added if-not-tree checks

Author: henrikac

contents/tree_traversal/code/javascript/tree.js

@@ -10,16 +10,28 @@ function createTree(rows, children) {
 }
 
 function dfsPreorder(tree) {
+  if (!tree) {
+    return;
+  }
+
   console.log(tree.id);
   tree.children.forEach(dfsPreorder);
 }
 
 function dfsPostorder(tree) {
+  if (!tree) {
+    return;
+  }
+
   tree.children.forEach(dfsPostorder);
   console.log(tree.id);
 }
 
 function dfsInorder(tree) {
+  if (!tree) {
+    return;
+  }
+
   switch (tree.children.length) {
     case 2:
       dfsInorder(tree.children[0]);

contents/tree_traversal/tree_traversal.md

@@ -60,7 +60,7 @@ Because of this, the most straightforward way to traverse the tree might be recu
 {% sample lang="java" %}
 [import:21-27, lang:"java"](code/java/Tree.java)
 {% sample lang="js" %}
-[import:12-15, lang:"javascript"](code/javascript/tree.js)
+[import:12-19, lang:"javascript"](code/javascript/tree.js)
 {% sample lang="py" %}
 [import:18-23, lang:"python"](code/python/Tree_example.py)
 {% sample lang="scratch" %}
@@ -114,7 +114,7 @@ Now, in this case the first element searched through is still the root of the tr
 {% sample lang="java" %}
 [import:34-41, lang:"java"](code/java/Tree.java)
 {% sample lang="js" %}
-[import:17-20, lang:"javascript"](code/javascript/tree.js)
+[import:21-28, lang:"javascript"](code/javascript/tree.js)
 {% sample lang="py" %}
 [import:26-31, lang:"python"](code/python/Tree_example.py)
 {% sample lang="scratch" %}
@@ -163,7 +163,7 @@ In this case, the first node visited is at the bottom of the tree and moves up t
 {% sample lang="java" %}
 [import:48-62, lang:"java"](code/java/Tree.java)
 {% sample lang="js" %}
-[import:22-39, lang:"javascript"](code/javascript/tree.js)
+[import:30-51, lang:"javascript"](code/javascript/tree.js)
 {% sample lang="py" %}
 [import:34-46, lang:"python"](code/python/Tree_example.py)
 {% sample lang="scratch" %}
@@ -221,7 +221,7 @@ In code, it looks like this:
 {% sample lang="java" %}
 [import:65-79, lang:"java"](code/java/Tree.java)
 {% sample lang="js" %}
-[import:41-48, lang:"javascript"](code/javascript/tree.js)
+[import:53-60, lang:"javascript"](code/javascript/tree.js)
 {% sample lang="py" %}
 [import:49-60, lang:"python"](code/python/Tree_example.py)
 {% sample lang="scratch" %}
@@ -272,7 +272,7 @@ And this is exactly what Breadth-First Search (BFS) does! On top of that, it can
 {% sample lang="java" %}
 [import:81-95, lang:"java"](code/java/Tree.java)
 {% sample lang="js" %}
-[import:50-57, lang:"javascript"](code/javascript/tree.js)
+[import:62-69, lang:"javascript"](code/javascript/tree.js)
 {% sample lang="py" %}
 [import:63-75, lang:"python"](code/python/Tree_example.py)
 {% sample lang="scratch" %}