尝试将属性从一个 object 分配给另一个时出现 TypeScript 错误

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?

Answer 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})

Answer 2

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];
    }
}