QTypeScript · Interview preparation
What does a question mark mean on a TypeScript property?
An optional property may be absent, so reading it typically produces its declared type or undefined.
The idea to remember.
An optional property may be absent, so reading it typically produces its declared type or undefined. Decide separately whether a present null value is allowed in your API contract.
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 what does a question mark mean on a TypeScript property without looking at the code. Then modify the intermediate example, describe one trade-off and identify when the real-world pattern fits.
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