fix: return TreeNode for first and last child (#894)

closes #893

Author: tobiasengelhardt

projects/angular-tree-component/src/lib/models/tree-node.model.ts

@@ -120,13 +120,13 @@ export class TreeNode implements ITreeNode {
   getFirstChild(skipHidden = false) {
     let children = skipHidden ? this.visibleChildren : this.children;
 
-    return [].concat(children || []).shift();
+    return children != null && children.length ? children[0] : null;
   }
 
   getLastChild(skipHidden = false) {
     let children = skipHidden ? this.visibleChildren : this.children;
 
-    return [].concat(children || []).pop();
+    return children != null && children.length ? children[children.length - 1] : null;
   }
 
   findNextNode(goInside = true, skipHidden = false) {

projects/angular-tree-component/src/lib/models/tree.model.ts

@@ -60,11 +60,13 @@ export class TreeModel implements ITreeModel, OnDestroy {
   }
 
   getFirstRoot(skipHidden = false) {
-    return [].concat(skipHidden ? this.getVisibleRoots() : this.roots).shift();
+    const root = skipHidden ? this.getVisibleRoots() : this.roots;
+    return root != null && root.length ? root[0] : null;
   }
 
   getLastRoot(skipHidden = false) {
-    return [].concat(skipHidden ? this.getVisibleRoots() : this.roots).pop();
+    const root = skipHidden ? this.getVisibleRoots() : this.roots;
+    return root != null && root.length ? root[root.length - 1] : null;
   }
 
   get isFocused() {