typescript: get an key map from object with generic

Question

I want to get the key map from any object,

I've implemented the function,

but I can't make typescript happy,

what should I do without using acc: any

const origin = {
  a: 1,
  b: 'string',
  c: () => {}
};

function getObjKeyMap<T>(obj: T) {
  return Object.keys(obj).reduce((acc, k) => {
 /* Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{}'.
  No index signature with a parameter of type 'string' was found on type '{}'.ts(7053) */
    acc[k] = k;
    return acc;
  }, {});
}

const keyMap = getObjKeyMap(origin);

Nishant · Accepted Answer · 2021-01-26 09:31:15Z

2

You need to tell the type of object that reduce uses for accumulator.

const originObj = {
  a: 1,
  b: 'string',
  c: () => {}
};

function getObjKeyMap<T>(obj: T) {
  return Object.keys(obj).reduce((acc, k) => {
    acc[k] = k;
    return acc;
  }, {} as Record<string, string>);
}

// or


function getObjKeyMap1<T>(obj: T) {
  return Object.keys(obj).reduce((acc: Record<string, string>, k: string) => {
    acc[k] = k;
    return acc;
  }, {});
}

const keyMap = getObjKeyMap(originObj);

TS Playground

However, I bet this is not the best solution. I bet there is a way to write a function declaration like this:

function getObjKeyMap<T, K extends keyof T>(obj: T): Record<K, K>

Where Typescript automatically tells you that getObjectMap({a: 1, b: 'string', c: () => {}} returns {a: "a", b: "b", c: "c"}.

I think this is much better:

const originObj = {
  a: 1,
  b: 'string',
  c: () => {}
};

type KeyMap<T> = {
  [key in keyof T]: key
};

function getObjKeyMap<T>(obj: T): KeyMap<T> {
  return Object.keys(obj).reduce((acc, k) => ({...acc, k: k}), {} as KeyMap<T>);
}


const keyMap = getObjKeyMap<typeof originObj>(originObj);

TS Playground

edited Jan 26, 2021 at 9:31

answered Jan 26, 2021 at 8:29

Nishant

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4 Comments

Kobe Over a year ago

Rather than using {} as T can't you just specify what the accumulator is inside of reduce's type arguments?

Nishant Over a year ago

Yes, if you see the first snippet, getObjKeyMap1 does that. In the second, I though the OP would understand that we can avoid as if we specify it in the parameter. I guess your form looks much neater. I'd rather edit the answer to do that. Thanks.

Kobe Over a year ago

Sorry, I meant something like .reduce<T>(...). I believe this is the intended way? Either way, it will still work :) I was just asking a question that I wasn't sure of

Nishant Over a year ago

Ah, where was my mind. :) Though, for some reason, it does not work that easily, see here: tsplay.dev/DWKA8N I tried casting Object.keys(obj) as [keyof T], no avail.

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