Allow Unicode extended escapes in ES5 and earlier

src/compiler/factory/nodeFactory.ts

@@ -874,7 +874,7 @@ namespace ts {
         }
 
         // @api
-        function createIdentifier(text: string, typeArguments?: readonly (TypeNode | TypeParameterDeclaration)[], originalKeywordKind?: SyntaxKind): Identifier {
+        function createIdentifier(text: string, typeArguments?: readonly (TypeNode | TypeParameterDeclaration)[], originalKeywordKind?: SyntaxKind, hasExtendedUnicodeEscape?: boolean): Identifier {
             const node = createBaseIdentifier(text, originalKeywordKind);
             if (typeArguments) {
                 // NOTE: we do not use `setChildren` here because typeArguments in an identifier do not contribute to transformations
@@ -883,6 +883,10 @@ namespace ts {
             if (node.originalKeywordKind === SyntaxKind.AwaitKeyword) {
                 node.transformFlags |= TransformFlags.ContainsPossibleTopLevelAwait;
             }
+            if (hasExtendedUnicodeEscape) {
+                node.hasExtendedUnicodeEscape = hasExtendedUnicodeEscape;
+                node.transformFlags |= TransformFlags.ContainsES2015;
+            }
             return node;
         }
 

src/compiler/parser.ts

@@ -2187,8 +2187,9 @@ namespace ts {
                 // Store original token kind if it is not just an Identifier so we can report appropriate error later in type checker
                 const originalKeywordKind = token();
                 const text = internIdentifier(scanner.getTokenValue());
+                const hasExtendedUnicodeEscape = scanner.hasExtendedUnicodeEscape();
                 nextTokenWithoutCheck();
-                return finishNode(factory.createIdentifier(text, /*typeArguments*/ undefined, originalKeywordKind), pos);
+                return finishNode(factory.createIdentifier(text, /*typeArguments*/ undefined, originalKeywordKind, hasExtendedUnicodeEscape), pos);
             }
 
             if (token() === SyntaxKind.PrivateIdentifier) {

src/compiler/scanner.ts

@@ -1486,7 +1486,7 @@ namespace ts {
 
 
         function peekExtendedUnicodeEscape(): number {
-            if (languageVersion >= ScriptTarget.ES2015 && codePointAt(text, pos + 1) === CharacterCodes.u && codePointAt(text, pos + 2) === CharacterCodes.openBrace) {
+            if (codePointAt(text, pos + 1) === CharacterCodes.u && codePointAt(text, pos + 2) === CharacterCodes.openBrace) {
                 const start = pos;
                 pos += 3;
                 const escapedValueString = scanMinimumNumberOfHexDigits(1, /*canHaveSeparators*/ false);

src/compiler/transformers/es2015.ts

@@ -643,11 +643,16 @@ namespace ts {
         }
 
         function visitIdentifier(node: Identifier): Identifier {
-            if (!convertedLoopState) {
-                return node;
+            if (convertedLoopState) {
+                if (resolver.isArgumentsLocalBinding(node)) {
+                    return convertedLoopState.argumentsName || (convertedLoopState.argumentsName = factory.createUniqueName("arguments"));
+                }
             }
-            if (resolver.isArgumentsLocalBinding(node)) {
-                return convertedLoopState.argumentsName || (convertedLoopState.argumentsName = factory.createUniqueName("arguments"));
+            if (node.hasExtendedUnicodeEscape) {
+                return setOriginalNode(setTextRange(
+                    factory.createIdentifier(unescapeLeadingUnderscores(node.escapedText)),
+                    node
+                ), node);
             }
             return node;
         }

src/compiler/types.ts

@@ -1427,6 +1427,7 @@ namespace ts {
         isInJSDocNamespace?: boolean;                             // if the node is a member in a JSDoc namespace
         /*@internal*/ typeArguments?: NodeArray<TypeNode | TypeParameterDeclaration>; // Only defined on synthesized nodes. Though not syntactically valid, used in emitting diagnostics, quickinfo, and signature help.
         /*@internal*/ jsdocDotPos?: number;                       // Identifier occurs in JSDoc-style generic: Id.<T>
+        /*@internal*/ hasExtendedUnicodeEscape?: boolean;
     }
 
     // Transient identifier node (marked by id === -1)
@@ -7651,7 +7652,7 @@ namespace ts {
         //
 
         createIdentifier(text: string): Identifier;
-        /* @internal */ createIdentifier(text: string, typeArguments?: readonly (TypeNode | TypeParameterDeclaration)[], originalKeywordKind?: SyntaxKind): Identifier; // eslint-disable-line @typescript-eslint/unified-signatures
+        /* @internal */ createIdentifier(text: string, typeArguments?: readonly (TypeNode | TypeParameterDeclaration)[], originalKeywordKind?: SyntaxKind, hasExtendedUnicodeEscape?: boolean): Identifier; // eslint-disable-line @typescript-eslint/unified-signatures
         /* @internal */ updateIdentifier(node: Identifier, typeArguments: NodeArray<TypeNode | TypeParameterDeclaration> | undefined): Identifier;
 
         /**