Define function, which transforms an object, keeping keys

[Strate]

I need to define a function, which accepts an object of this type:

interface Source<A> {
    [index: string]: A
}

and transforms that object, keeping the keys, but replaces a values:

interface Target<B> {
    [index: string]: B
}

and I also want to keep typechecking for that case. This is JS example:

function transform(source) {
    var result = {}
    Object.keys(source).forEach((key) => {
        result[key] = source[key] + "prefix"
    })
}

var target = transform({
    "key1": 1,
    "key2": 2,
})

// now target has a {"key1": "1prefix", "key2": "2prefix"}

var three = target.key3 // I want to get type error here on compile-time

Any way to do that?

[RyanCavanaugh]

There isn't a way to represent this in the type system today.

[Strate]

Any plans to future? Maybe some existing issue?

[saschanaz]

This may be close but the problem here is the value types of key1 and key2 is not string|number but number.

interface SourceOrTarget<T> {
    [index: string]: T
}

function transform<T extends SourceOrTarget<string|number>>(source: T) {
    var result = {} as T;
    Object.keys(source).forEach((key) => {
        result[key] = source[key] + "prefix"
    })
    return result;
}

var target = transform({
    "key1": 1,
    "key2": 2,
})

// now target has a {"key1": "1prefix", "key2": "2prefix"}

var three = target.key3 // error here

[Strate]

This is not about string|number, this is more about keeping object shape replacing types of values. The real-world case is trannsforming object of plain strings to object of complex objects, i.e.:

let out = defineMessages("idPrefix.", {
    id1: "Some message"
})

out === {
    id1: {
        id: "idPrefix.id1",
        defaultMessage: "Some message"
    }
}

This function transforms {[index: string]: string} to {[index: string]: { id: string, defaultMessage: string }}. I want to keep keys in this case.

I don't know how this can be achieved :)

[myitcv]

@Strate see #4967 (comment) for a related question