why this array type deduction failing

Issue

I write a function: createMyForm , I hope that fields[].prop must be the property of initialValue, but this code can’t do it,

type FieldConfig<K> = {
  prop: K;
  value?: any;
}
function createMyForm<
    V extends Record<string, any>,
    K extends string = Exclude<keyof V, symbol | number>,
>(
    { initialValue, fields = [] }: {
    initialValue: V;
    fields?: FieldConfig<K | `${K}.${string}` | `${K}[${string | number}]`>[];
}) { return; }


const initVal = { a: 1, b: '2' };

// why x2 not show error 😡
createMyForm({ initialValue: initVal, fields: [{ prop: 'x2' } as const]})

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Solution

Your type parameter for K:

K extends string = Exclude<keyof V, symbol | number>

Means that K can be any string, but by default is a string key of V. I think probably you just want:

K extends Exclude<keyof V, symbol | number>

K should always be a string key of V. You don't need a default case for K because it can always be inferred from required property initialValue, even though fields is optional.

Answered By - lawrence-witt