How can i overwrite a property type?

Question

Given that i have following structure

type AB = {
    A: string;
    B: string;
}

but now i want to create a new type which extends keys but also overwrite the 'A' property. So final type should be

{
    A: number;
    B: string;
}

So I tried to assign AB type to my final type and append the 'A' property with new type

type final = AB & {A: number};

But it doesn't work :( So my question is; how to extend type to other with changed property type?

Possible duplicate of [Overriding interface property type defined in Typescript d.ts file](https://stackoverflow.com/questions/41285211/overriding-interface-property-type-defined-in-typescript-d-ts-file) — Diodeus - James MacFarlane, Feb 19 '19 at 16:06
When you say "it doesn't work," what error message are you receiving? — Shaun Luttin, Feb 19 '19 at 16:34

score 2 · Accepted Answer · answered Feb 19 '19 at 19:56

I'm not sure if this is the most elegant solution, but you could use a mapped type to filter out keys that you want to override and then intersect it with override:

type Override<T1, T2> = {
    [K in Exclude<keyof T1, keyof T2>]: T1[K]
} & {
    [K in keyof T2]: T2[K]
};

type final = Override<AB, { A: number }>;

score 1 · Answer 2 · answered Feb 19 '19 at 22:43

Creating a simple intersection (AB & {A: number}) will not work, because it will make all individual properties intersect separately. It means A would need to be of type string & number, which is impossible do achieve.

Use this instead:

type Overwrite<T, U> = Pick<T, Exclude<keyof T, keyof U>> & U;

Usage:

type Final = Overwrite<AB, { A: number }>

score 0 · Answer 3 · answered May 25 '20 at 19:35

0

type Override<T1, T2> = Omit<T1, keyof T2> & T2;

answered May 25 '20 at 19:35

Viacheslav Borodulin

259
3
5

1

Hope It will solve issue but please add explanation of your code with it so user will get perfect understanding which he/she really wants. – Jaimil Patel May 26 '20 at 05:13
explanation: https://dev.to/vborodulin/ts-how-to-override-properties-with-type-intersection-554l – Viacheslav Borodulin Feb 09 '21 at 19:56

How can i overwrite a property type?

3 Answers3

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