Hierarchical json from flat with parent ID

Improved version, accepting parameters.

function convert(rows, options) {
    var options = options || {};
    var idProp = options.id || 'id';
    var parentProp = options.parent || 'parent';
    var childProp = options.children || 'children';

    function exists(rows, parent) {
        for (var i = 0; i < rows.length; i++) {
            if (rows[i][idProp] == parent) return true;
        }
        return false;
    }

    var nodes = [];

    // get the top level nodes
    for (var i = 0; i < rows.length; i++) {
        var row = rows[i];
        if (!exists(rows, row[parentProp])) {
            nodes.push(Object.assign({}, row));
        }
    }

    var toDo = [];
    for (var i = 0; i < nodes.length; i++) {
        toDo.push(nodes[i]);
    }

    while (toDo.length) {
        var node = toDo.shift();

        // the parent node
        // get the children nodes
        for (var i = 0; i < rows.length; i++) {
            var row = rows[i];
            if (row[parentProp] == node[idProp]) {
                var child = Object.assign({}, row);
                node[childProp] = node[childProp] || [];
                node[childProp].push(child);
                toDo.push(child);
            }
        }
    }

    return nodes;
}

// Usage
var data = [...];
convert(data);
// Or
convert(data, { id: 'Id', parent: 'Parent', children: 'Children' });

Try something like

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

function convert(array){
    var map = {};
    for(var i = 0; i < array.length; i++){
        var obj = array[i];
        obj.items= [];

        map[obj.Id] = obj;

        var parent = obj.Parent || '-';
        if(!map[parent]){
            map[parent] = {
                items: []
            };
        }
        map[parent].items.push(obj);
    }

    return map['-'].items;

}

var r = convert(arry)

Demo: Fiddle

Result

[{
    "Id" : "1",
    "Name" : "abc",
    "Parent" : "",
    "attr" : "abc",
    "children" : [{
                "Id" : "2",
                "Name" : "abc",
                "Parent" : "1",
                "attr" : "abc",
                "children" : [{
                            "Id" : "3",
                            "Name" : "abc",
                            "Parent" : "2",
                            "attr" : "abc",
                            "children" : []
                        }, {
                            "Id" : "4",
                            "Name" : "abc",
                            "Parent" : "2",
                            "attr" : "abc",
                            "children" : []
                        }]
            }]
}]

@Arun P Johny's answer is good, but it has a problem, when the array is not sorted, it will flush sub-trees. I updated it as this.

var arry = [
                { "Id": "5", "Name": "abc", "Parent": "3", "attr": "abc" },
               { "Id": "2", "Name": "abc", "Parent": "1", "attr": "abc" },
               { "Id": "4", "Name": "abc", "Parent": "2", "attr": "abc" },
               { "Id": "3", "Name": "abc", "Parent": "2", "attr": "abc" },
               { "Id": "1", "Name": "abc", "Parent": "", "attr": "abc" }
               ];

function convert(array){
    var map = {}
    for(var i = 0; i < array.length; i++){
        var obj = array[i]
        if(!(obj.Id in map)){
            map[obj.Id] = obj
            map[obj.Id].children = []
        }

        if(typeof map[obj.Id].Name == 'undefined'){
            map[obj.Id].Id = obj.Id
            map[obj.Id].Name = obj.Name
            map[obj.Id].attr = obj.attr
            map[obj.Id].Parent= obj.Parent
        }

        var parent = obj.Parent || '-';
        if(!(parent in map)){
            map[parent] = {}
            map[parent].children = []
        }

        map[parent].children.push(map[obj.Id])
    }
    return map['-']
}
console.log(JSON.stringify(convert(arry)))

The result is as this:

{
  "children": [
    {
      "children": [
        {
          "Id": "2",
          "Name": "abc",
          "Parent": "1",
          "attr": "abc",
          "children": [
            {
              "Id": "4",
              "Name": "abc",
              "Parent": "2",
              "attr": "abc",
              "children": []
            },
            {
              "children": [
                {
                  "Id": "5",
                  "Name": "abc",
                  "Parent": "3",
                  "attr": "abc",
                  "children": []
                }
              ],
              "Id": "3",
              "Name": "abc",
              "attr": "abc",
              "Parent": "2"
            }
          ]
        }
      ],
      "Id": "1",
      "Name": "abc",
      "attr": "abc",
      "Parent": ""
    }
  ]
}

function convert(rows) {
    function exists(rows, Parent) {
            for (var i = 0; i < rows.length; i++) {
                    if (rows[i].Id == Parent) return true;
            }
            return false;
    }
    var nodes = [];
    // get the top level nodes
    for (var i = 0; i < rows.length; i++) {
            var row = rows[i];
            if (!exists(rows, row.Parent)) {
                    nodes.push({
                            id: row.Id,
                            name: row.Name,
                            attr: row.attr
                    });
            }
    }
    var toDo = [];
    for (var i = 0; i < nodes.length; i++) {
            toDo.push(nodes[i]);
    }
    while (toDo.length) {
            var node = toDo.shift();
            // the parent node
            // get the children nodes
            for (var i = 0; i < rows.length; i++) {
                    var row = rows[i];
                    if (row.Parent == node.Id) {
                            var child = {
                                    Id: row.Id,
                                    Name: row.Name,
                                    attr: row.attr
                            };
                            if (node.options) {
                                    node.options.push(child);
                            } else {
                                    node.options = [child];
                            }
                            toDo.push(child);
                    }
            }
    }
    return nodes;}