Join Two Arrays by ID

 * @param {Array} arr1
 * @param {Array} arr2
 * @return {Array}
var join = function(arr1, arr2) {
  const combinedArray = arr1.concat(arr2);
  const merged = {};

  combinedArray.forEach((obj) => {
    const id =;
    if (!merged[id]) {
      merged[id] = { ...obj };
    } else {
      merged[id] = { ...merged[id], ...obj };

  const joinedArray = Object.values(merged);
  joinedArray.sort((a, b) => -;

  return joinedArray;

Problem Classification

Problem Domain: This problem is essentially a Data Manipulation problem in the domain of Computer Science, specifically involving arrays and dictionaries (or objects in general programming terminology).

‘What’ Components:

  1. Two arrays, arr1 and arr2, containing objects. Each object has at least one property, id, which is an integer.
  2. We need to merge the arrays into a new array, joinedArray, based on the id property of the objects.
  3. If an id exists in only one array, the corresponding object is included in joinedArray as is.
  4. If an id exists in both arrays, the corresponding objects are merged into a single object in joinedArray, with properties from the object in arr2 overriding those from the object in arr1 if there is a conflict.
  5. joinedArray should contain unique id values and be sorted in ascending order based on the id key.

Problem Classification: This is a Merge and Sort problem, with some aspects of Conflict Resolution (when an id is present in both input arrays). It involves Array Manipulation, Object Merging, Sorting, and Key-Value Pair Manipulation.

Explanation: This problem requires multiple coding concepts and data structures, including arrays, objects, key-value pair manipulation, sorting, and merging operations. It combines these tasks in a way that requires careful handling of the order of operations and conflict resolution strategy to ensure that the final result meets all specified conditions.

Language Agnostic Coding Drills

  1. Dissect the Code:

The code can be divided into several distinct concepts:

a. Array Concatenation: This concept involves merging two arrays into one, which is the first step in the provided code.

b. Looping over Arrays: The provided code then loops over the concatenated array, a basic operation that iterates through all elements in a collection.

c. Accessing Object Properties: Each iteration accesses a property (the ‘id’) of the current object, demonstrating how to read properties of objects.

d. Conditional Checking: The code checks if an object with a certain ‘id’ already exists in another collection (the ‘merged’ object), exemplifying conditional checking.

e. Object Spread Syntax: The code uses the object spread syntax (...) to clone objects and merge their properties. This allows us to combine or override properties from multiple objects.

f. Object to Array Transformation: The code then converts the ‘merged’ object back into an array. This involves using Object.values() to get an array of all values in an object.

g. Array Sorting: Finally, the code sorts the resulting array based on the ‘id’ property of each object.

  1. List the Coding Drills:

    a. Array Concatenation (Difficulty: 1/5): Join two arrays together. b. Looping over Arrays (Difficulty: 2/5): Iterate over all elements in an array. c. Accessing Object Properties (Difficulty: 2/5): Access and read property of an object. d. Conditional Checking (Difficulty: 2/5): Check the existence of a property in an object. e. Object Spread Syntax (Difficulty: 3/5): Create a clone or merge of two objects. f. Object to Array Transformation (Difficulty: 3/5): Convert an object to an array using Object.values(). g. Array Sorting (Difficulty: 4/5): Sort an array of objects based on a specific object property.

  2. Describe the Problem-solving Approach:

    a. Start by concatenating arr1 and arr2 to create a single combined array. This step prepares the data for further processing.

    b. Loop over the combined array. For each object, check if an object with the same ‘id’ already exists in the ‘merged’ object. This step groups objects by ‘id’.

    c. If an object with the same ‘id’ doesn’t exist in ‘merged’, create a new object with the same properties as the current object and add it to ‘merged’. If it does exist, merge the properties of the current object with the existing object in ‘merged’, with properties from the current object taking precedence. This step resolves conflicts and combines properties.

    d. Once all objects in the combined array have been processed, transform ‘merged’ back into an array by extracting all its values. This step brings the data back into the format specified by the problem statement.

    e. Sort the resulting array in ascending order of ‘id’. This step ensures the final array meets the specified order requirement.

    f. The sorted array is then the final result.

Targeted Drills in Python

  1. Python Drills for Each Concept:

    a. Array Concatenation:

    def concatenate_arrays(arr1, arr2):
        return arr1 + arr2

    b. Looping over Arrays:

    def loop_over_array(arr):
        for item in arr:

    c. Accessing Object Properties:

    def access_object_properties(obj):
        for key, value in obj.items():
            print(f"Key: {key}, Value: {value}")

    d. Conditional Checking:

    def check_key_in_object(key, obj):
        return key in obj

    e. Object Spread Syntax:

    def merge_objects(obj1, obj2):
        merged = {**obj1, **obj2}
        return merged

    f. Object to Array Transformation:

    def object_to_array(obj):
        return list(obj.values())

    g. Array Sorting:

    def sort_array(arr):
        return sorted(arr)
  2. Drills for Specific Needs of the Problem:

    a. Comparing Object Properties and Merging them:

    def merge_objects_based_on_property(obj1, obj2, property):
        if obj1[property] == obj2[property]:
            merged = {**obj1, **obj2}
            return merged
        return None

    b. Sorting Objects in Array based on Specific Property:

    def sort_objects_in_array(arr, property):
        return sorted(arr, key=lambda x: x[property])
  3. Merging these Drills for the Final Solution:

    a. Concatenate the two arrays arr1 and arr2 using the concatenate_arrays function.

    b. Create an empty dictionary merged.

    c. Loop over the combined array using the loop_over_array function. For each object in the array, check if an object with the same ‘id’ exists in merged using the check_key_in_object function.

    d. If an object with the same ‘id’ doesn’t exist in merged, add it to merged. If it does exist, merge the properties of the current object with the existing object in merged using the merge_objects function, ensuring properties from the current object take precedence.

    e. Once all objects in the combined array have been processed, transform merged into an array using the object_to_array function.

    f. Finally, sort the resulting array in ascending order of ‘id’ using the sort_objects_in_array function.

    g. The sorted array is then the final result.