Merge pull request #26112 from rhys-vdw/22685-array-flat-fix

Fix issue with Array#flatMap callback return type

Author: RyanCavanaugh

src/lib/esnext.array.d.ts

@@ -11,7 +11,7 @@ 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|U[],
+        callback: (this: This, value: T, index: number, array: T[]) => U|ReadonlyArray<U>,
         thisArg?: This
     ): U[]
 
@@ -125,7 +125,7 @@ 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|U[],
+        callback: (this: This, value: T, index: number, array: T[]) => U|ReadonlyArray<U>,
         thisArg?: This
     ): U[]
 

tests/baselines/reference/arrayFlatMap.js

@@ -0,0 +1,12 @@
+//// [arrayFlatMap.ts]
+const array: number[] = [];
+const readonlyArray: ReadonlyArray<number> = [];
+array.flatMap((): ReadonlyArray<number> => []); // ok
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+
+
+//// [arrayFlatMap.js]
+var array = [];
+var readonlyArray = [];
+array.flatMap(function () { return []; }); // ok
+readonlyArray.flatMap(function () { return []; }); // ok

tests/baselines/reference/arrayFlatMap.symbols

@@ -0,0 +1,20 @@
+=== tests/cases/compiler/arrayFlatMap.ts ===
+const array: number[] = [];
+>array : Symbol(array, Decl(arrayFlatMap.ts, 0, 5))
+
+const readonlyArray: ReadonlyArray<number> = [];
+>readonlyArray : Symbol(readonlyArray, Decl(arrayFlatMap.ts, 1, 5))
+>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.esnext.array.d.ts, --, --))
+
+array.flatMap((): ReadonlyArray<number> => []); // ok
+>array.flatMap : Symbol(Array.flatMap, Decl(lib.esnext.array.d.ts, --, --))
+>array : Symbol(array, Decl(arrayFlatMap.ts, 0, 5))
+>flatMap : Symbol(Array.flatMap, Decl(lib.esnext.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.esnext.array.d.ts, --, --))
+
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok
+>readonlyArray.flatMap : Symbol(ReadonlyArray.flatMap, Decl(lib.esnext.array.d.ts, --, --))
+>readonlyArray : Symbol(readonlyArray, Decl(arrayFlatMap.ts, 1, 5))
+>flatMap : Symbol(ReadonlyArray.flatMap, Decl(lib.esnext.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.esnext.array.d.ts, --, --))
+

tests/baselines/reference/arrayFlatMap.types

@@ -0,0 +1,25 @@
+=== tests/cases/compiler/arrayFlatMap.ts ===
+const array: number[] = [];
+>array : number[]
+>[] : undefined[]
+
+const readonlyArray: ReadonlyArray<number> = [];
+>readonlyArray : ReadonlyArray<number>
+>[] : undefined[]
+
+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 | ReadonlyArray<U>, thisArg?: This) => U[]
+>array : number[]
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | ReadonlyArray<U>, thisArg?: This) => U[]
+>(): ReadonlyArray<number> => [] : () => ReadonlyArray<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 | ReadonlyArray<U>, thisArg?: This) => U[]
+>readonlyArray : ReadonlyArray<number>
+>flatMap : <U, This = undefined>(callback: (this: This, value: number, index: number, array: number[]) => U | ReadonlyArray<U>, thisArg?: This) => U[]
+>(): ReadonlyArray<number> => [] : () => ReadonlyArray<number>
+>[] : undefined[]
+

tests/cases/compiler/arrayFlatMap.ts

@@ -0,0 +1,6 @@
+// @lib: esnext
+
+const array: number[] = [];
+const readonlyArray: ReadonlyArray<number> = [];
+array.flatMap((): ReadonlyArray<number> => []); // ok
+readonlyArray.flatMap((): ReadonlyArray<number> => []); // ok