2722 - Join Two Arrays By Id

The problem statement

Given two arrays arr1 and arr2, return a new array joinedArray. All the objects in each of the two inputs arrays will contain an id field that has an integer value. joinedArray is an array formed by merging arr1 and arr2 based on their id key. The length of joinedArray should be the length of unique values of id. The returned array should be sorted in ascending order based on the id key.

If a given id exists in one array but not the other, the single object with that id should be included in the result array without modification.

If two objects share an id, their properties should be merged into a single object:

If a key only exists in one object, that single key-value pair should be included in the object. If a key is included in both objects, the value in the object from arr2 should override the value from arr1.

Understanding the problem

This problem intends to really throw us off from the start. It gives us a JSONValue type which is just not fit for purpose to solve this problem.

The key takeaways from the problem statement are:

1. From the examples we can derive that the ids are in numerical order across arr1 and arr2.
2. If the id is in both arr1 and arr2, choose the value from arr2.
3. If the id is only in arr1 / arr2, just set that value.

Quite simple, but the instructions make it sound so much more complex and the example type is designed to throw you off.

Code

type JSONValue =
  | null
  | boolean
  | number
  | string
  | JSONValue[]
  | { [key: string]: JSONValue };

function join<T extends { id: number }>(arr1: T[], arr2: T[]): T[] {
  const res = {};
  for (const item of arr1) {
    res[item.id] = item;
  }
  for (const item of arr2) {
    if (res[item.id]) {
      res[item.id] = Object.assign(res[item.id], item);
    } else {
      res[item.id] = item;
    }
  }
  return Object.values(res);
}

Code Explanation

The approach:

1. Create a hashmap / object to store each id and its corresponding item.
2. Loop arr1 and store all items into this map.
3. Loop arr2
   1. If the item exists in our map, replace it.
   2. If the item doesnt exist, set it.
4. Return the hashmap's values as an array.

We can assume that the arrays are already sorted given the examples provided.

Key takeaways

1. Remember to use of to iterate object values. Use in to iterate object keys. Arrays are objects.
2. Read examples and understand any information they may leak.
3. Make it work -> make it better.
4. Object.assign is useful for blending two items of the same type. First param is target, second is source. Source will override target.