Build A Tree Array From A Flat Array - Recursion

I got an assignment to display comments in a recursive way, something like so:

The data I got from the server was flat, meaning:
every item in the array holds a reference to its parent, like so:

 const entries = [ { index: 1, parent: 0 }, { index: 2, parent: 1 }, { index: 3, parent: 2 }, ... ... ]; 

After thinking how to "attack" this problem, I realized
If I want a recursive object, then the easiest solution is a recursive one

Here is the function that converts a flat array to a tree array:

 const arrayToTree = (arr, parent = 0) => arr.filter(item => item.parent === parent) .map(child => ({ ...child, children: arrayToTree(arr, child.index) })); 

A quick rundown:

• We first filter the root parent's children.
• Then we do the same to each of the children we just grabbed

My answer to the question on Stack overflow

Here is a codepen if you want to play more with the data ot the solution:

Sources that helped me:

• Styling Comment Threads