Give an example of few metadata errors?

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},
  ]

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