When traversing fields, ensure we create them as we go to avoid accessing undefined object

Author: sadortun

src/ObjectStateMutations.js

@@ -138,7 +138,11 @@ export function estimateAttributes(
           data[first] = { ...serverData[first] };
           let object = { ...data };
           for (let i = 0; i < fields.length - 1; i++) {
-            object = object[fields[i]];
+            const key = fields[i];
+            if (!(key in object)) {
+              object[key] = {};
+            }
+            object = object[key];
           }
           object[last] = pendingOps[i][attr].applyTo(object[last]);
         } else {

src/__tests__/ObjectStateMutations-test.js

@@ -163,7 +163,7 @@ describe('ObjectStateMutations', () => {
   });
 
   it('can estimate attributes for nested documents', () => {
-    const serverData = { objectField: { counter: 10, letter: 'a' } };
+    let serverData = { objectField: { counter: 10, letter: 'a' } };
     let pendingOps = [{ 'objectField.counter': new ParseOps.IncrementOp(2) }];
     expect(
       ObjectStateMutations.estimateAttributes(serverData, pendingOps, 'someClass', 'someId')
@@ -182,6 +182,19 @@ describe('ObjectStateMutations', () => {
         letter: 'a',
       },
     });
+    serverData = {};
+    pendingOps = [{ 'objectField.subField.subField.counter': new ParseOps.IncrementOp(20) }];
+    expect(
+      ObjectStateMutations.estimateAttributes(serverData, pendingOps, 'someClass', 'someId')
+    ).toEqual({
+      objectField: {
+        subField: {
+          subField: {
+            counter: 20,
+          },
+        },
+      },
+    });
   });
 
   it('can commit changes from the server', () => {