How to define the union key of an object when clone itself

Question

Example:

interface A{
    a: string;
    b: number;
}

const a: A = {a: 'a', b: 1};
const b: A = {a: 'aa', b: 2};
Object.keys(a).forEach((key)=>{
    a[key as keyof A] = b[key as keyof A];
})

https://www.typescriptlang.org/play?#code/JYOwLgpgTgZghgYwgAgIIG8BQyfLgLmQGcwpQBzAbm1wCNCQBXAW1umoF9NMEB7EEnkKpkAXmToCyAORxpAGmT1kARg7U+AsEuFiJU2XMXKATOswB5WgCsICMADoA1hACeRABRwAlA5i8oAFFEAAsPDxdXb1EAPixcPABtSLwiZEjeGDQAXT1aZLdU9LdMnM5vIA