asked 3 months ago
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I know there are plenty of articles on this, but I still can't figure out when to use type vs interface in TypeScript in practice.
Specifically I'm confused about:
&)Can someone show concrete examples of when each is clearly the better choice?
The one concrete difference that matters in practice: declaration merging.
interface Window {
myCustomProp: string; // ✅ merges with the global Window interface
}
type Window = { // ❌ Error: duplicate identifier
myCustomProp: string;
};This is why you'll see interface used in .d.ts files and library augmentations. For application code, the difference rarely matters.
Here's my decision framework after 5 years of TypeScript:
Use interface when:
Use type when:
type Status = 'active' | 'inactive'type Nullable<T> = T | nulltype Point = [number, number]For the extending question: both work, but syntactically:
// interface
interface Animal { name: string }
interface Dog extends Animal { breed: string }
// type
type Animal = { name: string }
type Dog = Animal & { breed: string }They're functionally equivalent for object types. Pick one and be consistent.
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