QTypeScript · Interview preparation
When is the never type useful?
never represents a value that cannot occur, such as a function that always throws or an impossible branch after exhaustive narrowing.
The idea to remember.
never represents a value that cannot occur, such as a function that always throws or an impossible branch after exhaustive narrowing. Use it to make missing variants visible during compilation.
01Learn by doing
From first example to real project.
Start with the smallest working idea, examine a more detailed example, then look at a real application pattern. Adapt dependencies, error handling and data models to your project.
Basic: choose a union
01 / BEGINNERA value may be one of a fixed set of types.
type Status = 'idle' | 'loading' | 'success' | 'error';
let status: Status = 'idle';Intermediate: narrow unknown input
02 / INTERMEDIATEDo not assume an API response already satisfies your type.
function isUser(value: unknown): value is { id: number; name: string } {
if (typeof value !== 'object' || value === null) return false;
const item = value as Record<string, unknown>;
return typeof item.id === 'number' && typeof item.name === 'string';
}Real scenario: exhaustive state rendering
03 / REAL SCENARIODiscriminated unions model loading, success, and failure explicitly.
type Result =
| { state: 'loading' }
| { state: 'success'; data: string[] }
| { state: 'error'; message: string };
function label(result: Result) {
switch (result.state) {
case 'loading': return 'Loading';
case 'success': return result.data.join(', ');
case 'error': return result.message;
}
}02Check your understanding
Try it in your own words.
Explain when is the never type useful without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
Keep learning here.
Explore more in-depth guides, exercises and related interview questions in this library.
Browse TypeScript study guides ↗