adds option to ignore undefined values

Author: achingbrain

index.js

@@ -12,7 +12,8 @@ const defineProperty = (obj, name, value) => Object.defineProperty(obj, name, {
 
 const globalThis = this;
 const defaultMergeOpts = {
-	concatArrays: false
+	concatArrays: false,
+	ignoreUndefined: false
 };
 
 const getEnumerableOwnPropertyKeys = value => {
@@ -76,6 +77,10 @@ function cloneOptionObject(obj) {
  */
 const mergeKeys = (merged, source, keys, mergeOpts) => {
 	keys.forEach(key => {
+		if (typeof source[key] === 'undefined' && mergeOpts.ignoreUndefined) {
+			return;
+		}
+
 		// Do not recurse into prototype chain of merged
 		if (key in merged && merged[key] !== Object.getPrototypeOf(merged)) {
 			defineProperty(merged, key, merge(merged[key], source[key], mergeOpts));

readme.md

@@ -85,6 +85,25 @@ mergeOptions.apply({concatArrays: true}, [{src: ['src/**']}, {src: ['test/**']}]
 //=> {src: ['src/**', 'test/**']}
 ```
 
+##### config.ignoreUndefined
+
+Type: `boolean`<br/>Default: `false`
+
+Ignore undefined values:
+
+```js
+mergeOptions({foo: 'bar'}, {foo: undefined})
+//=> {foo: undefined}
+
+// Via call
+mergeOptions.call({ignoreUndefined: true}, {foo: 'bar'}, {foo: undefined})
+//=> {foo: 'bar'}
+
+// Via apply
+mergeOptions.apply({ignoreUndefined: true}, [{foo: 'bar'}, {foo: undefined}])
+//=> {foo: 'bar'}
+```
+
 
 ## Related
 

test/undefined-values.js

@@ -0,0 +1,16 @@
+import test from 'ava';
+import mergeOptions from '..';
+
+test('undefined values', t => {
+	t.deepEqual(
+		mergeOptions.call({ignoreUndefined: true}, {foo: 0}, {foo: undefined}),
+		{foo: 0}
+	);
+});
+
+test('deep undefined values', t => {
+	t.deepEqual(
+		mergeOptions.call({ignoreUndefined: true}, {nested: {unicorns: 'none'}}, {nested: {unicorns: undefined}}),
+		{nested: {unicorns: 'none'}}
+	);
+});