Type safe mapping of object properties

Question

I want to copy an object and map its properties in a type safe manner. I can describe the resulting type in TypeScript but I'm unable to achieve mapping the properties without casting.

So, consider I have an object that can be described like this:

interface MyObject {
    a: number;
    b: string;
}

I want to map all of its properties. In fact, I can describe the mapping with TypeScript correctly:

type MappedObject = {[K in keyof MyObject]: {meta: string; value: MyObject[K]}};

I have now extended all of the properties of MyObject. Fine. Now, the problem is, I can't apply that transformation in a type safe manner:

const object: MyObject = {
    a: 42,
    b: 'foo',
};

const mapped = Object.fromEntries(
    Object.entries(object).map(([k, v]) => [k, {meta: 'bar', value: v}]),
);
doSomethingWithMappedObject(mapped); // error

function doSomethingWithMappedObject(mapped: MappedObject): void {
    // ...
}

In my mind, the result of Object.fromEntries(...) should have been compatible with MappedObject but it isn't. How can we achieve this, ideally without casting?

Type safe mapping of object properties

0 Answers0