#185 change customMerge signature to allow for ignoring null or empty…

… string overwrites + a sweet little util function customMergeIgnoreEmptyValues for cuteness

index.d.ts

@@ -11,6 +11,7 @@ declare namespace deepmerge {
 
 	export function all (objects: object[], options?: Options): object;
 	export function all<T> (objects: Partial<T>[], options?: Options): T;
+	export function customMergeIgnoreEmptyValues (key: any, target: any, source: any): any;
 }
 
 export = deepmerge;

index.js

@@ -14,12 +14,12 @@ const defaultArrayMerge = (target, source, options) => {
 		.map(element => cloneUnlessOtherwiseSpecified(element, options))
 }
 
-const getMergeFunction = (key, options) => {
+const getMergeFunction = (key, options, target, source) => {
 	if (!options.customMerge) {
 		return deepmerge
 	}
 
-	const customMerge = options.customMerge(key)
+	const customMerge = options.customMerge(key, target, source)
 	return typeof customMerge === `function` ? customMerge : deepmerge
 }
 
@@ -61,7 +61,7 @@ const mergeObject = (target, source, options) => {
 		}
 
 		if (propertyIsOnObject(target, key) && options.isMergeable(source[key])) {
-			destination[key] = getMergeFunction(key, options)(target[key], source[key], options)
+			destination[key] = getMergeFunction(key, options, target[key], source[key])(target[key], source[key], options)
 		} else {
 			destination[key] = cloneUnlessOtherwiseSpecified(source[key], options)
 		}
@@ -114,4 +114,8 @@ deepmerge.all = (array, inputOptions) => {
 	return array.reduce((prev, next) => deepmerge(prev, next, options))
 }
 
+deepmerge.customMergeIgnoreEmptyValues = (key, target, source) => !target || target === ''
+	? () => source
+	: () => target;
+
 module.exports = deepmerge

test/merge.js

@@ -677,6 +677,21 @@ test('Falsey properties should be mergeable', function(t) {
 	t.end()
 })
 
+test('customMerge without overwriting with null or empty string', function(t) {
+	var src = { someNewVariable: "herp", very: { nested: { thing: "" } } };
+  var target = { very: { nested: { thing: "derp" } } };
+
+  var res = merge(target, src, {
+    customMerge: merge.customMergeIgnoreEmptyValues,
+  });
+
+  t.deepEqual(res, {
+    someNewVariable: "herp",
+    very: { nested: { thing: "derp" } },
+  });
+  t.end();
+})
+
 test('should not mutate options', function(t) {
 	var options = {}