Using generics in function typing

Question

I'd like to create an interface for a function using generics, such that the function takes in two arguments, and returns either the type of the first argument, or a union of the two. I had thought it was possible to do something like this:

declare interface CustomMerge {
  (x: Partial<T1>, y: Partial<T1 | T2>): T1 | T1 & T2;
}

But I get all sorts of errors about unexpected symbols here, so I suspect I'm quite a way off the mark with what I've done so far. Is this possible with typescript?

Igor Soloydenko · Accepted Answer · 2019-01-14 21:25:11Z

2

You must declare type variables before using them anywhere including function signatures. To do that, you have two options:

Declare on method level

declare interface CustomMerge {
   <T1, T2>(x: Partial<T1>, y: Partial<T1 | T2>): T1 | T1 & T2;
// ^^^^^^^^
}

Declare on interface level

declare interface CustomMerge<T1, T2> {
//                           ^^^^^^^^
  (x: Partial<T1>, y: Partial<T1 | T2>): T1 | T1 & T2;
}

answered Jan 14, 2019 at 21:25

Igor Soloydenko

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1 Comment

OliverRadini Over a year ago

This is great, and has solved my problem, thanks. This is all working, I'll accept the answer once I'm able to.

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