Indexed types and template literals - prefix every key

Question

Take type A. Create type B by prefixing every key in A with x using Typescript's latest Template Literal Types:

type A = {
  a: string;
  b: string;
};

// Create this automatically.
type Prefixed = {
  xa: string;
  xb: string;
};

How do I do so?

Answer 1

type B = { [T in keyof typeof A as `x${T}`]: A[T] }

Some helpers like Capitalize<T> exist as well.