extract-typescript-fields.ts

interface FullOfFields {
  x: number;
  y: string;
  z: () => void;
}

type FullOfFieldsX = FullOfFields["x"]

interface Example extends Pick<FullOfFields, "x" | "y"> {
  foo: string
}

const example: Example = {
  x: 1,
  y: "y",
  foo: "bar"
}