Inferring a type with a generic

generic-inference1.ts

// the example from typescript's docs
// The type T is never explicitly specified and can be any type that supports keyof
function getProperty<T, K extends keyof T>(obj: T, key: K) {
    return obj[key];
}

let x = { a: 1, b: 2, c: 3, d: 4 };

getProperty(x, "a"); // okay
getProperty(x, "m"); // error: Argument of type 'm' isn't assignable to 'a' | 'b' | 'c' | 'd'.

// new example not in the typescript docs:
const y = { totally: 'different' }

getProperty(y, "a"); // error: Argument of type 'a' isn't assignable to 'totally'.
getProperty(y, "totally"); // okay