diff --git a/lib/core.js b/lib/core.js
index 714334283a5..39db8be98bc 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -211,6 +211,9 @@ declare class $ReadOnlyArray<+T> {
     keys(): Iterator<number>;
     lastIndexOf(searchElement: mixed, fromIndex?: number): number;
     map<U>(callbackfn: (value: T, index: number, array: $ReadOnlyArray<T>) => U, thisArg?: any): Array<U>;
+    flatMap<U>(callbackfn: (value: T, index: number, array: $ReadOnlyArray<T>) => U|U[], thisArg? any): Array<U>;
+    flat(depth?: number): Array<T>;
+
 
     reduce(
       callbackfn: (previousValue: T, currentValue: T, currentIndex: number, array: $ReadOnlyArray<T>) => T,
@@ -245,6 +248,8 @@ declare class Array<T> extends $ReadOnlyArray<T> {
     findIndex(callbackfn: (value: T, index: number, array: Array<T>) => any, thisArg?: any): number;
     forEach(callbackfn: (value: T, index: number, array: Array<T>) => any, thisArg?: any): void;
     map<U>(callbackfn: (value: T, index: number, array: Array<T>) => U, thisArg?: any): Array<U>;
+    flatMap<U>(callbackfn: (value: T, index: number, array: Array<T>) => U|U[], thisArg? any): Array<U>;
+    flat(depth?: number): Array<T>;
     pop(): T;
     push(...items: Array<T>): number;
     reduce(