How to create a tree structure from JSON data

A solution for unordered nodes.

var arr = [
        { text: "text3", id: "3", parentId: "2" },
        { text: "text2", id: "2", parentId: "1" },
        { text: "text4", id: "4", parentId: "1" },
        { text: "text1", id: "1", /* no parent id */ },
        { text: "text5", id: "5", /* no parent id */ }
    ],
    data = arr.reduce(function (r, a) {
        function getParent(s, b) {
            return b.id === a.parentId ? b : (b.nodes && b.nodes.reduce(getParent, s));
        }

        var index = 0, node;
        if ('parentId' in a) {
            node = r.reduce(getParent, {});
        }
        if (node && Object.keys(node).length) {
            node.nodes = node.nodes || [];
            node.nodes.push(a);
        } else {
            while (index < r.length) {
                if (r[index].parentId === a.id) {
                    a.nodes = (a.nodes || []).concat(r.splice(index, 1));
                } else {
                    index++;
                }
            }
            r.push(a);
        }
        return r;
    }, []);

document.write('<pre>' + JSON.stringify(data, 0, 4) + '</pre>');

I used an object as a map here, otherwise you must iterate through all the array elements for search for ids of the nodes, and removing elements is easier too. The toString() functions are there, because the keys of the map must be strings.

// we use a map for search for the nodes by their ids
var nodes_map = {};

// we copy all the elements into the map, where the keys are the ids of the nodes
for ( var i=0; i<arr.length; ++i )
{
    // create nodes array for each node
    arr[ i ].nodes = [];

    // copy into the map
    nodes_map[ arr[i].id.toSting() ] = arr[ i ];
}    

// we iterate through all nodes, and add them into their parent's node array
for ( var key in nodes_map )
{
    // current node
    var node = nodes_map[ key ];

    // if the current node have idParent property, and the parent exists in the map
    if ( "idParent" in node && node.idParent.toSting() in nodes_map )
    {
        // we add the current node to the parent's nodes array 
        nodes_map[ node.idParent.toSting() ].nodes.push( node );
    }
}

// we remove all the nodes from the map, that has parents
for ( var key in nodes_map )
{
    // current node
    var node = nodes_map[ key ];

    // if it has idParent property
    if ( "idParent" in node )
    {
        // we remove from the map
        delete nodes_map[ key ];
    }
}    

// results array
var new_arr = [];
// copy back the nodes from the map into an array
for ( var key in nodes_map )
{
    new_arr.push( nodes_map[ key ] );
}