How to iterate through an object in TypeScipt - A proper way

Question

NOT LOOKING FOR A SOLUTION HERE, JUST NEED TO KNOW IF THIS IS A BUG IN TYPESCRIPT. THAT IS ALL.

Is this a bug in TypeScript? Should I report this bug? To my understanding this should work, unless I am doing something wrong here,

interface Something {
    key1: string;
    key2: number;
    key3: boolean;
}

const someObject: Something = {
    key1: '123',
    key2: 123,
    key3: false
}

for (const key in someObject) {
    console.log(someObject[key]); // TS error
}

Note: I am not looking for a workaround, because this works for me. Just looking to know if I am doing something wrong or there is actually a bug in TS.

for (const [key, value] of Object.entries(someObject)) {
    console.log(key, value);
}

or

for (const key in values) {
    console.log(key, values[key as keyof DirectDepositDTO]);
}

I do understand that the type of key is string, but shouldn't it be keyof Something?

TypeScript Playground Link

P.S. The question is marked as a duplicate, but I am not asking for a solution here, I already know how to fix it. All I need to know if that is a bug in TypeScript. Because for (const key in someObject) { means that the key will be keyof Something. I should not need to know re-define the types.

Answer 1

You have several ways.

If you want to use for .. in loop, you should allow TS to use indexed properties:

interface Something {
    key1: string;
    key2: number;
    key3: boolean;
}

type Values<T> = T[keyof T]
type MakeIndexed<T> = T & { [prop: string]: Values<T> }
type SomethingIndexed = MakeIndexed<Something>

const someObject: SomethingIndexed = {
    key1: '123',
    key2: 123,
    key3: false
}

for (const key in someObject) {
    const x = someObject[key] // Values<Something>
    const isString = someObject.key1 // string

    console.log(someObject[key]);
}

You can also use more functional approach without extra utility types but with type casting:

    const someObject: Something = {
        key1: '123',
        key2: 123,
        key3: false
    }

    // FP way with more explicit types
    const keys2 = (Object.keys(someObject) as Array<keyof typeof someObject>).forEach(key => {
        const result = someObject[key]; // string | number | boolean
    })