Below are some of the errors encountered in metadata,
Expression form not supported: Some of the language features outside of the compiler's restricted expression syntax used in angular metadata can produce this error. Let's see some of these examples,
1. export class User { ... }
const prop = typeof User; // typeof is not valid in metadata
2. { provide: 'token', useValue: { [prop]: 'value' } }; // bracket notation is not valid in metadata
Reference to a local (non-exported) symbol: The compiler encountered a referenced to a locally defined symbol that either wasn't exported or wasn't initialized. Let's take example of this error,
// ERROR
let username: string; // neither exported nor initialized
@Component({
selector: 'my-component',
template: ... ,
providers: [
{ provide: User, useValue: username }
]
})
export class MyComponent {}
You can fix this by either exporting or initializing the value,
export let username: string; // exported
(or)
let username = 'John'; // initialized
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 as below.
providers: [
{ provide: MyStrategy, useFactory: function() { ... } },
{ provide: OtherStrategy, useFactory: () => { ... } }
]
You can fix this with exported function
export function myStrategy() { ... }
export function otherStrategy() { ... }
... // metadata
providers: [
{ provide: MyStrategy, useFactory: myStrategy },
{ provide: OtherStrategy, useFactory: otherStrategy },
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:
import { user } from './user';
// destructured assignment to name and age
const {name, age} = user;
... //metadata
providers: [
{provide: Name, useValue: name},
{provide: Age, useValue: age},
]
You can fix this by non-destructured values
import { user } from './user';
... //metadata
providers: [
{provide: Name, useValue: user.name},
{provide: Age, useValue: user.age},
]
