TypeScript does not infer callback return type in an object-CodePudding

TypeScript infers type correctly in this case, without me explicitly giving a type.

function myFunction<T>(callback: () => T) {
     return callback();
}

const result = myFunction(() => 5); //TypeScript knows that "result" type is a Number.

But it's not working inside an object, TypeScript forces me to explicitly give a type.

type MyObject<T> = {
    callback: ()=>T
}

const myObject: MyObject = { //Error happens, the compiler ask for a type here.
    callback: ()=>5
}

But compiler should know that "callback" is a function that return a Number. Is there a way to fix this?

CodePudding user response：

TypeScript doesn't currently have the concept of inferring type arguments in generic types the way it does when you call generic functions. There is a feature request at microsoft/TypeScript#26242 which, if implemented, would allow the use of some sigil to tell the compiler to infer a particular type argument. So maybe, some day in the future, you'd be able to write const myObject: MyObject<infer> = ... or const myObject: MyObject<*> = ... or even const myObject: MyObject = ... and get the behavior you're looking for. But right now it's not part of the language.

The only way to get such behavior now is to use a generic helper identity function that returns its input:

const asMyObject = <T,>(m: MyObject<T>) => m;

So instead of const myObject: MyObject = ... you would write const myObject = asMyObject(...) and get the same effect:

const myObject = asMyObject({
    callback: () => 5
});

// const myObject: MyObject<number>

Here the compiler has inferred T to be number in the call to asMyObject(), and so the value returned from asMyObject() is of type MyObject<number>, and therefore so is the myObject variable.

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