Javascript: Building a hierarchical tree

Here's one solution:

var items = [
    {"Id": "1", "Name": "abc", "Parent": "2"},
    {"Id": "2", "Name": "abc", "Parent": ""},
    {"Id": "3", "Name": "abc", "Parent": "5"},
    {"Id": "4", "Name": "abc", "Parent": "2"},
    {"Id": "5", "Name": "abc", "Parent": ""},
    {"Id": "6", "Name": "abc", "Parent": "2"},
    {"Id": "7", "Name": "abc", "Parent": "6"},
    {"Id": "8", "Name": "abc", "Parent": "6"}
];

function buildHierarchy(arry) {

    var roots = [], children = {};

    // find the top level nodes and hash the children based on parent
    for (var i = 0, len = arry.length; i < len; ++i) {
        var item = arry[i],
            p = item.Parent,
            target = !p ? roots : (children[p] || (children[p] = []));

        target.push({ value: item });
    }

    // function to recursively build the tree
    var findChildren = function(parent) {
        if (children[parent.value.Id]) {
            parent.children = children[parent.value.Id];
            for (var i = 0, len = parent.children.length; i < len; ++i) {
                findChildren(parent.children[i]);
            }
        }
    };

    // enumerate through to handle the case where there are multiple roots
    for (var i = 0, len = roots.length; i < len; ++i) {
        findChildren(roots[i]);
    }

    return roots;
}

console.log(buildHierarchy(items));​

Here's another one. This should work for multiple root nodes:

function convertToHierarchy() { 

    var arry = [{ "Id": "1", "Name": "abc", "Parent": "" }, 
    { "Id": "2", "Name": "abc", "Parent": "1" },
    { "Id": "3", "Name": "abc", "Parent": "2" },
    { "Id": "4", "Name": "abc", "Parent": "2"}];

    var nodeObjects = createStructure(arry);

    for (var i = nodeObjects.length - 1; i >= 0; i--) {
        var currentNode = nodeObjects[i];

        //Skip over root node.
        if (currentNode.value.Parent == "") {
            continue;
        }

        var parent = getParent(currentNode, nodeObjects);

        if (parent == null) {
            continue;
        }

        parent.children.push(currentNode);
        nodeObjects.splice(i, 1);
    }

    //What remains in nodeObjects will be the root nodes.
    return nodeObjects;
}

function createStructure(nodes) {
    var objects = [];

    for (var i = 0; i < nodes.length; i++) {
        objects.push({ value: nodes[i], children: [] });
    }

    return objects;
}

function getParent(child, nodes) {
    var parent = null;

    for (var i = 0; i < nodes.length; i++) {
        if (nodes[i].value.Id == child.value.Parent) {
            return nodes[i];
        }
    }

    return parent;
}