In TypeScript, callback return type inference is not performed for objects

When working with TypeScript, I've noticed that it usually infers the type correctly without me having to explicitly define it. For example:

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

const result = myFunction(() => 5); //The type of "result" is automatically inferred as a Number.

However, I encountered an issue when trying to use this inference inside an object. TypeScript requires me to explicitly specify the type:

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

const myObject: MyObject = { //An error occurs here and the compiler asks for a type definition.
    callback: ()=>5
}

Even though it's clear that "callback" is a function that returns a Number, TypeScript doesn't seem to recognize this. Is there a way to work around this limitation?

Answer №1

Currently, TypeScript does not support inferring type arguments in generic types the same way it does for generic functions. There is a feature request on GitHub that proposes adding a sigil to allow the compiler to infer a specific type argument. This means that in the future, you might be able to use syntax like

const myObject: MyObject<infer> = ...
or
const myObject: MyObject<*> = ...
, but this capability is not yet part of the language.

For now, if you want to achieve similar behavior, you can create a generic helper function like the following:

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

Instead of writing const myObject: MyObject = ..., you would use const myObject = asMyObject(...) to achieve the desired effect:

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

// const myObject: MyObject<number>

In this example, the compiler infers T to be

number</code when calling <code>asMyObject()
, resulting in myObject being of type MyObject<number>.

Try this code in the TypeScript Playground

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