Add a helper function getOrUpdateProperty to prevent unprotected access to Maps.

src/compiler/core.ts

@@ -323,8 +323,12 @@ namespace ts {
         return keys;
     }
 
-    export function getProperty<T>(map: Map<T>, key: string): T {
-        return hasOwnProperty.call(map, key) ? map[key] : undefined;
+    export function getProperty<T>(map: Map<T>, key: string): T | undefined {
+        return hasProperty(map, key) ? map[key] : undefined;
+    }
+
+    export function getOrUpdateProperty<T>(map: Map<T>, key: string, makeValue: () => T): T {
+        return hasProperty(map, key) ? map[key] : map[key] = makeValue();
     }
 
     export function isEmpty<T>(map: Map<T>) {
@@ -941,7 +945,7 @@ namespace ts {
      *  [^./]                   # matches everything up to the first . character (excluding directory seperators)
      *  (\\.(?!min\\.js$))?     # matches . characters but not if they are part of the .min.js file extension
      */
-    const singleAsteriskRegexFragmentFiles = "([^./]|(\\.(?!min\\.js$))?)*";
+    const singleAsteriskRegexFragmentFiles = "([^./]|(\\.(?!min\\.js$))?)*";
     const singleAsteriskRegexFragmentOther = "[^/]*";
 
     export function getRegularExpressionForWildcard(specs: string[], basePath: string, usage: "files" | "directories" | "exclude") {

src/compiler/emitter.ts

@@ -6842,7 +6842,7 @@ const _super = (function (geti, seti) {
                                 // export { x, y }
                                 for (const specifier of (<ExportDeclaration>node).exportClause.elements) {
                                     const name = (specifier.propertyName || specifier.name).text;
-                                    (exportSpecifiers[name] || (exportSpecifiers[name] = [])).push(specifier);
+                                    getOrUpdateProperty(exportSpecifiers, name, () => []).push(specifier);
                                 }
                             }
                             break;

tests/baselines/reference/exportToString.js

@@ -0,0 +1,9 @@
+//// [exportToString.ts]
+const toString = 0;
+export { toString };
+
+
+//// [exportToString.js]
+"use strict";
+var toString = 0;
+exports.toString = toString;

tests/baselines/reference/exportToString.symbols

@@ -0,0 +1,7 @@
+=== tests/cases/compiler/exportToString.ts ===
+const toString = 0;
+>toString : Symbol(toString, Decl(exportToString.ts, 0, 5))
+
+export { toString };
+>toString : Symbol(toString, Decl(exportToString.ts, 1, 8))
+

tests/baselines/reference/exportToString.types

@@ -0,0 +1,8 @@
+=== tests/cases/compiler/exportToString.ts ===
+const toString = 0;
+>toString : number
+>0 : number
+
+export { toString };
+>toString : number
+

tests/cases/compiler/exportToString.ts

@@ -0,0 +1,2 @@
+const toString = 0;
+export { toString };