Typescript type mapping from Interface

Question

I'm trying to figure correct way to map Interface record value types to correct function type.

function stringCompose(): string {
    return ''
}
function numberCompose(): number {
    return 0
}

interface Demo {
    stringVal: string;
    numberVal: number;
}
// mapping type something like <T = any> = (() => T)
type ComposeMapper<T = any> = any;

const builder: ComposeMapper<Demo> = ({
    stringVal: stringCompose,
    numberVal: numberCompose,
});

So idea is that when creating builder it check that all Interface keys are in place and someway also do value mappings like Interface "string" => requires "() => string" which compose functions should be filling. Before I did similar setup, but there was tons of never checks and performance was really bad to solve those so I think there should be much easier way actually to do this.

Nishant · Accepted Answer · 2021-02-12 08:38:07Z

1

It seems like you need to create mapped type from the interface.

TS Playground link

function stringCompose(): string {
    return ''
}
function numberCompose(): number {
    return 0
}

interface Demo {
    stringVal: string;
    numberVal: number;
}

type ComposeMapper<T> = {
    [K in keyof T]: () =>  T[K]
}

// OK
const builder: ComposeMapper<Demo> = ({
    stringVal: stringCompose,
    numberVal: numberCompose,
});

// Error
const builder1: ComposeMapper<Demo> = ({
//    ~~~~~~~~ Property 'numberVal' is missing in type '{ stringVal: () => string; }' but required in type 'ComposeMapper<Demo>'.(2741)
    stringVal: stringCompose,
});


// Error
const builder2: ComposeMapper<Demo> = ({
    stringVal: numberCompose,
//  ~~~~~~~~~ Type '() => number' is not assignable to type '() => string'.
    numberVal: numberCompose,
});

answered Feb 12, 2021 at 8:38

Nishant

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