typescript conditional type on generic

Question

I have a type in a shape like this (simplified):

type AllContentType =
    | 'application/json'
    | 'application/octet-stream'
    | 'multipart/form-data';

type myType<T = AllContentType> = {
    body: T extends 'multipart/form-data' ? number : string;
    otherHeader: T;
};

and it cannot figure out which string has been put into otherHeader, it always returns


const obj: myType = { header: 'multipart/form-data', body: ""  };

body as string | number. Any way to get it to figure out what I actually have put in?

TSPlayground

daylily · Accepted Answer · 2021-10-16 12:16:19Z

1

By giving your type parameter a default value

type myType<T = AllContentType>

writing this

const obj: myType = { otherHeader: 'multipart/form-data', body: ""  };

is effectively

const obj: myType<AllContentType> = { otherHeader: 'multipart/form-data', body: ""  };

which makes the predicate T extends 'multipart/form-data' false.

If you leave out the default value for the type parameter and specify the type explicitly, it would work properly:

type myType<T extends AllContentType> = {
  body: T extends 'multipart/form-data' ? number : string;
  otherHeader: T;
};

const obj: myType<'multipart/form-data'> = { otherHeader: 'multipart/form-data', body: "" };

although that adds repetition.

answered Oct 16, 2021 at 12:16

daylily

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2 Comments

Twiggeh Over a year ago

And there is no way to get that repetition out, right?

daylily Over a year ago

@Twiggeh If you use myType<T> as a parameter of a function, then you can make type inference do the job. But if you're defining a property, variable, etc. you'll have to specify the type explicitly.

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