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The `useFactory` key tells Angular:

> "Create the dependency using my function (factory), not just through a class or a value."

---

### Example:

```ts
providers: [
  {
    provide: 'API_URL',
    useFactory: () => {
      return window.location.hostname === 'localhost'
        ? 'http://localhost:3000'
        : 'https://api.example.com';
    }
  }
]
```

Here Angular calls this function at startup and supplies its result (`API_URL`) as the dependency.

---

### When this is needed:

- when the value needs to be **computed dynamically**;
- when it depends on conditions (environment, settings, a token, and so on);
- when the factory needs to use other dependencies internally.

---

### Example with a dependency:

```ts
providers: [
  {
    provide: ConfigService,
    useFactory: (apiUrl: string) => new ConfigService(apiUrl),
    deps: ['API_URL'] // dependencies for the factory
  }
]
```

---

**Summary:**
`useFactory` is a way to tell Angular: *"get the dependency by calling my function, which decides on its own what to return."*

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