Generate the identity when running generate(a,a)

lib/generate.js

@@ -19,53 +19,33 @@ module.exports = function generate(before, after) {
   before = serialize(before);
   after = serialize(after);
 
+  if (!(before instanceof Object) &&
+      !(after instanceof Object) &&
+      before === after) { // Return no op when values match
+    return {}
+  }
+
   if (before === null || after === null ||
     typeof before !== 'object' || typeof after !== 'object' ||
-    Array.isArray(before) !== Array.isArray(after)) {
-    return after;
+    Array.isArray(before) || Array.isArray(after)) {
+    return serialize(after);
   }
 
-  if (Array.isArray(before)) {
-    if (!arrayEquals(before, after)) {
-      return after;
+  let patch = {};
+  for (let key of Object.keys(before)) {
+    let newVal = generate(before[key] ?? null, after[key] ?? null);
+    // Omit noops
+    if (equal(newVal, {})) {
+      continue;
     }
-    return undefined;
+    patch[key] = serialize(newVal);
   }
 
-  var patch = {};
-  var beforeKeys = Object.keys(before);
-  var afterKeys = Object.keys(after);
-
-  var key, i;
-
-  // new elements
-  var newKeys = {};
-  for (i = 0; i < afterKeys.length; i++) {
-    key = afterKeys[i];
-    if (beforeKeys.indexOf(key) === -1) {
-      newKeys[key] = true;
+  for (let key of Object.keys(after)) {
+    if (!(key in before)) {
       patch[key] = serialize(after[key]);
     }
   }
 
-  // removed & modified elements
-  var removedKeys = {};
-  for (i = 0; i < beforeKeys.length; i++) {
-    key = beforeKeys[i];
-    if (afterKeys.indexOf(key) === -1) {
-      removedKeys[key] = true;
-      patch[key] = null;
-    } else {
-      if (before[key] !== null && typeof before[key] === 'object') {
-        var subPatch = generate(before[key], after[key]);
-        if (subPatch !== undefined) {
-          patch[key] = subPatch;
-        }
-      } else if (before[key] !== after[key]) {
-        patch[key] = serialize(after[key]);
-      }
-    }
-  }
-
-  return (Object.keys(patch).length > 0 ? patch : undefined);
+  return (Object.keys(patch).length > 0 ? patch : {});
 };

package.json

@@ -1,6 +1,6 @@
 {
   "name": "json-merge-patch",
-  "version": "1.0.2",
+  "version": "2.1.0",
   "description": "Implementation of JSON Merge Patch (RFC 7396)",
   "main": "index.js",
   "directories": {

test/lib/generate.js

@@ -126,24 +126,24 @@ describe('generate', function() {
     );
   });
 
-  it('should return undefined if the object hasnt changed', function() {
+  it('should return {} if the object hasnt changed', function() {
     assert.deepEqual(
       generate({a: 'a'}, {a: 'a'}),
-      undefined
+      {}
     );
   });
 
-  it('should return undefined if the object with sub attributes hasnt changed', function() {
+  it('should return {} if the object with sub attributes hasnt changed', function() {
     assert.deepEqual(
       generate({a: {b: 'c'}}, {a: {b: 'c'}}),
-      undefined
+      {}
     );
   });
 
-  it('should return undefined if the array hasnt changed', function() {
+  it('should return the target if the output is an array', function() {
     assert.deepEqual(
       generate([1, 2, 3], [1, 2, 3]),
-      undefined
+      [1,2,3]
     );
   });