map function for objects (instead of arrays)

Question

I have an object:

myObject = { \'a\': 1, \'b\': 2, \'c\': 3 }

I am looking for a native method, similar to Array.prototype.map

Answer 1

Object Mapper in TypeScript

I like the examples that use Object.fromEntries such as this one, but still, they are not very easy to use. The answers that use Object.keys and then look up the key are actually doing multiple look-ups that may not be necessary.

I wished there was an Object.map function, but we can create our own and call it objectMap with the ability to modify both key and value:

Usage (JavaScript):

const myObject = { 'a': 1, 'b': 2, 'c': 3 };

// keep the key and modify the value
let obj = objectMap(myObject, val => val * 2);
// obj = { a: 2, b: 4, c: 6 }


// modify both key and value
obj = objectMap(myObject,
    val => val * 2 + '',
    key => (key + key).toUpperCase());
// obj = { AA: '2', BB: '4', CC: '6' }

Code (TypeScript):

interface Dictionary {
    [key: string]: T;
}

function objectMap(
    obj: Dictionary,
    valSelector: (val: TValue, obj: Dictionary) => TResult,
    keySelector?: (key: string, obj: Dictionary) => string,
    ctx?: Dictionary
) {
    const ret = {} as Dictionary;
    for (const key of Object.keys(obj)) {
        const retKey = keySelector
            ? keySelector.call(ctx || null, key, obj)
            : key;
        const retVal = valSelector.call(ctx || null, obj[key], obj);
        ret[retKey] = retVal;
    }
    return ret;
}

If you are not using TypeScript then copy the above code in TypeScript Playground to get the JavaScript code.

Also, the reason I put keySelector after valSelector in the parameter list, is because it is optional.

* Some credit go to alexander-mills' answer.