From 3b66e4cad6c5e9f6b1e0b8d7c3ec8a085a7c0dd1 Mon Sep 17 00:00:00 2001
From: Aaron Stone <c-aaron.stone@charter.com>
Date: Thu, 12 Nov 2020 12:42:49 -0700
Subject: [PATCH] Handle undefined target

---
 index.js      | 14 +++++++++-----
 test/merge.js | 12 ++++++++++++
 2 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/index.js b/index.js
index 1f614f1..962a6d9 100644
--- a/index.js
+++ b/index.js
@@ -56,16 +56,20 @@ function propertyIsUnsafe(target, key) {
 }
 
 // Retrieves either a new object or the appropriate target object to mutate.
-function getDestinationObject(target, options) {
-	if (options && options.mergeWithTarget) {
+function getDestinationObject(target, source, options) {
+	const targetDefined = typeof target !== 'undefined'
+	const isArray = Array.isArray(target) || Array.isArray(source)
+	const doMerge = options && (options.mergeWithTarget || options.clone === false)
+
+	if (targetDefined && doMerge) {
 		return Array.isArray(target) ? firstArrayEntry(target) : target
 	}
 
-	return {}
+	return isArray ? [] : {};
 }
 
 function mergeObject(target, source, options) {
-	var destination = getDestinationObject(target, options)
+	var destination = getDestinationObject(target, source, options)
 	if (options.isMergeableObject(target)) {
 		getKeys(target).forEach(function(key) {
 			destination[key] = cloneUnlessOtherwiseSpecified(target[key], options)
@@ -113,7 +117,7 @@ deepmerge.all = function deepmergeAll(array, options) {
 
 	return array.reduce(function(prev, next) {
 		return deepmerge(prev, next, options)
-	}, getDestinationObject(array, options))
+	}, getDestinationObject(array, undefined, options))
 }
 
 module.exports = deepmerge
diff --git a/test/merge.js b/test/merge.js
index c37b1fc..9cd3801 100644
--- a/test/merge.js
+++ b/test/merge.js
@@ -1,6 +1,18 @@
 var merge = require('../')
 var test = require('tape')
 
+test('should handle an undefined value in the target object when merging', function(t) {
+	var src = { key1: 'value1', key2: { key4: 'value4'}, key3: ['value3'] }
+	var target = { key1: 'value', key2: undefined, key3: undefined }
+
+	var notClonedRes = merge(target, src, {mergeWithTarget: true})
+
+	t.assert(notClonedRes === target, 'should successfully merge mutating target');
+	t.assert(notClonedRes.key2 instanceof Object, 'should successfully merge object');
+	t.assert(Array.isArray(notClonedRes.key3), 'should successfully merge array');
+	t.end()
+})
+
 test('result should retain target type information when mergeWithTarget set to true', function(t) {
 	var src = { key1: 'value1', key2: 'value2' }
 	class CustomType {}