嘗試將屬性從一個 object 分配給另一個時出現 TypeScript 錯誤

Question

我的代碼：

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]))

state[k] = p[k]處的錯誤：

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; }'

看起來k有string類型。 我有一些嘗試：

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'.

怎么了？

Answer 1

因為您正在嘗試像這樣評估 object ：

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

至：

{
  activeRequests: number, 
  test: boolean,
}

因為 partial 和 stateKeys 部分。

解決方案很簡單：

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

Answer 2

如果沒有 generics，您將無法做到這一點。 另外，您正在嘗試將部分未定義的值分配給不允許未定義的類型。 所以你需要處理這種情況。 這是一個潛在的解決方案：

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];
    }
}