AOT metadata errors

The following are metadata errors you may encounter, with explanations and suggested corrections.

Expression form not supported

Language features outside of the compiler's restricted expression syntax can produce this error, as seen in the following example:

      
// ERRORexport class Fooish { … }…const prop = typeof Fooish; // typeof is not valid in metadata  …  // bracket notation is not valid in metadata  { provide: 'token', useValue: { [prop]: 'value' } };  …
    

You can use typeof and bracket notation in normal application code. You just can't use those features within expressions that define Angular metadata.

Avoid this error by sticking to the compiler's restricted expression syntax when writing Angular metadata and be wary of new or unusual TypeScript features.

Reference to a local (non-exported) symbol

The compiler encountered a reference to a locally defined symbol that either wasn't exported or wasn't initialized.

Here's a provider example of the problem.

      
// ERRORlet foo: number; // neither exported nor initialized@Component({  selector: 'my-component',  template: … ,  providers: [    { provide: Foo, useValue: foo }  ]})export class MyComponent {}
    

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.

You could fix the problem by initializing foo.

      
let foo = 42; // initialized
    

The compiler will fold the expression into the provider as if you had written this.

      
providers: [  { provide: Foo, useValue: 42 }]
    

Alternatively, you can fix it by exporting foo with the expectation that foo will be assigned at runtime when you actually know its value.

      
// CORRECTEDexport let foo: number; // exported@Component({  selector: 'my-component',  template: … ,  providers: [    { provide: Foo, useValue: foo }  ]})export class MyComponent {}
    

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.

Adding export doesn't work when the compiler needs the actual value in order to generate code. For example, it doesn't work for the template property.

      
// ERRORexport let someTemplate: string; // exported but not initialized@Component({  selector: 'my-component',  template: someTemplate})export class MyComponent {}
    

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 and constants

The compiler found a reference to an exported variable or static field that wasn't initialized. It needs the value of that variable to generate code.

The following example tries to set the component's template property to the value of the exported someTemplate variable which is declared but unassigned.

      
// ERRORexport let someTemplate: string;@Component({  selector: 'my-component',  template: someTemplate})export class MyComponent {}
    

You'd also get this error if you imported someTemplate from some other module and neglected to initialize it there.

      
// ERROR - not initialized there eitherimport { someTemplate } from './config';@Component({  selector: 'my-component',  template: someTemplate})export class MyComponent {}
    

The compiler cannot wait until runtime to get the template information. It must statically derive the value of the someTemplate variable from the source code so that it can generate the component factory, which includes instructions for building the element based on the template.

To correct this error, provide the initial value of the variable in an initializer clause on the same line.

      
// CORRECTEDexport let someTemplate = '<h1>Greetings from Angular</h1>';@Component({  selector: 'my-component',  template: someTemplate})export class MyComponent {}
    

Reference to a non-exported class

Metadata referenced a class that wasn't exported.

For example, you may have defined a class and used it as an injection token in a providers array but neglected to export that class.

      
// ERRORabstract class MyStrategy { }  …  providers: [    { provide: MyStrategy, useValue: … }  ]  …
    

Angular generates a class factory in a separate module and that factory can only access exported classes. To correct this error, export the referenced class.

      
// CORRECTEDexport abstract class MyStrategy { }  …  providers: [    { provide: MyStrategy, useValue: … }  ]  …
    

Reference to a non-exported function

For example, you may have set a providers useFactory property to a locally defined function that you neglected to export.

      
// ERRORfunction myStrategy() { … }  …  providers: [    { provide: MyStrategy, useFactory: myStrategy }  ]  …
    

Angular generates a class factory in a separate module and that factory can only access exported functions. To correct this error, export the function.

      
// CORRECTEDexport function myStrategy() { … }  …  providers: [    { provide: MyStrategy, useFactory: myStrategy }  ]  …
    

Function calls are not supported

The compiler does not currently support function expressions or lambda functions. For example, you cannot set a provider's useFactory to an anonymous function or arrow function like this.

      
// ERROR  …  providers: [    { provide: MyStrategy, useFactory: function() { … } },    { provide: OtherStrategy, useFactory: () => { … } }  ]  …
    

You also get this error if you call a function or method in a provider's useValue.

      
// ERRORimport { calculateValue } from './utilities';  …  providers: [    { provide: SomeValue, useValue: calculateValue() }  ]  …
    

To correct this error, export a function from the module and refer to the function in a useFactory provider instead.

      
// CORRECTEDimport { calculateValue } from './utilities';export function myStrategy() { … }export function otherStrategy() { … }export function someValueFactory() {  return calculateValue();}  …  providers: [    { provide: MyStrategy, useFactory: myStrategy },    { provide: OtherStrategy, useFactory: otherStrategy },    { provide: SomeValue, useFactory: someValueFactory }  ]  …
    

Destructured variable or constant not supported

The compiler does not support references to variables assigned by destructuring.

For example, you cannot write something like this:

      
// ERRORimport { configuration } from './configuration';// destructured assignment to foo and barconst {foo, bar} = configuration;  …  providers: [    {provide: Foo, useValue: foo},    {provide: Bar, useValue: bar},  ]  …
    

To correct this error, refer to non-destructured values.

      
// CORRECTEDimport { configuration } from './configuration';  …  providers: [    {provide: Foo, useValue: configuration.foo},    {provide: Bar, useValue: configuration.bar},  ]  …
    

Could not resolve type

This can happen if you refer to an ambient type. For example, the Window type is an ambient type declared in the global .d.ts file.

You'll get an error if you reference it in the component constructor, which the compiler must statically analyze.

      
// ERROR@Component({ })export class MyComponent {  constructor (private win: Window) { … }}
    

TypeScript understands ambient types so you don't import them. The Angular compiler does not understand a type that you neglect to export or import.

In this case, the compiler doesn't understand how to inject something with the Window token.

Do not refer to ambient types in metadata expressions.

If you must inject an instance of an ambient type, you can finesse the problem in four steps:

1. Create an injection token for an instance of the ambient type.
2. Create a factory function that returns that instance.
3. Add a useFactory provider with that factory function.
4. Use @Inject to inject the instance.

Here's an illustrative example.

      
// CORRECTEDimport { Inject } from '@angular/core';export const WINDOW = new InjectionToken('Window');export function _window() { return window; }@Component({  …  providers: [    { provide: WINDOW, useFactory: _window }  ]})export class MyComponent {  constructor (@Inject(WINDOW) private win: Window) { … }}
    

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).

      
import { Inject }   from '@angular/core';import { DOCUMENT } from '@angular/common';@Component({ … })export class MyComponent {  constructor (@Inject(DOCUMENT) private doc: Document) { … }}
    

Name expected

This can happen if you use a number as a property name as in the following example.

      
// ERRORprovider: [{ provide: Foo, useValue: { 0: 'test' } }]
    

Change the name of the property to something non-numeric.

      
// CORRECTEDprovider: [{ provide: Foo, useValue: { '0': 'test' } }]
    

Unsupported enum member name

The compiler can understand simple enum values but not complex values such as those derived from computed properties.

      
// ERRORenum Colors {  Red = 1,  White,  Blue = "Blue".length // computed}  …  providers: [    { provide: BaseColor,   useValue: Colors.White } // ok    { provide: DangerColor, useValue: Colors.Red }   // ok    { provide: StrongColor, useValue: Colors.Blue }  // bad  ]  …
    

Avoid referring to enums with complicated initializers or computed properties.

Tagged template expressions are not supported

The compiler encountered a JavaScript ES2015 tagged template expression such as the following.

      
// ERRORconst expression = 'funky';const raw = String.raw`A tagged template ${expression} string`; … template: '<div>' + raw + '</div>' …
    

String.raw() is a tag function native to JavaScript ES2015.

The AOT compiler does not support tagged template expressions; avoid them in metadata expressions.

Symbol reference expected

This error can occur if you use an expression in the extends clause of a class.