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.
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:
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`.
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.
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).
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).
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).
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`.
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.
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.
The collector can evaluate references to module-local `const` declarations and initialized `var` and `let` declarations, effectively removing them from the `.metadata.json` file.
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`.
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.
The compiler understands all syntax forms that the collector supports, but it may reject _syntactically_ correct metadata if the _semantics_ violate compiler rules.
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.
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.
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")
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 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.
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)).
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 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:
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:
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.
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`.
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: `
<divclass="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-buttondisabled></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.
