Group Objects by Key

The main idea is to iterate over the array and place elements into an object, using the key’s value for grouping.

1. Initialization: Create an empty object to store the result (result = {}).
2. Iteration: Loop through each object (item) in the source array using a loop like forEach.
3. Get Key: For each object, get the value of the key specified in the key argument (groupKey = item[key]).
4. Grouping:
   • Check if a property with the name groupKey exists in result (if (!result[groupKey])).
   • If it doesn’t, create it and assign it an empty array: result[groupKey] = [].
   • Add the current item to the array corresponding to that key: result[groupKey].push(item).
5. Completion: After iterating through all elements, return result.

This approach can be elegantly implemented using the Array.prototype.reduce method.

const groupBy = (arr, key) => {
  // Use reduce to transform the array into an object.
  // accumulator (acc) is the object we will build.
  // current (obj) is the current element of the array.
  return arr.reduce((acc, obj) => {
    // Get the key value for the current object.
    const groupKey = obj[key];
 
    // If the accumulator doesn't have an array for this key yet, create one.
    if (!acc[groupKey]) {
      acc[groupKey] = [];
    }
 
    // Add the current object to the array corresponding to its key.
    acc[groupKey].push(obj);
 
    // Return the updated accumulator for the next iteration.
    return acc;
  }, {}); // The initial value for the accumulator is an empty object.
};

Solution Analysis:

• Time Complexity: O(n), where n is the number of elements in the array. We iterate through the array once.
• Space Complexity: O(n), because in the worst case (if all keys are unique), we will create a new object containing all the elements of the original array.