Better typings for Array.map()

src/lib/es5.d.ts

@@ -1138,7 +1138,7 @@ interface ReadonlyArray<T> {
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
-    map<U>(callbackfn: (value: T, index: number, array: readonly T[]) => U, thisArg?: any): U[];
+    map<U>(callbackfn: (value: T, index: number, array: readonly T[]) => U, thisArg?: any): { -readonly [P in keyof this]: U } extends U[] ? { -readonly [P in keyof this]: U } : U[];
     /**
      * Returns the elements of an array that meet the condition specified in a callback function.
      * @param callbackfn A function that accepts up to three arguments. The filter method calls the callbackfn function one time for each element in the array.
@@ -1308,7 +1308,7 @@ interface Array<T> {
      * @param callbackfn A function that accepts up to three arguments. The map method calls the callbackfn function one time for each element in the array.
      * @param thisArg An object to which the this keyword can refer in the callbackfn function. If thisArg is omitted, undefined is used as the this value.
      */
-    map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): U[];
+    map<U>(callbackfn: (value: T, index: number, array: T[]) => U, thisArg?: any): { -readonly [P in keyof this]: U } extends U[] ? { -readonly [P in keyof this]: U } : U[];
     /**
      * Returns the elements of an array that meet the condition specified in a callback function.
      * @param callbackfn A function that accepts up to three arguments. The filter method calls the callbackfn function one time for each element in the array.

tests/baselines/reference/arityAndOrderCompatibility01.errors.txt

@@ -1,4 +1,5 @@
 tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(15,12): error TS2493: Tuple type '[string, number]' of length '2' has no element at index '2'.
+tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(16,5): error TS2461: Type 'StrNum' is not an array type.
 tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(17,5): error TS2461: Type '{ 0: string; 1: number; length: 2; }' is not an array type.
 tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(18,5): error TS2741: Property '2' is missing in type '[string, number]' but required in type '[number, number, number]'.
 tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(19,5): error TS2741: Property '2' is missing in type 'StrNum' but required in type '[number, number, number]'.
@@ -28,7 +29,7 @@ tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(31,5): error
 tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(32,5): error TS2322: Type '{ 0: string; 1: number; length: 2; }' is not assignable to type '[number, string]'.
 
 
-==== tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts (17 errors) ====
+==== tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts (18 errors) ====
     interface StrNum extends Array<string|number> {
         0: string;
         1: number;
@@ -47,6 +48,8 @@ tests/cases/conformance/types/tuple/arityAndOrderCompatibility01.ts(32,5): error
                ~
 !!! error TS2493: Tuple type '[string, number]' of length '2' has no element at index '2'.
     var [d, e, f] = y;
+        ~~~~~~~~~
+!!! error TS2461: Type 'StrNum' is not an array type.
     var [g, h, i] = z;
         ~~~~~~~~~
 !!! error TS2461: Type '{ 0: string; 1: number; length: 2; }' is not an array type.

tests/baselines/reference/arityAndOrderCompatibility01.types

@@ -36,9 +36,9 @@ var [a, b, c] = x;
 >x : [string, number]
 
 var [d, e, f] = y;
->d : string
->e : number
->f : string | number
+>d : any
+>e : any
+>f : any
 >y : StrNum
 
 var [g, h, i] = z;

tests/baselines/reference/arrayOfSubtypeIsAssignableToReadonlyArray.errors.txt

@@ -1,12 +1,29 @@
 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(13,1): error TS2322: Type 'A[]' is not assignable to type 'readonly B[]'.
   Property 'b' is missing in type 'A' but required in type 'B'.
+tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(15,1): error TS2322: Type 'C<A>' is not assignable to type 'readonly A[]'.
+  The types returned by 'map(...)' are incompatible between these types.
+    Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]' is not assignable to type 'U[]'.
+      Type 'U[] | { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+        Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+          Types of property 'length' are incompatible.
+            Type 'U' is not assignable to type 'number'.
+tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(16,1): error TS2322: Type 'C<B>' is not assignable to type 'readonly A[]'.
+  The types returned by 'map(...)' are incompatible between these types.
+    Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]' is not assignable to type 'U[]'.
+      Type 'U[] | { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+        Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+          Types of property 'length' are incompatible.
+            Type 'U' is not assignable to type 'number'.
+tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(17,1): error TS2322: Type 'C<B>' is not assignable to type 'readonly B[]'.
+  The types returned by 'map(...)' are incompatible between these types.
+    Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]' is not assignable to type 'U[]'.
 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error TS2322: Type 'C<A>' is not assignable to type 'readonly B[]'.
   The types returned by 'concat(...)' are incompatible between these types.
     Type 'A[]' is not assignable to type 'B[]'.
       Type 'A' is not assignable to type 'B'.
 
 
-==== tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts (2 errors) ====
+==== tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts (5 errors) ====
     class A { a }
     class B extends A { b }
     class C<T> extends Array<T> { c }
@@ -26,8 +43,28 @@ tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts(18,1): error T
 !!! related TS2728 tests/cases/compiler/arrayOfSubtypeIsAssignableToReadonlyArray.ts:2:21: 'b' is declared here.
 
     rra = cra;
+    ~~~
+!!! error TS2322: Type 'C<A>' is not assignable to type 'readonly A[]'.
+!!! error TS2322:   The types returned by 'map(...)' are incompatible between these types.
+!!! error TS2322:     Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]' is not assignable to type 'U[]'.
+!!! error TS2322:       Type 'U[] | { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+!!! error TS2322:         Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+!!! error TS2322:           Types of property 'length' are incompatible.
+!!! error TS2322:             Type 'U' is not assignable to type 'number'.
     rra = crb; // OK, C<B> is assignable to ReadonlyArray<A>
+    ~~~
+!!! error TS2322: Type 'C<B>' is not assignable to type 'readonly A[]'.
+!!! error TS2322:   The types returned by 'map(...)' are incompatible between these types.
+!!! error TS2322:     Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]' is not assignable to type 'U[]'.
+!!! error TS2322:       Type 'U[] | { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+!!! error TS2322:         Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; }' is not assignable to type 'U[]'.
+!!! error TS2322:           Types of property 'length' are incompatible.
+!!! error TS2322:             Type 'U' is not assignable to type 'number'.
     rrb = crb;
+    ~~~
+!!! error TS2322: Type 'C<B>' is not assignable to type 'readonly B[]'.
+!!! error TS2322:   The types returned by 'map(...)' are incompatible between these types.
+!!! error TS2322:     Type '{ [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; c: U; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]' is not assignable to type 'U[]'.
     rrb = cra; // error: 'A' is not assignable to 'B'
     ~~~
 !!! error TS2322: Type 'C<A>' is not assignable to type 'readonly B[]'.

tests/baselines/reference/bestChoiceType.errors.txt

@@ -0,0 +1,36 @@
+tests/cases/compiler/bestChoiceType.ts(3,23): error TS2349: This expression is not callable.
+  Each member of the union type '(<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]) | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.
+tests/cases/compiler/bestChoiceType.ts(10,15): error TS2349: This expression is not callable.
+  Each member of the union type '(<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]) | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.
+tests/cases/compiler/bestChoiceType.ts(16,15): error TS2349: This expression is not callable.
+  Each member of the union type '(<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]) | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.
+
+
+==== tests/cases/compiler/bestChoiceType.ts (3 errors) ====
+    // Repro from #10041
+
+    (''.match(/ /) || []).map(s => s.toLowerCase());
+                          ~~~
+!!! error TS2349: This expression is not callable.
+!!! error TS2349:   Each member of the union type '(<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]) | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.
+
+    // Similar cases
+
+    function f1() {
+        let x = ''.match(/ /);
+        let y = x || [];
+        let z = y.map(s => s.toLowerCase());
+                  ~~~
+!!! error TS2349: This expression is not callable.
+!!! error TS2349:   Each member of the union type '(<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]) | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.
+    }
+
+    function f2() {
+        let x = ''.match(/ /);
+        let y = x ? x : [];
+        let z = y.map(s => s.toLowerCase());
+                  ~~~
+!!! error TS2349: This expression is not callable.
+!!! error TS2349:   Each member of the union type '(<U>(callbackfn: (value: string, index: number, array: string[]) => U, thisArg?: any) => { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } extends U[] ? { [x: number]: U; index?: U | undefined; input?: U | undefined; length: U; toString: U; toLocaleString: U; pop: U; push: U; concat: U; join: U; reverse: U; shift: U; slice: U; sort: U; splice: U; unshift: U; indexOf: U; lastIndexOf: U; every: U; some: U; forEach: U; map: U; filter: U; reduce: U; reduceRight: U; } : U[]) | (<U>(callbackfn: (value: never, index: number, array: never[]) => U, thisArg?: any) => U[])' has signatures, but none of those signatures are compatible with each other.
+    }
+