Check interface type in TypeScript

Question

interface IData {
  firstName: string;
  lastName: string;
}

interface IDemo {
    Events: {
      GetItem: (callback: (data: IData) => void) => void;
    }
}

const item = {
  Events: {
    GetItem: //mock function
  }
}

if (item is of type IDemo)

In the above scenario, I have IDemo interface that takes in an object of Events, which in turn has an object called GetItem - a function.

I want to check if const item is of type IDemo. How can I achieve this?

https://stackoverflow.com/questions/13487765/how-instanceof-will-work-on-an-interface — Nestor, Aug 15 '18 at 04:19
Do you want a runtime type check or compile-time type check? — eddiewould, Aug 15 '18 at 04:19
Possible duplicate of [Interface type check with Typescript](https://stackoverflow.com/questions/14425568/interface-type-check-with-typescript) — Matt McCutchen, Aug 15 '18 at 04:20
Where did "item" originally come from? Ajax response? A non-typescript library? Or is it defined as an object-literal (in which case we _know_ it is of type IDemo) — eddiewould, Aug 15 '18 at 04:25
Maybe the answer here will help: https://stackoverflow.com/questions/48442746/check-if-object-correctly-implements-interface/48444302#48444302 — Titian Cernicova-Dragomir, Aug 15 '18 at 05:24
I have made some changes to the original question. Please have a look. — blankface, Aug 15 '18 at 08:22

score 3 · Answer 1 · answered Aug 15 '18 at 05:17

3

Since item comes from ajax response, you are looking for some kind of runtime typecheck. TS interfaces are compile time entities, so you need to create your own runtime checks using your own type guard.

Something like:

function isIDemo(item: any): item is IDemo {
    return typeof item.first === 'string' && typeof item.last === 'string';
}

answered Aug 15 '18 at 05:17

jh314

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how would i do it if it wasn't an ajax response? – blankface Aug 15 '18 at 05:28
I have made some changes to the original question. Please have a look. – blankface Aug 15 '18 at 08:22

Check interface type in TypeScript

1 Answers1