Add logic to set deleted values in rhs to persist keys to undefined

Author: mattphillips

src/index.js

@@ -5,15 +5,21 @@ const diff = (lhs, rhs) => {
 
   if (typeof rhs !== 'object' || rhs === null) return rhs;
 
-  return Object.keys(rhs).reduce((acc, key) => {
+  const rhsKeys = Object.keys(rhs);
+
+  const deletedValues = Object.keys(lhs).reduce((acc, key) => {
+    return rhsKeys.indexOf(key) !== -1 ? acc : { ...acc, [key]: undefined };
+  }, {});
+
+  return rhsKeys.reduce((acc, key) => {
     if (!lhs.hasOwnProperty(key)) return { ...acc, [key]: rhs[key] };
 
     const lhsValue = lhs[key];
     const rhsValue = rhs[key];
     if (isEqual(lhsValue, rhsValue)) return acc;
 
     return { ...acc, [key]: diff(lhsValue, rhsValue) };
-  }, {});
+  }, deletedValues);
 };
 
 export default diff;

src/index.spec.js

@@ -54,12 +54,16 @@ describe('.diff', () => {
       });
 
       it('returns subset of right hand side value when a key value has been deleted', () => {
-        expect(diff({ a: { b: 1 }, c: 2, d: { e: 100 } }, { a: { b: 1 }, c: 2, d: {} })).to.deep.equal({ d: {} });
+        expect(diff({ a: { b: 1 }, c: 2, d: { e: 100 } }, { a: { b: 1 }, c: 2, d: {} })).to.deep.equal({ d: { e: undefined } });
       });
 
       it('returns subset of right hand side value when a key value has been added', () => {
         expect(diff({ a: 1 }, { a: 1, b: 2 })).to.deep.equal({ b: 2 });
       });
+
+      it('returns keys as undefined when deleted from right hand side', () => {
+        expect(diff({ a: 1, b: { c: 2 }}, { a: 1 })).to.deep.equal({ b: undefined });
+      });
     });
 
     describe('arrays', () => {
@@ -72,7 +76,7 @@ describe('.diff', () => {
       });
 
       it('returns subset of right hand side array as object of indices to value when right hand side array has deletions', () => {
-        expect(diff([1, 2, 3], [1, 3])).to.deep.equal({ 1: 3 });
+        expect(diff([1, 2, 3], [1, 3])).to.deep.equal({ 1: 3, 2: undefined });
       });
 
       it('returns subset of right hand side array as object of indices to value when right hand side array has additions', () => {