How to Group Nested Arrays of Objects by Child Object Value in JavaScript

Learn how to efficiently group a nested array of objects by a child object's value using JavaScript. This guide provides clear steps and code examples for achieving the desired results.
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Grouping Nested Arrays of Objects by Child Object Value in JavaScript

When working with complex data structures in JavaScript, you might encounter a need to group arrays of objects based on the values of their nested properties. One common scenario arises when you're trying to categorize objects that contain an array of statuses. In this guide, we'll address a specific example: how to group a nested array of objects by the value of one of the child objects.

The Problem Statement

Imagine you have an array of objects, each representing an "issue" with a corresponding array of "status" values. For example:

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

You want to group these objects so that each status (like "test", "prod", or "dev") becomes a key in your resulting object, with its associated issues listed as values. The existing reduce method you have implemented doesn’t yield the desired results—it groups statuses together rather than separating them.

The current output looks like this:

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

But you want it to resemble this instead:

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

The Solution

To achieve this grouping correctly, we can utilize the reduce method in conjunction with forEach. This ensures that each status within the status array is processed individually. Here's a step-by-step breakdown of the solution:

Step 1: Using reduce

We start with the reduce function to iterate through each object in the original array.

Step 2: Loop Through the Status Array

Inside the reduce method, we'll use forEach to go through each status of the current object.

Step 3: Initialize Grouping

We check if the status exists as a key in the result object. If it doesn’t, we initialize it as an empty array.

Step 4: Push a Copy of the Object

Since JavaScript objects are reference types, it's important to avoid mutation when pushing objects to the new array. Therefore, we can utilize JSON.parse(JSON.stringify(o)) to push a clone of the object.

Final Code Example

Here’s how the complete code looks:

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

Explanation of the Code

const names: This is our initial array of issues and statuses.

reduce: We use this method to create a new object that accumulates the grouped results.

forEach: Each status is looped through and processed individually.

r[name] ??=: This ES2021 feature checks if the key exists; if not, it initializes it.

JSON.parse(JSON.stringify(o)): This line serves to create a deep copy of the original object, preventing modification during the upcoming iterations.

Conclusion

In conclusion, when you need to group an array of objects by a nested child property in JavaScript, using both reduce and forEach offers a straightforward and efficient way to achieve your desired output. By following the steps outlined in this post, you can manipulate similar data structures with ease.

If you have any questions or need further assistance, feel free to reach out. Happy coding!