Apply clone option to target object

index.js

@@ -4,6 +4,10 @@ function emptyTarget(val) {
 	return Array.isArray(val) ? [] : {}
 }
 
+function firstArrayEntry(arr) {
+	return arr && arr.length ? arr[0] : []
+}
+
 function cloneUnlessOtherwiseSpecified(value, options) {
 	return (options.clone !== false && options.isMergeableObject(value))
 		? deepmerge(emptyTarget(value), value, options)
@@ -51,8 +55,17 @@ function propertyIsUnsafe(target, key) {
 			&& Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
 }
 
+// Retrieves either a new object or the appropriate target object to mutate.
+function getDestinationObject(target, options) {
+	if (options && options.mergeWithTarget) {
+		return Array.isArray(target) ? firstArrayEntry(target) : target
+	}
+
+	return {}
+}
+
 function mergeObject(target, source, options) {
-	var destination = {}
+	var destination = getDestinationObject(target, options)
 	if (options.isMergeableObject(target)) {
 		getKeys(target).forEach(function(key) {
 			destination[key] = cloneUnlessOtherwiseSpecified(target[key], options)
@@ -100,7 +113,7 @@ deepmerge.all = function deepmergeAll(array, options) {
 
 	return array.reduce(function(prev, next) {
 		return deepmerge(prev, next, options)
-	}, {})
+	}, getDestinationObject(array, options))
 }
 
 module.exports = deepmerge

test/merge.js

@@ -1,6 +1,42 @@
 var merge = require('../')
 var test = require('tape')
 
+test('result should retain target type information when mergeWithTarget set to true', function(t) {
+	var src = { key1: 'value1', key2: 'value2' }
+	class CustomType {}
+	var target = new CustomType()
+
+	var res = merge(target, src, {mergeWithTarget: true})
+	t.not(src instanceof CustomType)
+	t.assert(target instanceof CustomType)
+	t.assert(res instanceof CustomType)
+	t.end()
+})
+
+test('modify target object if mergeWithTarget set to true', function(t) {
+	var src = { key1: 'value1', key2: 'value2' }
+	var target = { key3: 'value3'}
+
+	var clonedRes = merge(target, src)
+	var notClonedRes = merge(target, src, {mergeWithTarget: true})
+
+	t.assert(clonedRes !== target, 'result should be cloned')
+	t.assert(notClonedRes === target, 'result should maintain target reference')
+	t.end()
+})
+
+test('merge.all mutates target object when mergeWithTarget set to true', function(t) {
+	var src = { key1: 'value1', key2: 'value2' }
+	var target = { key3: 'value3'}
+
+	var clonedRes = merge.all([target, src])
+	var notClonedRes = merge.all([target, src], {mergeWithTarget: true})
+
+	t.assert(clonedRes !== target, 'result should be cloned')
+	t.assert(notClonedRes === target, 'result should maintain first array entry reference')
+	t.end()
+})
+
 test('add keys in target that do not exist at the root', function(t) {
 	var src = { key1: 'value1', key2: 'value2' }
 	var target = {}