基于其他属性动态限制 TypeScript 中 object 属性的类型

Question

type MatchOperator = "==" | ">" | "<";

type Criteria<T, P extends keyof T> = {
    field: P,
    value: T[P],
    operator: MatchOperator,
}

interface User {
    name: string;
    age: number;
    id: number;
}

const adultCriteria: Criteria<User, "age"> = {
    field: "age",
    operator: ">",
    value: 18
}

Is there a better way to restrict the type of value based on field using Typescript as mentioned below?

const adultCriteria: Criteria<User> = {
    field: "age",
    operator: ">",
    value: 18
}

Answer 1

Yeah, it's possible:

type Criteria<T> = {
    [P in keyof T]: {
        field: P,
        value: T[P],
        operator: MatchOperator
    }
}[keyof T]

This way you get a union type composite of 3 possible types:

type OnePossibleCriteria = Criteria<User>

type OnePossibleCriteria = {
    field: "name";
    value: string;
    operator: MatchOperator;
} | {
    field: "age";
    value: number;
    operator: MatchOperator;
} | {
    field: "id";
    value: number;
    operator: MatchOperator;
}

And when you assign a solid value to it, it's narrowed down to one of them.

const adultCriteria: OnePossibleCriteria = {
    field: "age",
    value: 18,
    operator: ">"
}

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