TypeScript: Function as object key

Question

For sane reasons, I need to have an object that has functions as key in an object, e.g:

function a() {}
function b() {}

const obj = {
  [a]: b
}

The reason for this is that I want to map the values of function a to function b and be able to remember and remove mappings again.

Now I wonder how to write typings for this in TypeScript. If I do

type MapFunctions = { [key: Function]: Function };

I will get the error

An index signature parameter type must be 'string' or 'number'.ts(1023)

But how would I write a type for this?

Those reasons are not sane and what you're trying is impossible. — Roberto Zvjerković
– Roberto Zvjerković, Commented Jul 16, 2019 at 9:47
Agreeing with @ritaj, if you look into what index type you have for your property [a], it's being converted to a string and doesn't remain a Function: console.log(typeof Object.keys(obj)[0] === 'string');. Your approach does not work in Javascript, and giving it a type declaration does not change that. — fjc
– fjc, Commented Jul 16, 2019 at 9:57
@ritaj first of all it is definitely possible, try this in your browser: const a = () => {}; const obj = {[a]: "val"}; console.log(obj[a]); second of all, can you elaborate on why you believe this is not sane? — Lukas
– Lukas, Commented Jul 16, 2019 at 10:24
@fjc thank you for your insight, I did not know the functions will bne stringified! — Lukas
– Lukas, Commented Jul 16, 2019 at 10:25
that is typescript question... the question is how to typescript follow the signature of a & b functions when they updated — pery mimon
– pery mimon, Commented Jul 22, 2020 at 8:07

Asaf Aviv · Accepted Answer · 2019-07-16 10:33:29Z

5

An object can't take a function reference as the key, but you can use a Map which can take a function as a key

function a() {}
function b() {}

const map = new Map<() => void, () => void>();

map.set(a, b);

map.get(a)();

answered Jul 16, 2019 at 10:08

Asaf Aviv

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1 Comment

that is typescript question... the question is how to typescript follow the signature of a & b functions when they updated