Make a property mandatory based on another property of the same object in typescript

Question

I have an use case

  interface MakeMandatory {
    name: string;
    age?: number; //
    isHiringPossible?: boolean
  }

Here, age and isHiringPossible is optional but if age contains value then isHiringPossible should also contain value and vice versa. Any thoughts?

Answer 1

You actually have two types. One where age and isHiringPossible are both missing, and one where both are required. And what you want is a type that is the union of those two types.

interface MakeMandatoryMinimal {
    name: string;
    age?: undefined;
    isHiringPossible?: undefined
}

interface MakeMandatoryComplete {
    name: string;
    age: number;
    isHiringPossible: boolean
}

type MakeMandatory = MakeMandatoryMinimal | MakeMandatoryComplete

Now you can test age which will prove that isHiringPossible exists, like so:

declare const data: MakeMandatory // pretend we have object of type MakeMandatory

data.isHiringPossible // type: boolean | undefined

if (data.age) {
    data.isHiringPossible // type: boolean
}

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