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.
See the [CLI documentation](https://github.com/angular/angular-cli/wiki) for details, especially the [`build` topic](https://github.com/angular/angular-cli/wiki/build).
You can control your app compilation by providing template compiler options in the `tsconfig.json` file along with the options supplied to the TypeScript compiler. The template compiler options are specified as members of
This option tells the compiler not to produce `.metadata.json` files.
The option is `false` by default.
`.metadata.json` files contain infomration needed by the template compiler from a `.ts`
file that is not included in the `.d.ts` file produced by the TypeScript compiler. This information contains,
for example, the content of annotations (such as a component's template) which TypeScript
emits to the `.js` file but not to the `.d.ts` file.
This option should be set to `true` if using TypeScript's `--outFile` option, as the metadata files
are not valid for this style of TypeScript output. It is not recommeded to use `--outFile` with
Angular. Use a bundler, such as [webpack](https://webpack.js.org/), instead.
This option can also be set to `true` when using factory summaries as the factory summaries
include a copy of the information that is in the `.metadata.json` file.
### *strictMetadataEmit*
This option tells the template compiler to report an error to the `.metadata.json`
file if `"skipMetadataEmit"` is `false` . This option is `false` by default. This should only be used when `"skipMetadataEmit"` is `false` and `"skipTemplateCodeGen"` is `true`.
It is intended to validate the `.metadata.json` files emitted for bundling with an `npm` package. The validation is overly strict and can emit errors for metadata that would never produce an error when used by the template compiler. You can choose to suppress the error emitted by this option for an exported symbol by including `@dynamic` in the comment documenting the symbol.
It is valid for `.metadata.json` files to contain errors. The template compiler reports these errors
if the metadata is used to determine the contents of an annotation. The metadata
collector cannot predict the symbols that are designed to use in an annotation, so it will preemptively
include error nodes in the metadata for the exported symbols. The template compiler can then use the error
nodes to report an error if these symbols are used. If the client of a library intends to use a symbol in an annotation, the template compiler will not normally report
this until the client uses the symbol. This option allows detecting these errors during the build phase of
the library and is used, for example, in producing Angular libraries themselves.
### *skipTemplateCodegen*
This option tells the compiler to suppress emitting `.ngfactory.js` and `.ngstyle.js` files. When set,
this turns off most of the template compiler and disables reporting template diagnostics.
This option can be used to instruct the
template compiler to produce `.metadata.json` files for distribution with an `npm` package while
avoiding the production of `.ngfactory.js` and `.ngstyle.js` files that cannot be distributed to
`npm`.
### *strictInjectionParameters*
When set to `true`, this options tells the compiler to report an error for a parameter supplied
whose injection type cannot be determined. When this value option is not provided or is `false`, constructor parameters of classes marked with `@Injectable` whose type cannot be resolved will
produce a warning.
*Note*: It is recommended to change this option explicitly to `true` as this option will default to `true` in the future.
### *flatModuleOutFile*
When set to `true`, this option tells the template compiler to generate a flat module
index of the given file name and the corresponding flat module metadata. Use this option when creating
flat modules that are packaged similarly to `@angular/core` and `@angular/common`. When this option
is used, the `package.json` for the library should refer
to the generated flat module index instead of the library index file. With this
option only one `.metadata.json` file is produced that contains all the metadata necessary
for symbols exported from the library index. In the generated `.ngfactory.js` files, the flat
module index is used to import symbols that includes both the public API from the library index
as well as shrowded internal symbols.
By default the `.ts` file supplied in the `files` field is assumed to be library index.
If more than one `.ts` file is specified, `libraryIndex` is used to select the file to use.
If more than one `.ts` file is supplied without a `libraryIndex`, an error is produced. A flat module
index `.d.ts` and `.js` will be created with the given `flatModuleOutFile` name in the same
location as the library index `.d.ts` file. For example, if a library uses
`public_api.ts` file as the library index of the module, the `tsconfig.json``files` field
would be `["public_api.ts"]`. The `flatModuleOutFile` options could then be set to, for
example `"index.js"`, which produces `index.d.ts` and `index.metadata.json` files. The
library's `package.json`'s `module` field would be `"index.js"` and the `typings` field
would be `"index.d.ts"`.
### *flatModuleId*
This option specifies the preferred module id to use for importing a flat module.
References generated by the template compiler will use this module name when importing symbols
from the flat module.
This is only meaningful when `flatModuleOutFile` is also supplied. Otherwise the compiler ignores
this option.
### *generateCodeForLibraries*
This option tells the template compiler to generate factory files (`.ngfactory.js` and `.ngstyle.js`)
for `.d.ts` files with a corresponding `.metadata.json` file. This option defaults to
`true`. When this option is `false`, factory files are generated only for `.ts` files.
This option should be set to `false` when using factory summaries.
`decorators` | Leave the Decorators in-place. This makes compilation faster. TypeScript will emit calls to the __decorate helper. Use `--emitDecoratorMetadata` for runtime reflection. However, the resulting code will not properly tree-shake.
`static fields` | Replace decorators with a static field in the class. Allows advanced tree-shakers like [Closure Compiler](https://github.com/google/closure-compiler) to remove unused classes.
### *trace*
This tells the compiler to print extra information while compiling templates.
### *disableExpressionLowering*
The Angular template compiler transforms code that is used, or could be used, in an annotation
to allow it to be imported from template factory modules. See
[metadata rewriting](#metadata-rewriting) for more information.
Setting this option to `false` disables this rewriting, requiring the rewriting to be
done manually.
### *preserveWhitespaces*
This option tells the compiler whether to remove blank text nodes from compiled templates.
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.
It helps to think of the AOT compiler as having two phases: a code analysis phase in which it simply records a representation of the source; and a code generation phase in which the compiler's `StaticReflector` handles the interpretation as well as places restrictions on what it interprets.
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).
Spread in literal array | `['apples', 'flour', ...the_rest]`
Calls | `bake(ingredients)`
New | `new Oven()`
Property access | `pie.slice`
Array index | `ingredients[0]`
Identifier reference | `Component`
A template string | <code>`pie is ${multiplier} times better than cake`</code>
Literal string | `'pi'`
Literal number | `3.14153265`
Literal boolean | `true`
Literal null | `null`
Supported prefix operator | `!cake`
Supported Binary operator | `a + b`
Conditional operator | `a ? b : c`
Parentheses | `(a + b)`
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.
If you want `ngc` to report syntax errors immediately rather than produce a `.metadata.json` file with errors, set the `strictMetadataEmit` option in `tsconfig`.
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 _collector_ can represent a function call or object creation with `new` as long as the syntax is valid. The _collector_ only cares about proper syntax.
But beware. The compiler may later refuse to generate a call to a _particular_ function or creation of a _particular_ object.
The compiler only supports calls to a small set of functions and will use `new` for only a few designated classes. These functions and classes are in a table of [below](#supported-functions).
The _collector_ may be able to evaluate an expression during collection and record the result in the `.metadata.json` instead of the original expression.
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 _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.
The compiler understands all syntax forms that the _collector_ supports, but it may reject _syntactically_ correct metadata if the _semantics_ violate compiler rules.
Most importantly, the compiler only generates code to create instances of certain classes, support certain decorators, and call certain functions from the following lists.
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. The compiler converts the expression initializing one of these fields into an exported variable, which 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—it just needs to be able to generate a reference to the value.
The compiler does the rewriting during the emit of the `.js` file. This doesn't rewrite the `.d.ts` file, however, so TypeScript doesn't recognize it as being an export. Thus, it does not pollute the ES module's exported API.
The compiler generates the component factory, which includes the `useValue` provider code, in a separate module. _That_ factory module can't reach back to _this_ source module to access the local (non-exported) `foo` variable.
Adding `export` often works for variables referenced in metadata such as `providers` and `animations` because the compiler can generate _references_ to the exported variables in these expressions. It doesn't need the _values_ of those variables.
The compiler needs the value of the `template` property _right now_ to generate the component factory.
The variable reference alone is insufficient.
Prefixing the declaration with `export` merely produces a new error, "[`Only initialized variables and constants can be referenced`](#only-initialized-variables)".
_Referencing an exported destructured variable or constant is not supported by the template compiler. Consider simplifying this to avoid destructuring._
The compiler does not support references to variables assigned by [destructuring](https://www.typescriptlang.org/docs/handbook/variable-declarations.html#destructuring).
The `Window` type in the constructor is no longer a problem for the compiler because it
uses the `@Inject(WINDOW)` to generate the injection code.
Angular does something similar with the `DOCUMENT` token so you can inject the browser's `document` object (or an abstraction of it, depending upon the platform in which the application runs).
Avoid referring to enums with complicated initializers or computed properties.
<hr>
{@a tagged-template-expressions-not-supported}
<h3class="no-toc">Tagged template expressions are not supported</h3>
<divclass="alert is-helpful">
_Tagged template expressions are not supported in metadata._
</div>
The compiler encountered a JavaScript ES2015 [tagged template expression](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals#Tagged_template_literals) such as,
```
// ERROR
const expression = 'funky';
const raw = String.raw`A tagged template ${expression} string`;
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.
