Bump to v4.0.0.

internal/getTag.js

@@ -0,0 +1,55 @@
+import Map from './Map';
+import Set from './Set';
+
+/** `Object#toString` result references. */
+var mapTag = '[object Map]',
+    objectTag = '[object Object]',
+    setTag = '[object Set]';
+
+/** Used for built-in method references. */
+var objectProto = Object.prototype;
+
+/** Used to resolve the decompiled source of functions. */
+var funcToString = Function.prototype.toString;
+
+/**
+ * Used to resolve the [`toStringTag`](http://ecma-international.org/ecma-262/6.0/#sec-object.prototype.tostring)
+ * of values.
+ */
+var objectToString = objectProto.toString;
+
+/** Used to detect maps and sets. */
+var mapCtorString = Map ? funcToString.call(Map) : '',
+    setCtorString = Set ? funcToString.call(Set) : '';
+
+/**
+ * Gets the `toStringTag` of `value`.
+ *
+ * @private
+ * @param {*} value The value to query.
+ * @returns {string} Returns the `toStringTag`.
+ */
+function getTag(value) {
+  return objectToString.call(value);
+}
+
+// Fallback for IE 11 providing `toStringTag` values for maps and sets.
+if ((Map && getTag(new Map) != mapTag) || (Set && getTag(new Set) != setTag)) {
+  getTag = function(value) {
+    var result = objectToString.call(value),
+        Ctor = result == objectTag ? value.constructor : null,
+        ctorString = typeof Ctor == 'function' ? funcToString.call(Ctor) : '';
+
+    if (ctorString) {
+      if (ctorString == mapCtorString) {
+        return mapTag;
+      }
+      if (ctorString == setCtorString) {
+        return setTag;
+      }
+    }
+    return result;
+  };
+}
+
+export default getTag;