How to create a generic type for an arrow function in Typescript

Question

I try to write typescript functions in the style that is most close to functional. For simple functions I can write:

type A = (value: number) => string;
const a: A = value => value.toString();

But what can I do with generic types? How can I type in that simple way following function?

function a<T>(value: T): T {
  return value;
}

If I try to simply add a generic type, it gives nothing:

type A = <T>(value: T) => T;
const a: A = value => value; // `value` implicitly has an `any` type

Is there any way to do it?

Nitzan Tomer · Accepted Answer · 2017-01-26 14:19:36Z

10

In your last snippet:

type A = <T>(value: T) => T;
const a: A = value => value;

You tell the compiler that a is of type A, but you don't bind it to a specific generic type which is why it uses any.

For example, you can set the generic type like so:

const a: A = (value: string) => value;

You can also do this:

type A<T> = (value: T) => T;
const a: A<string> = value => value;

If you want a to be specific.

If you want a to stay generic you'll need to declare the generic constraint on it as well:

const a: A = <T>(value: T) => value;

answered Jan 26, 2017 at 14:19

Nitzan Tomer

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5 Comments

Lodin Over a year ago

So, the clear callback that is not specific without parenthesis is not allowed, right?

Nitzan Tomer Over a year ago

It is allowed, you're not getting any errors right?

Lodin Over a year ago

Well, except the value becames any, and it is not allowed by my tsconfig.json. Anyway, thank you, the answer helped me.

smac89 Over a year ago

What version of typescript does this work with? I am on the typescript playground and your solution still shows errors. I have tried down to version 3.3, and they all show errors...

Nitzan Tomer Over a year ago

@smac89 yeah, getting errors as well. based on the date of the answer i'd say that it was using version 2.2

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