Typescript I can't index in the object ,which has many maybe object type-CodePudding

interface type1 {
    ttis: string,
    time: string
}

interface type2 {
    time: string,
    c2: number
}

type nameType1 = 'ttis' | 'time';

type nameType2 = 'c2' | 'time';

function testFun (v: type1[] | type2[], yKey: nameType1 | nameType2) {
    for (const item of v) {
        console.log(item[yKey]) // Property 'ttis' does not exist on type 'type1 | type2'.
    }
}

The v array has many types of element that is an object, and I want to index these objects using a yKey, but the error always up show

CodePudding user response：

Create a generic constraint for the value type, and then use keyof to restrict the allowed keys:

interface Type1 {
    ttis: string,
    time: string
}

interface Type2 {
    time: string,
    c2: number
}

function testFun<T extends Type1 | Type2>(v: T[], yKey: keyof T) {
    for (const item of v) {
        console.log(item[yKey]);
    }
}

CodePudding user response：

I'm still thinking about this as whenever unions are involved in index signatures, it can be very tricky but one workaround (without an explicit type) is as follows:

function testFun(v: type1[] | type2[], yKey: string) {
    for (const item of v) {
        console.log(item[yKey as keyof typeof item]);
    }
}

This way it types to the correct expected object. It's honestly not a solution I prefer though since you can't provide an explicit, usable type ahead of time so I'll think on this some more but it does get you going right now.

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