Partial Type Predicate in TypeScript

partialTypePredicate.ts

interface Base {
  base1: string;
  base2: string;
  a: string;
}

interface Interface1 extends Base {
  b: number;
}

interface Interface2 extends Base {
  c: boolean;
}

type UnionInterface = Interface1 | Interface2;

/**
 * When the input is a partial object, the type predicate must also be a partial check.
 */
const isI1 = (i: Omit<UnionInterface, 'base1'>): i is Omit<Interface1, 'base1'> => (i as Interface1).b != null;

function fn({base1, ...params}: UnionInterface): void {
  if (isI1(params)) {
    console.log(params.b);
  } else {
    console.log(params.c);
  }
}