Updating nested properties from an object with bracket notation in Typescript

Question

So say I've got this interface and object with nested properties:

interface Iobj {
  a: { a2:string };
  b: string;
}

const obj: Iobj = {
  a:{
    a2: "hello"
  }
  b: "world"
};

And I've got a strings that identify properties in obj:

const prop = "a.a2"
// or
const prop = "b"

I'm trying to update obj with bracket notation but these statements give me the error Type 'string' is not assignable to type 'never'.

obj[prop] = "newString";
obj[prop as keyof Iobj] = "newString";

Seems like obj[prop] isn't being recognized as valid. Something I'm doing wrong here?

obj["a.a2"] won't work even in vanilla JS; you would have to use something like lodash.get. — iz_
– iz_, Commented May 19, 2021 at 16:09
Valid prop (e.g. "b") works fine typescriptlang.org/play?#code/… — Aleksey L.
– Aleksey L., Commented May 19, 2021 at 16:14

omidh · Accepted Answer · 2021-05-19 16:09:08Z

1

The problem is that with obj['a.a2'] the Javascript expects obj to have been defined like this:

obj = {
  "a.a2": "hello"
}

However in you case a2 is child of a so first you need to access a then access a2, That's why in your case obj['a']['a2'] works instead. However if you still insist to use a.a2 then you can use Lodash library which understands that key.

https://lodash.com/docs/4.17.15#set

answered May 19, 2021 at 16:09

omidh

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