I'm trying to get the type of an property from within a generic type:

export type TestType = {
    key1: string,
    key2: number,
    key3: Array<string>
};

const test:TestType = {
    key1: 'hello',
    key2: 56,
    key3: ['hi']
};


function getProperty<T> (obj:T, property:keyof T): T[typeof property]{
    return obj[property];
}

const thisVariableShouldBeAString:string = getProperty<TestType>(test, 'key1');
//thisVariableShouldBeAString has type "string | number | string[]" which is not "string"

typescript playground

But it does not work! thisVariableShouldBeAString ends up with an union types of all the properties within TestType

Doing test['key1'] instead of getProperty<TestType>(test, 'key1') is not sufficient for my usecase.

Is it possible to properly type the getProperty function?

CodePudding user response：

You can add another type parameter to getProperty that extends keyof T:

export type TestType = {
    key1: string,
    key2: number,
    key3: Array<string>
};

const test: TestType = {
    key1: 'hello',
    key2: 56,
    key3: ['hi']
};


function getProperty<T, K extends keyof T>(obj: T, property: K): T[K] {
    return obj[property];
}

const thisVariableShouldBeAString = getProperty(test, 'key1');