Bump to v4.0.0.

internal/indexKeys.js

@@ -0,0 +1,22 @@
+import baseTimes from './baseTimes';
+import isArguments from '../isArguments';
+import isArray from '../isArray';
+import isLength from '../isLength';
+import isString from '../isString';
+
+/**
+ * Creates an array of index keys for `object` values of arrays,
+ * `arguments` objects, and strings, otherwise `null` is returned.
+ *
+ * @private
+ * @param {Object} object The object to query.
+ * @returns {Array|null} Returns index keys, else `null`.
+ */
+function indexKeys(object) {
+  var length = object ? object.length : undefined;
+  return (isLength(length) && (isArray(object) || isString(object) || isArguments(object)))
+    ? baseTimes(length, String)
+    : null;
+}
+
+export default indexKeys;