Bump to v4.0.0.

internal/isKeyable.js

@@ -0,0 +1,14 @@
+/**
+ * Checks if `value` is suitable for use as unique object key.
+ *
+ * @private
+ * @param {*} value The value to check.
+ * @returns {boolean} Returns `true` if `value` is suitable, else `false`.
+ */
+function isKeyable(value) {
+  var type = typeof value;
+  return type == 'number' || type == 'boolean' ||
+    (type == 'string' && value !== '__proto__') || value == null;
+}
+
+export default isKeyable;