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Learn how to efficiently `group child objects` under their parents in a flat array using JavaScript with a recursive approach.
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How to Recursively Group Children Objects Under Parent in JavaScript

When dealing with hierarchical data in JavaScript, you might find yourself faced with the challenge of organizing flat arrays of objects into a nested structure. For instance, imagine you have an array of topics, each linked to a parent. Your task is to structure these topics into a parent-child hierarchy. In this post, we'll demonstrate how to achieve this recursively using JavaScript.

The Problem

Consider an array of topics where each topic has an id, a name, and a parent_id that indicates its parent topic. Here’s what the data might look like:

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The goal is to restructure this flat array into a hierarchical format where each parent topic contains its children under a children property, resulting in an output array that looks like this:

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The Solution

To transform this flat structure into a nested hierarchy, we’ll need to create two functions: one for creating an index based on the parent IDs and another for constructing the tree using this index. Below is a detailed breakdown of the solution.

Step 1: Create an Index

The first step is to index the topics by their parent_id. We’ll create a new structure that maps each parent to its children. This allows for efficient grouping of children under their respective parents.

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Step 2: Construct the Tree

Now that we have an index, the next step is to recursively construct the tree. We’ll define a function that takes the index and looks for children of a node based on its ID.

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Step 3: Putting It All Together

With the indexing and tree-building functions in place, we can combine them to produce the desired output structure.

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Expected Output

When you run the above code, you should see the nested structure printed as desired:

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Conclusion

Constructing a tree from a flat array of objects can be effectively managed using indexing and recursion. This method provides a clear, organized approach to building relationships between parent and child topics in JavaScript. By following the steps outlined above, you ensure that your code is flexible and reusable across various data structures. Give it a try in your next project and see how easy it becomes to handle hierarchical data!