--- /dev/null
+/*! *****************************************************************************
+Copyright (c) Microsoft Corporation. All rights reserved.
+Licensed under the Apache License, Version 2.0 (the "License"); you may not use
+this file except in compliance with the License. You may obtain a copy of the
+License at http://www.apache.org/licenses/LICENSE-2.0
+
+THIS CODE IS PROVIDED ON AN *AS IS* BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+KIND, EITHER EXPRESS OR IMPLIED, INCLUDING WITHOUT LIMITATION ANY IMPLIED
+WARRANTIES OR CONDITIONS OF TITLE, FITNESS FOR A PARTICULAR PURPOSE,
+MERCHANTABLITY OR NON-INFRINGEMENT.
+
+See the Apache Version 2.0 License for specific language governing permissions
+and limitations under the License.
+***************************************************************************** */
+
+
+
+/// <reference no-default-lib="true"/>\r
+
+
+type FlatArray<Arr, Depth extends number> = {\r
+ "done": Arr,\r
+ "recur": Arr extends ReadonlyArray<infer InnerArr>\r
+ ? FlatArray<InnerArr, [-1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20][Depth]>\r
+ : Arr\r
+}[Depth extends -1 ? "done" : "recur"];\r
+\r
+interface ReadonlyArray<T> {\r
+\r
+ /**\r
+ * Calls a defined callback function on each element of an array. Then, flattens the result into\r
+ * a new array.\r
+ * This is identical to a map followed by flat with depth 1.\r
+ *\r
+ * @param callback A function that accepts up to three arguments. The flatMap method calls the\r
+ * callback function one time for each element in the array.\r
+ * @param thisArg An object to which the this keyword can refer in the callback function. If\r
+ * thisArg is omitted, undefined is used as the this value.\r
+ */\r
+ flatMap<U, This = undefined> (\r
+ callback: (this: This, value: T, index: number, array: T[]) => U | ReadonlyArray<U>,\r
+ thisArg?: This\r
+ ): U[]\r
+\r
+\r
+ /**\r
+ * Returns a new array with all sub-array elements concatenated into it recursively up to the\r
+ * specified depth.\r
+ *\r
+ * @param depth The maximum recursion depth\r
+ */\r
+ flat<A, D extends number = 1>(\r
+ this: A,\r
+ depth?: D\r
+ ): FlatArray<A, D>[]\r
+ }\r
+\r
+interface Array<T> {\r
+\r
+ /**\r
+ * Calls a defined callback function on each element of an array. Then, flattens the result into\r
+ * a new array.\r
+ * This is identical to a map followed by flat with depth 1.\r
+ *\r
+ * @param callback A function that accepts up to three arguments. The flatMap method calls the\r
+ * callback function one time for each element in the array.\r
+ * @param thisArg An object to which the this keyword can refer in the callback function. If\r
+ * thisArg is omitted, undefined is used as the this value.\r
+ */\r
+ flatMap<U, This = undefined> (\r
+ callback: (this: This, value: T, index: number, array: T[]) => U | ReadonlyArray<U>,\r
+ thisArg?: This\r
+ ): U[]\r
+\r
+ /**\r
+ * Returns a new array with all sub-array elements concatenated into it recursively up to the\r
+ * specified depth.\r
+ *\r
+ * @param depth The maximum recursion depth\r
+ */\r
+ flat<A, D extends number = 1>(\r
+ this: A,\r
+ depth?: D\r
+ ): FlatArray<A, D>[]\r
+}\r