Count by Condition

Medium Important

Prompt

We have a nested array that can contain any type of items including other nested arrays. We want to create a JavaScript function that counts the number of items in this array that pass a certain condition. This condition will be given to you as a JavaScript callback function that takes an item as input and returns a boolean value.

Here's an example for clarity: If we have the array [1, 'a', [2, 'b', [3, 'c', [4]]], 'd', [5, 'e']] and the callback function is (item) => typeof item === 'string', the function should return 4, because there are four strings in the array 'a', 'b', 'c', and 'e'.

Playground

Hint

Use recursion to handle the nested arrays. When you come across an item that is also an array, you can call the same function on this nested array.

Hint

Use Array.isArray() to check if an item is an array. This will help you decide if you need to apply the callback function or make a recursive call.

Solution

Explanation

Let's say we have an array like this: [1, 'a', [2, 'b', [3, 'c', [4]]], 'd', [5, 'e']]. If the callback function checks for strings, the expected output would be 4 because there are four strings in the array: 'a', 'b', 'c', and 'e'.

Here's how we can approach this problem:

  • Define the Function: First, we define a function called countNestedArray that takes two parameters: arr (the nested array) and callback (the function that defines the condition).
  • Input Validation: Before doing any processing, we need to make sure that the inputs are valid. We check if arr is an array and if callback is a function. If either of these conditions is not met, we throw an error with an appropriate message.
  • Initialize a Counter: We initialize a variable count to 0. This variable will keep track of the number of items that satisfy the condition.
  • Iterate Over the Array: We use the forEach method to iterate over each item in the arr array.
  • Check for Nested Arrays: For each item, we check if it is an array using Array.isArray(item). If it is an array, we recursively call the countNestedArray function with the nested array and the callback function. The result of this recursive call is added to the count variable.
  • Check the Condition: If the current item is not an array, we call the callback function with the item as an argument. If the callback function returns true, it means the item satisfies the condition, and we increment the count variable.
  • Return the Result: After iterating over all items in the array, we return the final count.

The key aspects of this solution are recursion and callback functions.

Note

The use of the forEach method to iterate over the array and the recursive approach to handle nested arrays are both valid techniques. However, some developers may prefer using a for...of loop or a separate recursive helper function to enhance readability and modularity.

Common Pitfalls

Nested Loops vs Recursion: Some candidates may attempt to solve the problem using nested loops instead of recursion. While this approach can work for simple cases, it may become less efficient and harder to maintain for deeply nested arrays.

Array Methods: Candidates may not be familiar with array methods like Array.isArray()

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