Assign value only when key is a valid property of a type in TypeScript

I have a type, e.g.:

type Output = {
    fieldA?: string;
    fieldB?: string;

And I have an object, e.g.:

const input = { fieldA: "A", fieldB: "B", other: "C" };

Now I want to map the value to another object of type Output, so I want to loop through the object keys and assign only when the key is a valid property of type Output, e.g.:

let output: Output = {};
Object.keys(input).forEach(k => {
    if (k is keyof Output) {
        output[k] = input[k];

How do you achieve the above?