具有特定键和值的通用类型

Question

我正在研究接受两个泛型类型参数的函数。其中之一是Object 类型，另一个是扩展keyof 传递对象类型的字符串。我需要以某种方式确定提供的Object 必须包含第二个参数中提供的键，并且它的值也应该是指定的类型。

我尝试过这样的事情：

const someFn = <
  T extends {[key: string]: number | null},
  K extends keyof T
>(data: T[], dataField: K) => {

  // expecting data values to be (number | null)[] for further purposes
  const dataValues = data.map(datum => datum[dataField]);

}

然而，上面的代码只有在传递的对象中的所有键都是number | null时才有效

type WorkingType = { value: number | null }
const workingArray = [{value: 1}, {value: null}]

someFn<WorkingType, "value">(workingArray, "value") // this works just fine

但是如果对象包含不同类型的字段我得到错误：

type FailingType = { value: number | null, sthElse: string }
const workingArray = [{value: 1, sthElse: ""}, {value: null, sthElse}]

//. Property 'sthElse' is incompatible with index signature.
//   Type 'string' is not assignable to type 'number | null'
someFn<FailingType, "value">(workingArray, "value") 

所以我需要做的是以某种方式提供信息，即只有作为K 提供的密钥需要是number | null 我尝试了类似的方法：

const someFn<
  T extends { [key: T]: number | null },
  K extends keyof T
>(data: T[], dataKey: K) => {/* ... */}

但这似乎是 TS 不允许的，我得到了An index signature parameter type must be either 'string' or 'number'。有没有可能达到我的目标的方法？

Answer 1

我希望我明白你的意思，无论如何这对我有用：

    interface someFnI {
    <T,K extends keyof T>(data: T[], dataField: K);
}



const someFn = <
    T,
    K extends keyof T
>(data: T[], dataField: K) => {

    // expecting data values to be (number | null)[] for further purposes
const dataValues: T[K][] = data.map(datum => {
    return datum[dataField]});
    console.log(dataValues);

}

let myIdentity: someFnI = someFn;


type NotFailingType = { value: number | null, sthElse: string }
const workingArray = [
    { value: 1, sthElse: "" }, 
    { value: null, sthElse: null },
    //this will break as expected : uncomment below to check
    // { value: "notallowed", sthElse: null }
]

//. Property 'sthElse' is incompatible with index signature.
//   Type 'string' is not assignable to type 'number | null'
myIdentity<NotFailingType, "value">(workingArray, "value")



type WorkingType = { value: number | null }
const workingArray2 = [{ value: 1 }, { value: null }]

myIdentity<WorkingType, "value">(workingArray2, "value")