Maintain object value types for each key when reassigning values

Question

const obj = {
    a: 1,
    b: 'foo',
};

for (const k of (Object.keys(obj) as (keyof typeof obj)[])) {
    obj[k] = obj[k];
}

TS Playground: https://www.typescriptlang.org/play?#code/MYewdgzgLgBCBGArGBeGBvAUDHMCGAXDAIwA02u8RA5AGYgjXkC+A3JpvQE4wAUokWAGs4tPgHkkAU2BQAdEKkBPCLwSIAlPgh9FSkGKhKADlINwkGgNoBdDVqy4LiK0Jupnrm+2ZA

I'm getting:

Type 'string | number' is not assignable to type 'never'.
  Type 'string' is not assignable to type 'never'.

obj has different value types for each key and I want to maintain the value type for each key. In other words, I want obj.a to remain number and obj.b to remain string. I'm assuming this error is occurring because TS expects the type of obj[k] to be string | number.

Is there a way to make TS maintain the correct value type for each key?

While not a strict duplicate (as far as I know) see this and this for explanation and solutions. Remember too that in order to pull out a type from a collection the type must be statically knowable, and obj could have properties of multiple types at runtime. — Jared Smith
– Jared Smith, Commented Jun 16, 2021 at 3:00
why don't use Object.assign. Like this: Object.assign(obj, {a:20}); Or Object.assign(obj, {a:20, b:"another foo"}); — Alireza Ahmadi
– Alireza Ahmadi, Commented Jun 16, 2021 at 8:09

edemaine · Accepted Answer · 2021-06-16 14:11:57Z

1

Here is one way, using generics to get the key as a type K instead of as a runtime value k:

const obj = {
    a: 1,
    b: 'foo',
};

function set<K extends keyof typeof obj>(k : K) {
    obj[k] = obj[k];
}

for (const k of (Object.keys(obj) as (keyof typeof obj)[])) {
    set(k);
}

Playground link

answered Jun 16, 2021 at 14:11

edemaine

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