[Typescript] “keyof”, Generics and Lookup Types

const person = {
    name: "wan",
    age: 28
}

type Person = typeof person 
type PersonKeys = keyof person // "name" | "age"
type PersonKTypes = Person[PersonKeys] // string | number

 

"keyof" results in a unit type.

"extends" means a subtype of certain type.

It is common to combine those keywords together:

K extends keyof T

 

We can use "keyof" (a unit type) to build a lookup:

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

const personName = getProperty(person, 'name')

 

'name' extends keyof Person

'name' extends "name" | "age" -> true statement