1. What is keyof operator?

keyof operator takes interface or type and returns literal or numerical union of keys, which can be used as a type.

type ExampleType = {
    name: string,
    age: number
}

type KeysOfExampleType = keyof ExampleType // "name" | "age"

type TypeWithSameKeysAsExample = {
    [key in keyof ExampleType]: string
}

const obj: {value: KeysOfExampleType}: {
    value: "something" // Type 'something' is not assignable to type 'keyof ExampleType``
}

const obj2: TypeWithSameKeysAsExample = {} // Type '{}' is missing the following properties from type 'TypeWithSameKeysAsExample': name, age