Allow falsey values to be merged

Fixes #170

changelog.md

@@ -1,3 +1,7 @@
+# [4.2.1](https://github.com/TehShrike/deepmerge/releases/tag/v4.2.1)
+
+- Fix: falsey values can now be merged.  [#170](https://github.com/TehShrike/deepmerge/issues/170)
+
 # [4.2.0](https://github.com/TehShrike/deepmerge/releases/tag/v4.2.0)
 
 - Properties are now only overwritten if they exist on the target object and are enumerable.  [#164](https://github.com/TehShrike/deepmerge/pull/164)

index.js

@@ -36,19 +36,21 @@ function getKeys(target) {
 	return Object.keys(target).concat(getEnumerableOwnPropertySymbols(target))
 }
 
-// Protects from prototype poisoning and unexpected merging up the prototype chain.
-function propertyIsUnsafe(target, key) {
+function propertyIsOnObject(object, property) {
 	try {
-		return (key in target) // Properties are safe to merge if they don't exist in the target yet,
-			&& !(Object.hasOwnProperty.call(target, key) // unsafe if they exist up the prototype chain,
-				&& Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
-	} catch (unused) {
-		// Counterintuitively, it's safe to merge any property on a target that causes the `in` operator to throw.
-		// This happens when trying to copy an object in the source over a plain string in the target.
+		return property in object
+	} catch(_) {
 		return false
 	}
 }
 
+// Protects from prototype poisoning and unexpected merging up the prototype chain.
+function propertyIsUnsafe(target, key) {
+	return propertyIsOnObject(target, key) // Properties are safe to merge if they don't exist in the target yet,
+		&& !(Object.hasOwnProperty.call(target, key) // unsafe if they exist up the prototype chain,
+			&& Object.propertyIsEnumerable.call(target, key)) // and also unsafe if they're nonenumerable.
+}
+
 function mergeObject(target, source, options) {
 	var destination = {}
 	if (options.isMergeableObject(target)) {
@@ -61,7 +63,7 @@ function mergeObject(target, source, options) {
 			return
 		}
 
-		if (!options.isMergeableObject(source[key]) || !target[key]) {
+		if (!options.isMergeableObject(source[key]) || !propertyIsOnObject(target, key)) {
 			destination[key] = cloneUnlessOtherwiseSpecified(source[key], options)
 		} else {
 			destination[key] = getMergeFunction(key, options)(target[key], source[key], options)

readme.md

@@ -2,7 +2,7 @@
 
 Merges the enumerable properties of two or more objects deeply.
 
-> UMD bundle is 667B minified+gzipped
+> UMD bundle is 723B minified+gzipped
 
 ## Getting Started
 

test/merge.js

@@ -637,3 +637,33 @@ test('copy symbol keys in target that do exist on the target', function(t) {
 	t.equal(res[mySymbol], 'value1')
 	t.end()
 })
+
+test('Falsey properties should be mergeable', function(t) {
+	var uniqueValue = {}
+
+	var target = {
+		wat: false
+	}
+
+	var source = {
+		wat: false
+	}
+
+	var customMergeWasCalled = false
+
+	var result = merge(target, source, {
+		isMergeableObject: function() {
+			return true
+		},
+		customMerge: function() {
+			return function() {
+				customMergeWasCalled = true
+				return uniqueValue
+			}
+		}
+	})
+
+	t.equal(result.wat, uniqueValue)
+	t.ok(customMergeWasCalled, 'custom merge function was called')
+	t.end()
+})