What is the best way to use a generic to reference the value of an object in Typescript?

I've been working on creating a signature for this particular function:

export function objToArray(obj){
    let result = [];
    for(const prop of Object.keys(obj)){
        result.push(Object.assign({objProp: prop.toString()}, obj[prop]));
    }
    return result;
}

In the case of an object of type T that holds values of type U, my goal is to generate

Array<U & {objKey: string}>
. I'm struggling with how to achieve this using TypeScript.

Answer №1

If you want to transform an object into an array with keys and values, you can utilize indexed access types. In TypeScript, a type T consists of keys of type keyof T and values of type T[keyof T]. Here's how you can type your function:

export function convertObjToArray<T extends object>(
 obj: T): Array<T[keyof T] & { key: string }>{
    let result = [];
    // Object.keys() returns `string[]` but we will assert as `Array<keyof T>`
    for(const key of Object.keys(obj) as Array<keyof T>){
        result.push(Object.assign({key: key.toString()}, obj[key]));
    }
    return result;
}

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