root node parent can be undefined

dist/list-to-tree.js

@@ -27,6 +27,7 @@ module.exports = class LTT{
 
   constructor(list, options = {}) {
     const _list = list.map((item) => item);
+    _list.map(item=>item.parent=item.parent||0)
 
     options = Object.assign({}, defaultOptions, options);
     this.options = options;

spec/undefined-root-node-parent-spec.js

@@ -0,0 +1,80 @@
+const LTT = require('../dist/list-to-tree');
+
+describe('Root node parent undefined:', function() {
+
+    var tree = null;
+
+    beforeEach(function() {
+        var list = [
+            {
+                id: 1
+            }, {
+                id: 2,
+                parent: 1
+            }, {
+                id: 3,
+                parent: 1
+            }, {
+                id: 4,
+                parent: 2
+            }, {
+                id: 5,
+                parent: 2
+            }, {
+                id: 6
+            }, {
+                id: 7
+            }, {
+                id: 8,
+                parent: 7
+            }, {
+                id: 9,
+                parent: 8
+            }, {
+                id: 10
+            }
+        ];
+        var ltt = new LTT(list);
+        tree = ltt.GetTree();
+    });
+
+
+    it('It is workly', function() {
+        expect( tree.length ).toBe(4);
+
+    });
+
+    it('First node check id', function() {
+        var firstNode = tree[0];
+        expect( firstNode.id ).toBe(1);
+    });
+
+    it('First node check parent', function() {
+        var firstNode = tree[0];
+        expect( firstNode.parent ).toBe(0);
+    });
+
+    it('First node check child', function() {
+        var child = tree[0].child;
+        expect( child.length ).toBe(2);
+    });
+
+    it('First child - check id', function() {
+        var child = tree[0].child;
+        var node = child[0];
+        expect( node.id ).toBe(2);
+    });
+
+    it('First child - check parent', function() {
+        var child = tree[0].child;
+        var node = child[0];
+        expect( node.parent ).toBe(1);
+    });
+
+    it('Child node have a child key', function() {
+        var child = tree[0].child;
+        var node = child[0];
+        expect( 'child' in node ).toBe(true);
+    });
+
+});