Address PR

Author: Yui T

src/compiler/emitter.ts

@@ -3889,6 +3889,10 @@ var __awaiter = (this && this.__awaiter) || function (thisArg, _arguments, P, ge
                         // We create a synthetic copy of the identifier in order to avoid the rewriting that might
                         // otherwise occur when the identifier is emitted.
                         index = <Identifier | LiteralExpression>createSynthesizedNode(propName.kind);
+                        // We need to escape identifier here because when parsing an identifier prefixing with "__"
+                        // the parser need to append "_" in order to escape colliding with magic identifiers such as "__proto__"
+                        // Therefore, in order to correctly emit identifiers that are written in original TypeScript file,
+                        // we will unescapeIdentifier to remove additional underscore (if no underscore is added, the function will return original input string)
                         (<Identifier | LiteralExpression>index).text = unescapeIdentifier((<Identifier | LiteralExpression>propName).text);
                     }
 

src/compiler/utilities.ts

@@ -230,18 +230,10 @@ namespace ts {
         return getSourceTextOfNodeFromSourceFile(getSourceFileOfNode(node), node, includeTrivia);
     }
 
-    function prefixWithUnderscoreUnderscore(identifier: string): boolean {
-        if (identifier.length <= 2) {
-            return false;
-        }
-        else {
-            return identifier.charCodeAt(0) === CharacterCodes._ && identifier.charCodeAt(1) === CharacterCodes._ ? true : false;
-        }
-    }
-
     // Add an extra underscore to identifiers that start with two underscores to avoid issues with magic names like '__proto__'
     export function escapeIdentifier(identifier: string): string {
-        return prefixWithUnderscoreUnderscore(identifier) ? "_" + identifier : identifier;
+        const prefixWithUnderscore = identifier.length > 2 && identifier.charCodeAt(0) === CharacterCodes._ && identifier.charCodeAt(1) === CharacterCodes._;
+        return prefixWithUnderscore ? "_" + identifier : identifier;
     }
 
     // Remove extra underscore from escaped identifier