How to update an object with keys from another object in Typescript?

Question

i am newcomer to typescript and have a following problem

I have an object of type Man with set of keys

type Man = { name: string; surname: string; age: number }
type ManKey = keyof Man

const johnSmith: Man = { name: 'John', surname: 'Smith', age: 99 }

now i have an object with arbitrary properties, some of them can intersect with Man's keys, then we should use their values to update johnSmith values. For instance:

const newProps = { religion: 'hindu', age: 30 }

in this example age should be recognized as Man's property and 30 should be written to johnSmith variable. Something like this (such code of course doesn't work)

for (let key in newProps) 
  if (key instanceof ManKey)
    johnSmith[key] = newProps[key]

Thanks in advance

PS sandbox with example https://codesandbox.io/s/elated-ardinghelli-db6kv?file=/src/App.tsx:0-478

Aplet123 · Accepted Answer · 2020-12-25 12:18:41Z

1

Yuo're close, but you shoul duse key in johnSmith instead of key instanceof ManKey:

for (const key in newProps) {
    if (key in johnSmith) {
        johnSmith[key] = newProps[key];
    }
}

EDIT: It appears that typescript still suffers from the inability to infer for...in types, so for now you'll have to do the ugly method of casting the key:

for (let key in newProps) {
  if (key in johnSmith) {
    let castKey = key as (keyof typeof newProps & keyof typeof johnSmith);
    johnSmith[castKey] = newProps[castKey];
  }
}

edited Dec 25, 2020 at 12:18

answered Dec 25, 2020 at 12:04

Aplet123

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6 Comments

Dmitry Reutov Over a year ago

Unfortunately not so easy, for both sides of assignment i get ts errors. For a first one "Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Man'. No index signature with a parameter of type 'string' was found on type 'Man'

Dmitry Reutov Over a year ago

sandbox: codesandbox.io/s/elated-ardinghelli-db6kv?file=/src/…

Aplet123 Over a year ago

@DmitryReutov I've updated my answer with some working code.

Dmitry Reutov Over a year ago

Looks quite tricky since i'm not used to it. Thanks!

Aplet123 Over a year ago

No, that is something that typescript never aims to do (see non-goal #5). The typesystem should never influence the behavior of the running program (the keys in a type should not be able to influence a keyof call).

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