diff --git a/src/compiler/core.ts b/src/compiler/core.ts
index 896bf7d849c..c93f6ccb3ab 100644
--- a/src/compiler/core.ts
+++ b/src/compiler/core.ts
@@ -600,25 +600,18 @@ namespace ts {
      *
      * @param array The array to flatten.
      */
-    export function flatten<T>(array: T[][]): T[];
-    export function flatten<T>(array: ReadonlyArray<T | ReadonlyArray<T> | undefined>): T[];
-    export function flatten<T>(array: ReadonlyArray<T | ReadonlyArray<T> | undefined> | undefined): T[] | undefined;
-    export function flatten<T>(array: ReadonlyArray<T | ReadonlyArray<T> | undefined> | undefined): T[] | undefined {
-        let result: T[] | undefined;
-        if (array) {
-            result = [];
-            for (const v of array) {
-                if (v) {
-                    if (isArray(v)) {
-                        addRange(result, v);
-                    }
-                    else {
-                        result.push(v);
-                    }
+    export function flatten<T>(array: T[][] | ReadonlyArray<T | ReadonlyArray<T> | undefined>): T[] {
+        const result = [];
+        for (const v of array) {
+            if (v) {
+                if (isArray(v)) {
+                    addRange(result, v);
+                }
+                else {
+                    result.push(v);
                 }
             }
         }
-
         return result;
     }