Bump to v4.0.0.

internal/assignMergeValue.js

@@ -0,0 +1,18 @@
+import eq from '../eq';
+
+/**
+ * This function is like `assignValue` except that it doesn't assign `undefined` values.
+ *
+ * @private
+ * @param {Object} object The object to modify.
+ * @param {string} key The key of the property to assign.
+ * @param {*} value The value to assign.
+ */
+function assignMergeValue(object, key, value) {
+  if ((value !== undefined && !eq(object[key], value)) ||
+      (typeof key == 'number' && value === undefined && !(key in object))) {
+    object[key] = value;
+  }
+}
+
+export default assignMergeValue;