Narrowing types with predicates

example.ts

const A_TYPE = "a";
const B_TYPE = "b";

type AProps = {
  type: typeof A_TYPE;
  foo: string;
  bar: number;
};

type BProps = {
  type: typeof B_TYPE;
  foo: number;
  frob: string;
};

type AllProps = AProps | BProps;

const fn = ({ foo, type }: AllProps) => {
  if (type === A_TYPE) {
    return foo;
  }
};

const isA = (props: AllProps): props is AProps => {
  return props.type === A_TYPE;
};

const fn2 = (props: AllProps) => {
  if (isA(props)) {
    return props.foo;
  }
};