How to Sum Values from Nested Arrays in JavaScript

Learn how to effectively sum values from an array inside an object within an array in JavaScript, using maps and reduce.
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How to Sum Values from Nested Arrays in JavaScript

When working with complex data structures in JavaScript, such as arrays nested inside objects, it can sometimes be challenging to extract and manipulate the information you need. One common task is summing values that are stored in a nested structure. In this guide, we’ll tackle a specific example involving an array of objects, where each object contains an array of presents and we want to calculate the total value of those presents.

The Problem

Consider the following scenario: You have an array of people, and each person has a set of presents they want. There is also a separate list of present values. The goal is to iterate over the people, look up the present values, and calculate the total value of presents for each person.

Below is a sample structure that demonstrates the problem:

[[See Video to Reveal this Text or Code Snippet]]

In the sample above, the presentsData array holds objects that each contain a name, an array of presents, and some amount of money. The prices array holds the values of each present.

What you need to do is create a new field sumP in each object, which contains the total value of the presents listed.

The Solution

To solve this problem, we can utilize JavaScript’s Map to transform the prices array for easier lookups. Then, we will use Array.map() alongside Array.reduce() to calculate the total for each person's presents.

Step 1: Create a Price Map

We will first create a Map that allows us to quickly access present prices by their name:

[[See Video to Reveal this Text or Code Snippet]]

This setup will allow for O(1) time complexity lookups for each present's price.

Step 2: Compute the Total Value

Next, we utilize the map() function on the presentsData array to create a new array where we also include the sumP field:

[[See Video to Reveal this Text or Code Snippet]]

Here's what’s happening in this code:

Iterate through each person: The map() function generates a new array with each person included.

Calculate the sum: For each person's presents, we use reduce() to accumulate the total value of presents.

Using the map: The pricesMap.get(present) retrieves the price for each present and adds it to the accumulator total.

Full Code Implementation

Putting it all together, here’s the complete implementation:

[[See Video to Reveal this Text or Code Snippet]]

Conclusion

By following the steps outlined in this post, you can efficiently sum values from arrays nested inside objects, creating a new field dynamically as needed. This method leverages the power of JavaScript's functional programming features, providing a clean and effective solution to the problem.

Keep practicing with nested data structures, and you'll be more comfortable working with complex data in JavaScript!

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