Allow exports assignments (#23319)

1. Allow assignment to `exports`.
2. The type of the rhs is not checked against the type of `exports`
since they are aliased declarations.

To support more complex patterns like `exports = c.name = c`, we may
have to treat `c.name` as a declaration. That will be more complicated
than this PR.

src/compiler/checker.ts

@@ -13882,7 +13882,8 @@ namespace ts {
             const assignmentKind = getAssignmentTargetKind(node);
 
             if (assignmentKind) {
-                if (!(localOrExportSymbol.flags & SymbolFlags.Variable)) {
+                if (!(localOrExportSymbol.flags & SymbolFlags.Variable) &&
+                    !(isInJavaScriptFile(node) && localOrExportSymbol.flags & SymbolFlags.ValueModule)) {
                     error(node, Diagnostics.Cannot_assign_to_0_because_it_is_not_a_variable, symbolToString(symbol));
                     return unknownType;
                 }
@@ -19858,8 +19859,9 @@ namespace ts {
                     //    VarExpr = ValueExpr
                     // requires VarExpr to be classified as a reference
                     // A compound assignment furthermore requires VarExpr to be classified as a reference (section 4.1)
-                    // and the type of the non - compound operation to be assignable to the type of VarExpr.
-                    if (checkReferenceExpression(left, Diagnostics.The_left_hand_side_of_an_assignment_expression_must_be_a_variable_or_a_property_access)) {
+                    // and the type of the non-compound operation to be assignable to the type of VarExpr.
+                    if (checkReferenceExpression(left, Diagnostics.The_left_hand_side_of_an_assignment_expression_must_be_a_variable_or_a_property_access)
+                        && (!isIdentifier(left) || unescapeLeadingUnderscores(left.escapedText) !== "exports")) {
                         // to avoid cascading errors check assignability only if 'isReference' check succeeded and no errors were reported
                         checkTypeAssignableTo(valueType, leftType, left, /*headMessage*/ undefined);
                     }

tests/baselines/reference/exportNestedNamespaces2.errors.txt

@@ -1,7 +1,5 @@
-tests/cases/conformance/salsa/first.js(1,1): error TS2539: Cannot assign to '"tests/cases/conformance/salsa/first"' because it is not a variable.
 tests/cases/conformance/salsa/first.js(1,11): error TS2304: Cannot find name 'require'.
 tests/cases/conformance/salsa/first.js(2,9): error TS2339: Property 'formatters' does not exist on type 'typeof import("tests/cases/conformance/salsa/first")'.
-tests/cases/conformance/salsa/second.js(1,1): error TS2539: Cannot assign to '"tests/cases/conformance/salsa/second"' because it is not a variable.
 tests/cases/conformance/salsa/second.js(1,11): error TS2304: Cannot find name 'require'.
 tests/cases/conformance/salsa/second.js(2,9): error TS2339: Property 'formatters' does not exist on type 'typeof import("tests/cases/conformance/salsa/second")'.
 
@@ -9,10 +7,8 @@ tests/cases/conformance/salsa/second.js(2,9): error TS2339: Property 'formatters
 ==== tests/cases/conformance/salsa/mod.js (0 errors) ====
     // Based on a pattern from adonis
     exports.formatters = {}
-==== tests/cases/conformance/salsa/first.js (3 errors) ====
+==== tests/cases/conformance/salsa/first.js (2 errors) ====
     exports = require('./mod')
-    ~~~~~~~
-!!! error TS2539: Cannot assign to '"tests/cases/conformance/salsa/first"' because it is not a variable.
               ~~~~~~~
 !!! error TS2304: Cannot find name 'require'.
     exports.formatters.j = function (v) {
@@ -20,10 +16,8 @@ tests/cases/conformance/salsa/second.js(2,9): error TS2339: Property 'formatters
 !!! error TS2339: Property 'formatters' does not exist on type 'typeof import("tests/cases/conformance/salsa/first")'.
         return v
     }
-==== tests/cases/conformance/salsa/second.js (3 errors) ====
+==== tests/cases/conformance/salsa/second.js (2 errors) ====
     exports = require('./mod')
-    ~~~~~~~
-!!! error TS2539: Cannot assign to '"tests/cases/conformance/salsa/second"' because it is not a variable.
               ~~~~~~~
 !!! error TS2304: Cannot find name 'require'.
     exports.formatters.o = function (v) {

tests/baselines/reference/exportNestedNamespaces2.types

@@ -10,7 +10,7 @@ exports.formatters = {}
 === tests/cases/conformance/salsa/first.js ===
 exports = require('./mod')
 >exports = require('./mod') : typeof import("tests/cases/conformance/salsa/mod")
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/first")
 >require('./mod') : typeof import("tests/cases/conformance/salsa/mod")
 >require : any
 >'./mod' : "./mod"
@@ -31,7 +31,7 @@ exports.formatters.j = function (v) {
 === tests/cases/conformance/salsa/second.js ===
 exports = require('./mod')
 >exports = require('./mod') : typeof import("tests/cases/conformance/salsa/mod")
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/second")
 >require('./mod') : typeof import("tests/cases/conformance/salsa/mod")
 >require : any
 >'./mod' : "./mod"

tests/baselines/reference/moduleExportAlias.types

@@ -147,7 +147,7 @@ module.exports.func4 = function () { };
 var multipleDeclarationAlias1 = exports = module.exports;
 >multipleDeclarationAlias1 : any
 >exports = module.exports : any
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >module.exports : any
 >module : any
 >exports : any
@@ -212,7 +212,7 @@ var multipleDeclarationAlias5 = module.exports = exports = {};
 >module : any
 >exports : any
 >exports = {} : {}
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >{} : {}
 
 multipleDeclarationAlias5.func9 = function () { };
@@ -225,7 +225,7 @@ multipleDeclarationAlias5.func9 = function () { };
 var multipleDeclarationAlias6 = exports = module.exports = {};
 >multipleDeclarationAlias6 : { [x: string]: any; }
 >exports = module.exports = {} : { [x: string]: any; }
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >module.exports = {} : { [x: string]: any; }
 >module.exports : any
 >module : any
@@ -241,7 +241,7 @@ multipleDeclarationAlias6.func10 = function () { };
 
 exports = module.exports = someOtherVariable = {};
 >exports = module.exports = someOtherVariable = {} : {}
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >module.exports = someOtherVariable = {} : {}
 >module.exports : any
 >module : any
@@ -268,7 +268,7 @@ module.exports.func12 = function () { };
 
 exports = module.exports = someOtherVariable = {};
 >exports = module.exports = someOtherVariable = {} : {}
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >module.exports = someOtherVariable = {} : {}
 >module.exports : any
 >module : any
@@ -295,7 +295,7 @@ module.exports.func12 = function () { };
 
 exports = module.exports = {};
 >exports = module.exports = {} : { [x: string]: any; }
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >module.exports = {} : { [x: string]: any; }
 >module.exports : any
 >module : any
@@ -320,7 +320,7 @@ module.exports.func14 = function () { };
 
 exports = module.exports = {};
 >exports = module.exports = {} : { [x: string]: any; }
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >module.exports = {} : { [x: string]: any; }
 >module.exports : any
 >module : any
@@ -349,7 +349,7 @@ module.exports = exports = {};
 >module : any
 >exports : any
 >exports = {} : {}
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/b")
 >{} : {}
 
 exports.func17 = function () { };

tests/baselines/reference/moduleExportAlias2.symbols

@@ -0,0 +1,51 @@
+=== tests/cases/conformance/salsa/index.js ===
+/// <reference path='node.d.ts' />
+const C = require("./semver")
+>C : Symbol(C, Decl(index.js, 1, 5))
+>require : Symbol(require, Decl(node.d.ts, 0, 0))
+>"./semver" : Symbol("tests/cases/conformance/salsa/semver", Decl(semver.js, 0, 0))
+
+var two = C.f(1)
+>two : Symbol(two, Decl(index.js, 2, 3))
+>C.f : Symbol(f, Decl(semver.js, 1, 28))
+>C : Symbol(C, Decl(index.js, 1, 5))
+>f : Symbol(f, Decl(semver.js, 1, 28))
+
+var c = new C
+>c : Symbol(c, Decl(index.js, 3, 3))
+>C : Symbol(C, Decl(index.js, 1, 5))
+
+=== tests/cases/conformance/salsa/node.d.ts ===
+declare function require(name: string): any;
+>require : Symbol(require, Decl(node.d.ts, 0, 0))
+>name : Symbol(name, Decl(node.d.ts, 0, 25))
+
+declare var exports: any;
+>exports : Symbol(exports, Decl(node.d.ts, 1, 11))
+
+declare var module: { exports: any };
+>module : Symbol(module, Decl(node.d.ts, 2, 11))
+>exports : Symbol(exports, Decl(node.d.ts, 2, 21))
+
+=== tests/cases/conformance/salsa/semver.js ===
+/// <reference path='node.d.ts' />
+exports = module.exports = C
+>exports : Symbol("tests/cases/conformance/salsa/semver", Decl(semver.js, 0, 0))
+>module.exports : Symbol(exports, Decl(node.d.ts, 2, 21))
+>module : Symbol(export=, Decl(semver.js, 1, 9))
+>exports : Symbol(export=, Decl(semver.js, 1, 9))
+>C : Symbol(C, Decl(semver.js, 2, 22))
+
+exports.f = n => n + 1
+>exports.f : Symbol(f, Decl(semver.js, 1, 28))
+>exports : Symbol(f, Decl(semver.js, 1, 28))
+>f : Symbol(f, Decl(semver.js, 1, 28))
+>n : Symbol(n, Decl(semver.js, 2, 11))
+>n : Symbol(n, Decl(semver.js, 2, 11))
+
+function C() {
+>C : Symbol(C, Decl(semver.js, 2, 22))
+
+    this.p = 1
+>p : Symbol(C.p, Decl(semver.js, 3, 14))
+}

tests/baselines/reference/moduleExportAlias2.types

@@ -0,0 +1,65 @@
+=== tests/cases/conformance/salsa/index.js ===
+/// <reference path='node.d.ts' />
+const C = require("./semver")
+>C : typeof C
+>require("./semver") : typeof C
+>require : (name: string) => any
+>"./semver" : "./semver"
+
+var two = C.f(1)
+>two : any
+>C.f(1) : any
+>C.f : (n: any) => any
+>C : typeof C
+>f : (n: any) => any
+>1 : 1
+
+var c = new C
+>c : C
+>new C : C
+>C : typeof C
+
+=== tests/cases/conformance/salsa/node.d.ts ===
+declare function require(name: string): any;
+>require : (name: string) => any
+>name : string
+
+declare var exports: any;
+>exports : any
+
+declare var module: { exports: any };
+>module : { exports: any; }
+>exports : any
+
+=== tests/cases/conformance/salsa/semver.js ===
+/// <reference path='node.d.ts' />
+exports = module.exports = C
+>exports = module.exports = C : typeof C
+>exports : typeof import("tests/cases/conformance/salsa/semver")
+>module.exports = C : typeof C
+>module.exports : any
+>module : { exports: any; }
+>exports : any
+>C : typeof C
+
+exports.f = n => n + 1
+>exports.f = n => n + 1 : (n: any) => any
+>exports.f : (n: any) => any
+>exports : typeof import("tests/cases/conformance/salsa/semver")
+>f : (n: any) => any
+>n => n + 1 : (n: any) => any
+>n : any
+>n + 1 : any
+>n : any
+>1 : 1
+
+function C() {
+>C : typeof C
+
+    this.p = 1
+>this.p = 1 : 1
+>this.p : any
+>this : any
+>p : any
+>1 : 1
+}

tests/baselines/reference/typeFromPropertyAssignment19.errors.txt

@@ -1,20 +0,0 @@
-tests/cases/conformance/salsa/semver.js(2,1): error TS2539: Cannot assign to '"tests/cases/conformance/salsa/semver"' because it is not a variable.
-
-
-==== tests/cases/conformance/salsa/index.js (0 errors) ====
-    /// <reference path='./types.d.ts'/>
-    const C = require("./semver")
-    var two = C.f(1)
-    
-==== tests/cases/conformance/salsa/types.d.ts (0 errors) ====
-    declare var require: any;
-    declare var module: any;
-==== tests/cases/conformance/salsa/semver.js (1 errors) ====
-    /// <reference path='./types.d.ts'/>
-    exports = module.exports = C
-    ~~~~~~~
-!!! error TS2539: Cannot assign to '"tests/cases/conformance/salsa/semver"' because it is not a variable.
-    C.f = n => n + 1
-    function C() {
-        this.p = 1
-    }

tests/baselines/reference/typeFromPropertyAssignment19.types

@@ -25,7 +25,7 @@ declare var module: any;
 /// <reference path='./types.d.ts'/>
 exports = module.exports = C
 >exports = module.exports = C : typeof C
->exports : any
+>exports : typeof import("tests/cases/conformance/salsa/semver")
 >module.exports = C : typeof C
 >module.exports : any
 >module : any

tests/cases/conformance/salsa/moduleExportAlias2.ts

@@ -0,0 +1,19 @@
+// @checkJs: true
+// @allowJS: true
+// @noEmit: true
+// @Filename: node.d.ts
+declare function require(name: string): any;
+declare var exports: any;
+declare var module: { exports: any };
+// @Filename: semver.js
+/// <reference path='node.d.ts' />
+exports = module.exports = C
+exports.f = n => n + 1
+function C() {
+    this.p = 1
+}
+// @filename: index.js
+/// <reference path='node.d.ts' />
+const C = require("./semver")
+var two = C.f(1)
+var c = new C