--- /dev/null
+var eq = require('./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;
+ }
+}
+
+module.exports = assignMergeValue;