Improve code quality and write tests

node_modules/lodash/_listCacheHas.js

@@ -0,0 +1,16 @@
+var assocIndexOf = require('./_assocIndexOf');
+
+/**
+ * Checks if a list cache value for `key` exists.
+ *
+ * @private
+ * @name has
+ * @memberOf ListCache
+ * @param {string} key The key of the entry to check.
+ * @returns {boolean} Returns `true` if an entry for `key` exists, else `false`.
+ */
+function listCacheHas(key) {
+  return assocIndexOf(this.__data__, key) > -1;
+}
+
+module.exports = listCacheHas;