TypeScript errors while trying to assign properties from one object to another

Question

My code:

const state = { activeRequests: 0, test: true };
type state = typeof state;
type pState = Partial<state>;
type stateKeys = keyof state; // "activeRequests" | "test"
...
set: (state: state, p: pState) => Object.keys(p).forEach(k => (state[k] = p[k]))

Error at state[k] = p[k]:

Element implicitly has an 'any' type because expression of type 'string' can't be used to index type '{ activeRequests: number; test: boolean; }'.
No index signature with a parameter of type 'string' was found on type '{ activeRequests: number; test: boolean; }'

Looks like k has string type. There are some of my attempts:

Object.keys(p).forEach((k: stateKeys) => state[k].push(p[k]))
Error: Type 'string' is not assignable to type '"activeRequests" | "test"'`.

Object.keys(p).forEach(k => (state[k as stateKeys] = p[k])
Error: Type 'any' is not assignable to type 'never'`

(Object.keys(p) as stateKeys[]).forEach(k => (state[k] = p[k]))
Error: Type 'number | boolean | undefined' is not assignable to type 'never'.
  Type 'undefined' is not assignable to type 'never'.

What is wrong?

Tim · Accepted Answer · 2021-04-04 03:17:28Z

1

You can't do that cleanly without generics. Plus you are trying to assign a partially undefined value to a type that doesn't allow undefined. So you need to handle that scenario. Here's a potential solution:

var t = function<TState>(state: TState, p: Partial<TState>) {
    let keys = Object.keys(p) as (keyof TState)[];
    for (let key of keys) {
        state[key] = p[key] ?? state[key];
    }
}

answered Apr 4, 2021 at 3:17

Tim

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Comments

a2best · Accepted Answer · 2021-03-26 07:41:14Z

1

because you are trying to assing object like this:

{
  activeRequests: number | boolean | undefined, 
  test: number | boolean | undefined
}

to:

{
  activeRequests: number, 
  test: boolean,
}

because of the partial and stateKeys part.

Solution is simple like that:

const set = (state: state, p: pState) => ({...state, ...p})

edited Mar 26, 2021 at 7:41

answered Mar 26, 2021 at 7:23

a2best

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3 Comments

a2best Over a year ago

I added solution to main answer

SoundOfTheSky Over a year ago

Thank you for your example of solution, but this will create new object. I'd really like to mutate existing state. Is there no way to make this work my way?

a2best Over a year ago

You would need to add if statement working as type guards for typescript:

const set = (state: state, p: pState): void => {     Object.keys(p).forEach(k => {       if (k === 'activeRequests') {         const a = p[k];         if (a) {           state[k] = a;         }       }     }) }

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