#185 change customMerge signature for merging empty values

index.js

@@ -16,11 +16,11 @@ function defaultArrayMerge(target, source, options) {
 	})
 }
 
-function getMergeFunction(key, options) {
+function getMergeFunction(key, options, target, source) {
 	if (!options.customMerge) {
 		return deepmerge
 	}
-	var customMerge = options.customMerge(key)
+	var customMerge = options.customMerge(key, target, source);
 	return typeof customMerge === 'function' ? customMerge : deepmerge
 }
 
@@ -64,11 +64,10 @@ function mergeObject(target, source, options) {
 		}
 
 		if (propertyIsOnObject(target, key) && options.isMergeableObject(source[key])) {
-			destination[key] = getMergeFunction(key, options)(target[key], source[key], options)
-		} else {
-			destination[key] = cloneUnlessOtherwiseSpecified(source[key], options)
-		}
-	})
+			destination[key] = getMergeFunction(key, options, target[key], source[key])(target[key], source[key], options);
+		} else
+			destination[key] = cloneUnlessOtherwiseSpecified(source[key], options);
+	});
 	return destination
 }
 
@@ -103,4 +102,8 @@ deepmerge.all = function deepmergeAll(array, options) {
 	}, {})
 }
 
+deepmerge.customMergeIgnoreEmptyValues = (key, target, source) => !target || target === ''
+	? () => source
+	: () => target;
+
 module.exports = deepmerge

test/merge.js

@@ -667,3 +667,15 @@ test('Falsey properties should be mergeable', function(t) {
 	t.ok(customMergeWasCalled, 'custom merge function was called')
 	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()
+})