diff --git a/src/lib/es2019.array.d.ts b/src/lib/es2019.array.d.ts
index 9e3e04e58158d..494bfa6b1fbc3 100644
--- a/src/lib/es2019.array.d.ts
+++ b/src/lib/es2019.array.d.ts
@@ -11,9 +11,9 @@ interface ReadonlyArray<T> {
      * thisArg is omitted, undefined is used as the this value.
      */
     flatMap<U, This = undefined> (
-        callback: (this: This, value: T, index: number, array: T[]) => U | ReadonlyArray<U>,
+        callback: (this: This, value: T, index: number, array: T[]) => U,
         thisArg?: This
-    ): U[]
+    ): (U extends readonly (infer V)[] ? V : U)[];
 
 
     /**
@@ -125,9 +125,9 @@ interface Array<T> {
      * thisArg is omitted, undefined is used as the this value.
      */
     flatMap<U, This = undefined> (
-        callback: (this: This, value: T, index: number, array: T[]) => U | ReadonlyArray<U>,
+        callback: (this: This, value: T, index: number, array: T[]) => U,
         thisArg?: This
-    ): U[]
+    ): (U extends readonly (infer V)[] ? V : U)[];
 
     /**
      * Returns a new array with all sub-array elements concatenated into it recursively up to the
diff --git a/tests/baselines/reference/arrayFlatMap.js b/tests/baselines/reference/arrayFlatMap.js
index 3c35cedc1f3be..0344776d9c245 100644
--- a/tests/baselines/reference/arrayFlatMap.js
+++ b/tests/baselines/reference/arrayFlatMap.js
@@ -3,6 +3,11 @@ const array: number[] = [];
 const readonlyArray: ReadonlyArray<number> = [];
 array.flatMap((): ReadonlyArray<number> => []); // ok
 readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+
+// #19535
+
+const [x] = [""].flatMap(undefined as () => string[] | string[][]);
+x == "";
 
 
 //// [arrayFlatMap.js]
@@ -10,3 +15,6 @@ var array = [];
 var readonlyArray = [];
 array.flatMap(function () { return []; }); // ok
 readonlyArray.flatMap(function () { return []; }); // ok
+// #19535
+var x = [""].flatMap(undefined)[0];
+x == "";
diff --git a/tests/baselines/reference/arrayFlatMap.symbols b/tests/baselines/reference/arrayFlatMap.symbols
index b3a8d55721acd..2c08f13dc12d3 100644
--- a/tests/baselines/reference/arrayFlatMap.symbols
+++ b/tests/baselines/reference/arrayFlatMap.symbols
@@ -18,3 +18,14 @@ readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
 >flatMap : Symbol(ReadonlyArray.flatMap, Decl(lib.es2019.array.d.ts, --, --))
 >ReadonlyArray : Symbol(ReadonlyArray, Decl(lib.es5.d.ts, --, --), Decl(lib.es2015.core.d.ts, --, --), Decl(lib.es2015.iterable.d.ts, --, --), Decl(lib.es2016.array.include.d.ts, --, --), Decl(lib.es2019.array.d.ts, --, --))
 
+// #19535
+
+const [x] = [""].flatMap(undefined as () => string[] | string[][]);
+>x : Symbol(x, Decl(arrayFlatMap.ts, 7, 7))
+>[""].flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>flatMap : Symbol(Array.flatMap, Decl(lib.es2019.array.d.ts, --, --))
+>undefined : Symbol(undefined)
+
+x == "";
+>x : Symbol(x, Decl(arrayFlatMap.ts, 7, 7))
+
diff --git a/tests/baselines/reference/arrayFlatMap.types b/tests/baselines/reference/arrayFlatMap.types
index 631ae04ef982f..10a9931a70d71 100644
--- a/tests/baselines/reference/arrayFlatMap.types
+++ b/tests/baselines/reference/arrayFlatMap.types
@@ -9,17 +9,34 @@ const readonlyArray: ReadonlyArray<number> = [];
 
 array.flatMap((): ReadonlyArray<number> => []); // ok
 >array.flatMap((): ReadonlyArray<number> => []) : number[]
->array.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[]
+>array.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >array : number[]
->flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[]
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >(): ReadonlyArray<number> => [] : () => readonly number[]
 >[] : undefined[]
 
 readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
 >readonlyArray.flatMap((): ReadonlyArray<number> => []) : number[]
->readonlyArray.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[]
+>readonlyArray.flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >readonlyArray : readonly number[]
->flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | readonly U[], thisArg?: This) => U[]
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
 >(): ReadonlyArray<number> => [] : () => readonly number[]
 >[] : undefined[]
 
+// #19535
+
+const [x] = [""].flatMap(undefined as () => string[] | string[][]);
+>x : string | string[]
+>[""].flatMap(undefined as () => string[] | string[][]) : (string | string[])[]
+>[""].flatMap : <U, This = undefined>(callback: (this: This, value: string, index: number, array: string[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
+>[""] : string[]
+>"" : ""
+>flatMap : <U, This = undefined>(callback: (this: This, value: string, index: number, array: string[]) => U, thisArg?: This) => (U extends readonly (infer V)[] ? V : U)[]
+>undefined as () => string[] | string[][] : () => string[] | string[][]
+>undefined : undefined
+
+x == "";
+>x == "" : boolean
+>x : string | string[]
+>"" : ""
+
diff --git a/tests/cases/compiler/arrayFlatMap.ts b/tests/cases/compiler/arrayFlatMap.ts
index 98f84e7877363..a02998b9a607f 100644
--- a/tests/cases/compiler/arrayFlatMap.ts
+++ b/tests/cases/compiler/arrayFlatMap.ts
@@ -4,3 +4,8 @@ const array: number[] = [];
 const readonlyArray: ReadonlyArray<number> = [];
 array.flatMap((): ReadonlyArray<number> => []); // ok
 readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+
+// #19535
+
+const [x] = [""].flatMap(undefined as () => string[] | string[][]);
+x == "";