declarations and initialized `var` and `let` declarations, effectively removing them from the `.metadata.json` file.

Consider the following component definition:

```typescript
const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template
})
export class HeroComponent {
  @Input() hero: Hero;
}
```

The compiler could not refer to the `template` constant because it isn't exported.
The collector, however, can fold the `template` constant into the metadata definition by in-lining its contents.
The effect is the same as if you had written:

```typescript
@Component({
  selector: 'app-hero',
  template: '<div>{{hero.name}}</div>'
})
export class HeroComponent {
  @Input() hero: Hero;
}
```

There is no longer a reference to `template` and, therefore, nothing to trouble the compiler when it later interprets the _collector's_ output in `.metadata.json`.

You can take this example a step further by including the `template` constant in another expression:

```typescript
const template = '<div>{{hero.name}}</div>';

@Component({
  selector: 'app-hero',
  template: template + '<div>{{hero.title}}</div>'
})
export class HeroComponent {
  @Input() hero: Hero;
}
```

The collector reduces this expression to its equivalent _folded_ string:

```
'<div>{{hero.name}}</div><div>{{hero.title}}</div>'
```

#### Foldable syntax

The following table describes which expressions the collector can and cannot fold:

<style>
  td, th {vertical-align: top}
</style>

<table>
  <tr>
    <th>Syntax</th>
    <th>Foldable</th>
  </tr>
  <tr>
    <td>Literal object </td>
    <td>yes</td>
  </tr>
  <tr>
    <td>Literal array  </td>
    <td>yes</td>
  </tr>
  <tr>
    <td>Spread in literal array</td>
    <td>no</td>
  </tr>
   <tr>
    <td>Calls</td>
    <td>no</td>
  </tr>
   <tr>
    <td>New</td>
    <td>no</td>
  </tr>
   <tr>
    <td>Property access</td>
    <td>yes, if target is foldable</td>
  </tr>
   <tr>
    <td>Array index</td>
    <td> yes, if target and index are foldable</td>
  </tr>
   <tr>
    <td>Identity reference</td>
    <td>yes, if it is a reference to a local</td>
  </tr>
   <tr>
    <td>A template with no substitutions</td>
    <td>yes</td>
  </tr>
   <tr>
    <td>A template with substitutions</td>
    <td>yes, if the substitutions are foldable</td>
  </tr>
   <tr>
    <td>Literal string</td>
    <td>yes</td>
  </tr>
   <tr>
    <td>Literal number</td>
    <td>yes</td>
  </tr>
   <tr>
    <td>Literal boolean</td>
    <td>yes</td>
  </tr>
   <tr>
    <td>Literal null</td>
    <td>yes</td>
  </tr>
   <tr>
    <td>Supported prefix operator </td>
    <td>yes, if operand is foldable</td>
  </tr>
   <tr>
    <td>Supported binary operator </td>
    <td>yes, if both left and right are foldable</td>
  </tr>
   <tr>
    <td>Conditional operator</td>
    <td>yes, if condition is foldable </td>
  </tr>
   <tr>
    <td>Parentheses</td>
    <td>yes, if the expression is foldable</td>
  </tr>
</table>


If an expression is not foldable, the collector writes it to `.metadata.json` as an [AST](https://en.wikipedia.org/wiki/Abstract_syntax_tree) for the compiler to resolve.


## Phase 2: code generation

The collector makes no attempt to understand the metadata that it collects and outputs to `.metadata.json`.
It represents the metadata as best it can and records errors when it detects a metadata syntax violation.
It's the compiler's job to interpret the `.metadata.json` in the code generation phase.

The compiler understands all syntax forms that the collector supports, but it may reject _syntactically_ correct metadata if the _semantics_ violate compiler rules.

### Public symbols

The compiler can only reference _exported symbols_.

* Decorated component class members must be public. You cannot make an `@Input()` property private or protected.
* Data bound properties must also be public.

```typescript
// BAD CODE - title is private
@Component({
  selector: 'app-root',
  template: '<h1>{{title}}</h1>'
})
export class AppComponent {
  private title = 'My App'; // Bad
}
```

{@a supported-functions}

### Supported classes and functions

The collector can represent a function call or object creation with `new` as long as the syntax is valid.
The compiler, however, can later refuse to generate a call to a _particular_ function or creation of a _particular_ object.

The compiler can only create instances of certain classes, supports only core decorators, and only supports calls to macros (functions or static methods) that return expressions.
* New instances

   The compiler only allows metadata that create instances of the class `InjectionToken` from `@angular/core`.

* Supported decorators

   The compiler only supports metadata for the [Angular decorators in the `@angular/core` module](api/core#decorators).

* Function calls

   Factory functions must be exported, named functions.
   The AOT compiler does not support lambda expressions ("arrow functions") for factory functions.

{@a function-calls}
### Functions and static method calls

The collector accepts any function or static method that contains a single `return` statement.
The compiler, however, only supports macros in the form of functions or static methods that return an *expression*.

For example, consider the following function:

```typescript
export function wrapInArray<T>(value: T): T[] {
  return [value];
}
```

You can call the `wrapInArray` in a metadata definition because it returns the value of an expression that conforms to the compiler's restrictive JavaScript subset.

You might use  `wrapInArray()` like this:

```typescript
@NgModule({
  declarations: wrapInArray(TypicalComponent)
})
export class TypicalModule {}
```

The compiler treats this usage as if you had written:

```typescript
@NgModule({
  declarations: [TypicalComponent]
})
export class TypicalModule {}
```
The Angular [`RouterModule`](api/router/RouterModule) exports two macro static methods, `forRoot` and `forChild`, to help declare root and child routes.
Review the [source code](https://github.com/angular/angular/blob/master/packages/router/src/router_module.ts#L139 "RouterModule.forRoot source code")
for these methods to see how macros can simplify configuration of complex [NgModules](guide/ngmodules).

{@a metadata-rewriting}

### Metadata rewriting

The compiler treats object literals containing the fields `useClass`, `useValue`, `useFactory`, and `data` specially, converting the expression initializing one of these fields into an exported variable that replaces the expression.
This process of rewriting these expressions removes all the restrictions on what can be in them because
the compiler doesn't need to know the expression's value&mdash;it just needs to be able to generate a reference to the value.

You might write something like:

```typescript
class TypicalServer {

}

@NgModule({
  providers: [{provide: SERVER, useFactory: () => TypicalServer}]
})
export class TypicalModule {}
```

Without rewriting, this would be invalid because lambdas are not supported and `TypicalServer` is not exported.
To allow this, the compiler automatically rewrites this to something like:

```typescript
class TypicalServer {

}

export const ɵ0 = () => new TypicalServer();

@NgModule({
  providers: [{provide: SERVER, useFactory: ɵ0}]
})
export class TypicalModule {}
```

This allows the compiler to generate a reference to `ɵ0` in the factory without having to know what the value of `ɵ0` contains.

The compiler does the rewriting during the emit of the `.js` file.
It does not, however, rewrite the `.d.ts` file, so TypeScript doesn't recognize it as being an export. and it does not interfere with the ES module's exported API.


{@a binding-expression-validation}
## Phase 3: Template type checking

In the template type-checking phase, the Angular template compiler uses the TypeScript compiler to validate the binding expressions in templates.
Enable this phase explicitly by adding the compiler option `"fullTemplateTypeCheck"` in the `"angularCompilerOptions"` of the project's `tsconfig.json`
(see [Angular Compiler Options](guide/angular-compiler-options)).

Template validation produces error messages when a type error is detected in a template binding
expression, similar to how type errors are reported by the TypeScript compiler against code in a `.ts`
file.

For example, consider the following component:

```typescript
  @Component({
    selector: 'my-component',
    template: '{{person.addresss.street}}'
  })
  class MyComponent {
    person?: Person;
  }
```

This produces the following error:

```
  my.component.ts.MyComponent.html(1,1): : Property 'addresss' does not exist on type 'Person'. Did you mean 'address'?
 ```

The file name reported in the error message, `my.component.ts.MyComponent.html`, is a synthetic file
generated by the template compiler that holds contents of the `MyComponent` class template.
The compiler never writes this file to disk.
The line and column numbers are relative to the template string in the `@Component` annotation of the class, `MyComponent` in this case.
If a component uses `templateUrl` instead of `template`, the errors are reported in the HTML file referenced by the `templateUrl` instead of a synthetic file.

The error location is the beginning of the text node that contains the interpolation expression with the error.
If the error is in an attribute binding such as `[value]="person.address.street"`, the error
location is the location of the attribute that contains the error.

The validation uses the TypeScript type checker and the options supplied to the TypeScript compiler to control how detailed the type validation is.
For example, if the `strictTypeChecks` is specified, the error
```my.component.ts.MyComponent.html(1,1): : Object is possibly 'undefined'```
is reported as well as the above error message.

### Type narrowing

The expression used in an `ngIf` directive is used to narrow type unions in the Angular
template compiler, the same way the `if` expression does in TypeScript.
For example, to avoid `Object is possibly 'undefined'` error in the template above, modify it to only emit the interpolation if the value of `person` is initialized as shown below:

```typescript
  @Component({
    selector: 'my-component',
    template: '<span *ngIf="person"> {{person.addresss.street}} </span>'
  })
  class MyComponent {
    person?: Person;
  }
```

Using `*ngIf` allows the TypeScript compiler to infer that the `person` used in the binding expression will never be `undefined`.

#### Custom `ngIf` like directives

Directives that behave like `*ngIf` can declare that they want the same treatment by including a static member marker that is a signal to the template compiler to treat them like `*ngIf`. This static member for `*ngIf` is:

```typescript
    public static ngIfUseIfTypeGuard: void;
```

This declares that the input property `ngIf` of the `NgIf` directive should be treated as a guard to the use of its template, implying that the template will only be instantiated if the `ngIf` input property is true.


### Non-null type assertion operator

Use the [non-null type assertion operator](guide/template-syntax#non-null-assertion-operator) to suppress the `Object is possibly 'undefined'` error when it is inconvenient to use `*ngIf` or when some constraint in the component ensures that the expression is always non-null when the binding expression is interpolated.

In the following example, the `person` and `address` properties are always set together, implying that `address` is always non-null if `person` is non-null.
There is no convenient way to describe this constraint to TypeScript and the template compiler, but the error is suppressed in the example by using `address!.street`.

```typescript
  @Component({
    selector: 'my-component',
    template: '<span *ngIf="person"> {{person.name}} lives on {{address!.street}} </span>'
  })
  class MyComponent {
    person?: Person;
    address?: Address;

    setData(person: Person, address: Address) {
      this.person = person;
      this.address = address;
    }
  }
```

The non-null assertion operator should be used sparingly as refactoring of the component might break this constraint.

In this example it is recommended to include the checking of `address` in the `*ngIf`as shown below:

```typescript
  @Component({
    selector: 'my-component',
    template: '<span *ngIf="person && address"> {{person.name}} lives on {{address.street}} </span>'
  })
  class MyComponent {
    person?: Person;
    address?: Address;

    setData(person: Person, address: Address) {
      this.person = person;
      this.address = address;
    }
  }
```

### Input setter coercion

Occasionally it is desirable for the `@Input` of a directive or component to alter the value bound to it, typically using a getter/setter pair for the input. As an example, consider this custom button component:

Consider the following directive:

```typescript
@Component({
  selector: 'submit-button',
  template: `
    <div class="wrapper">
      <button [disabled]="disabled">Submit</button>'
    </div>
  `,
})
class SubmitButton {
  private _disabled: boolean;

  get disabled(): boolean {
    return this._disabled;
  }

  set disabled(value: boolean) {
    this._disabled = value;
  }
}
```

Here, the `disabled` input of the component is being passed on to the `<button>` in the template. All of this works as expected, as long as a `boolean` value is bound to the input. But, suppose a consumer uses this input in the template as an attribute:

```html
<submit-button disabled></submit-button>
```

This has the same effect as the binding:

```html
<submit-button [disabled]="''"></submit-button>
```

At runtime, the input will be set to the empty string, which is not a `boolean` value. Angular component libraries that deal with this problem often "coerce" the value into the right type in the setter:

```typescript
set disabled(value: boolean) {
  this._disabled = (value === '') || value;
}
```

It would be ideal to change the type of `value` here, from `boolean` to `boolean|''`, to match the set of values which are actually accepted by the setter. Unfortunately, TypeScript requires that both the getter and setter have the same type, so if the getter should return a `boolean` then the setter is stuck with the narrower type.

If the consumer has Angular's strictest type checking for templates enabled, this creates a problem: the empty string `''` is not actually assignable to the `disabled` field, which will create a type error when the attribute form is used.

As a workaround for this problem, Angular supports checking a wider, more permissive type for `@Input`s than is declared for the input field itself. This is enabled by adding a static property with the `ngAcceptInputType_` prefix to the component class:

```typescript
class SubmitButton {
  private _disabled: boolean;

  get disabled(): boolean {
    return this._disabled;
  }

  set disabled(value: boolean) {
    this._disabled = (value === '') || value;
  }

  static ngAcceptInputType_disabled: boolean|'';
}
```

This field does not need to have a value. Its existence communicates to the Angular type checker that the `disabled` input should be considered as accepting bindings that match the type `boolean|''`. The suffix should be the `@Input` _field_ name.

Care should be taken that if an `ngAcceptInputType_` override is present for a given input, then the setter should be able to handle any values of the overridden type.

### Disabling type checking using `$any()`

Disable checking of a binding expression by surrounding the expression in a call to the [`$any()` cast pseudo-function](guide/template-syntax).
The compiler treats it as a cast to the `any` type just like in TypeScript when a `<any>` or `as any` cast is used.

In the following example, the error `Property addresss does not exist` is suppressed by casting `person` to the `any` type.

```typescript
  @Component({
    selector: 'my-component',
    template: '{{$any(person).addresss.street}}'
  })
  class MyComponent {
    person?: Person;
  }
```
