From a003f3a0b9ea13624de3d863b1553aa957e984be Mon Sep 17 00:00:00 2001
From: Tim Branyen <tim@tabdeveloper.com>
Date: Sat, 22 Jul 2017 22:37:05 -0700
Subject: [PATCH] Error if duplicate keys are present

---
 packages/diffhtml/lib/tree/sync.js |  6 ++++++
 packages/diffhtml/test/tree.js     | 25 +++++++++++++++++++++++++
 2 files changed, 31 insertions(+)

diff --git a/packages/diffhtml/lib/tree/sync.js b/packages/diffhtml/lib/tree/sync.js
index 90748d49..6f71b58b 100644
--- a/packages/diffhtml/lib/tree/sync.js
+++ b/packages/diffhtml/lib/tree/sync.js
@@ -44,6 +44,12 @@ export default function syncTree(oldTree, newTree, patches, parentTree, specialC
         const vTree = nodes[i];
 
         if (vTree.key) {
+          if (process.env.NODE_ENV !== 'production') {
+            if (map.has(vTree.key)) {
+              throw new Error(`Key: ${vTree.key} cannot be duplicated`);
+            }
+          }
+
           map.set(vTree.key, vTree);
         }
       }
diff --git a/packages/diffhtml/test/tree.js b/packages/diffhtml/test/tree.js
index 6e51fbb2..5ce55bab 100644
--- a/packages/diffhtml/test/tree.js
+++ b/packages/diffhtml/test/tree.js
@@ -750,6 +750,31 @@ describe('Tree', function() {
     });
 
     describe('Elements (keyed)', () => {
+      it('will error if duplicate keys are provided in development', () => {
+        const referenceNode = createTree('div', { key: '0' });
+        const duplicateNode = createTree('div', { key: '0' });
+        const oldTree = createTree('div', null, [referenceNode]);
+        const newTree = createTree('div', null, [
+          referenceNode,
+          duplicateNode,
+        ]);
+
+        throws(() => syncTree(oldTree, newTree));
+      });
+
+      it('will not error if duplicate keys are provided in production', () => {
+        const referenceNode = createTree('div', { key: '0' });
+        const duplicateNode = createTree('div', { key: '0' });
+        const oldTree = createTree('div', null, [referenceNode]);
+        const newTree = createTree('div', null, [
+          referenceNode,
+          duplicateNode,
+        ]);
+
+        process.env.NODE_ENV = 'production';
+        doesNotThrow(() => syncTree(oldTree, newTree));
+      });
+
       describe('insertBefore', () => {
         it('will prepend an element', () => {
           const referenceNode = createTree('div', { key: '0' });