assign.ts

type Defined<T extends object> = { [K in keyof T]: Exclude<T[K], undefined> };

type Assign<T1, T2> = {
  [K in keyof T1 | keyof T2]: K extends keyof T2
    ? T2[K] extends undefined
      ? K extends keyof T1
        ? T1[K]
        : never
      : T2[K]
    : K extends keyof T1
    ? T1[K]
    : never
};

/**
 * Assign values of obj2 to shallow copy of obj1 if they're not undefined
 */
function assign<T1 extends Record<string, any>, T2 extends Record<string, any>>(
  obj1: T1,
  obj2: T2
): Assign<T1, T2> {
  const result = { ...obj1 };
  for (const [key, val] of Object.entries(obj2)) {
    if (val !== undefined) {
      result[key as keyof T1] = val;
    }
  }
  return result as Assign<T1, T2>;
}