Give an example of few metadata errors?

Below are some of the errors encountered in metadata,

  1. 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
  1. 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,
let username: string; // neither exported nor initialized
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
let username = 'John'; // initialized
  1. 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 },
  1. 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:},
{provide: Age, useValue: user.age},

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