JavaScript loop nested array

Question

I'm building an array with data received from an API.

The result from the API contains a flat array of all pages on a site. I want to create a new array by making it multidimensional so that a page can have children, which can have their own children etc.

Expected array if I can get this to work:

array = 
  [{id:1, children:[] }                   // No children
  ,{id:2, children:[{id:3, children:[]}]}           // One level of children
  ,{id:4, children:[{id:5, children:[{id:6, children:[]}]}]} // Two levels of children etc...
  ]

So what I'm doing now when looping through the API data is first checking if the element has children. If it doesn't have any I just append it to the array as it is but with an empty children[] as extra.

If the element has a parent I will get the parentId which I have to search for in my custom array.

if(!element.parent){
   array.push(element);
   continue;
}

// Now I know this element has a parent. So the fun begins.

var parentId = element.parent.id; // Here I have the parent ID 

// Example of a none working append (because I dont know the depth)
for(var i = 0; i < array.length; i++){
  if(array[i].id === parentId){
    array[i].children.push(element);
  }
}

So the question is how to loop through all these possible levels of children to find a match and append it correctly to my custom array?

Maybe you can take a look at Array.prototype.flatMap(), can be useful. developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/… — norbitrial
– norbitrial, Commented Feb 23, 2020 at 13:56
Welcome to Stack Overflow! Please take the tour (you get a badge!) and read through the help center, in particular How do I ask a good question? Your best bet here is to do your research, search for related topics on SO, and give it a go. If you get stuck and can't get unstuck after doing more research and searching, post a minimal reproducible example of your attempt and say specifically where you're stuck. People will be glad to help. — palaѕн
– palaѕн, Commented Feb 23, 2020 at 13:58
I don't fully understand your question but most likely you want to use Recursion — Dom
– Dom, Commented Feb 23, 2020 at 13:59
This answer can guide you to build your desired array recursively. stackoverflow.com/questions/47746154/… — Anuj Shah
– Anuj Shah, Commented Feb 23, 2020 at 14:02
Hi, yes I'm looking at recursion, but I still cant wrap my head around a solution. I understand a basic recursion but in this example with keeping track of what level down in the array I am makes me confused. Updated my question with a little bit more code to maybe explain better the expected result. — Hopstad
– Hopstad, Commented Feb 23, 2020 at 14:17

wg1k · Accepted Answer · 2020-02-24 06:56:50Z

Perhaps this code sample can lead you in the right direction:

var buildTree = function (arr, parent) {
    var result = [];
    var subTree = arr.filter((e) => e.parent === parent);
    for (var i = 0; i < subTree.length; ++i) {
        result[i] = {
            ...subTree[i],
            children: buildTree(arr, subTree[i].id)
        }
    };
    return result;
};

var arr = [{
        id: 1,
        parent: 0,
        name: "item-1"
    },
    {
        id: 2,
        parent: 1,
        name: "item-1.1"
    },
    {
        id: 3,
        parent: 2,
        name: "item-1.1.1"
    },
    {
        id: 4,
        parent: 2,
        name: "item-1.1.2"
    },
    {
        id: 5,
        parent: 1,
        name: "item-1.2"
    },
    {
        id: 6,
        parent: 0,
        name: "item-2"
    },
];

var result = buildTree(arr, 0);

console.log(result);

The recursive buildTree method is returning each time the subtree from a given parent id (first time is expected to be the root of the tree).

BTW: in recursion, the most important thing you need to care about is, besides what you want your method to do (one level each time), the stop condition for your method (when you won't call it again). In this case when there are no more children (handled by the for loop over the filtered data).

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