Review changes

src/language/lexer.ts

@@ -151,10 +151,6 @@ function encodeSurrogatePair(point: number): string {
   );
 }
 
-function decodeSurrogatePair(leading: number, trailing: number): number {
-  return 0x10000 + (((leading & 0x03ff) << 10) | (trailing & 0x03ff));
-}
-
 /**
  * Prints the code point (or end of file reference) at a given location in a
  * source for use in error messages.
@@ -163,22 +159,18 @@ function decodeSurrogatePair(leading: number, trailing: number): number {
  * code point form (ie. U+1234).
  */
 function printCodePointAt(lexer: Lexer, location: number): string {
-  const body = lexer.source.body;
-  if (location >= body.length) {
+  const code = lexer.source.body.codePointAt(location);
+
+  if (code === undefined) {
     return TokenKind.EOF;
+  } else if (code >= 0x0020 && code <= 0x007e) {
+    // Printable ASCII
+    const char = String.fromCodePoint(code);
+    return char === '"' ? "'\"'" : `"${char}"`;
   }
-  const code = body.charCodeAt(location);
-  // Printable ASCII
-  if (code >= 0x0020 && code <= 0x007e) {
-    return code === 0x0022 ? "'\"'" : `"${body[location]}"`;
-  }
+
   // Unicode code point
-  const point = isSupplementaryCodePoint(body, location)
-    ? decodeSurrogatePair(code, body.charCodeAt(location + 1))
-    : code;
-  const zeroPad =
-    point > 0xfff ? '' : point > 0xff ? '0' : point > 0xf ? '00' : '000';
-  return `U+${zeroPad}${point.toString(16).toUpperCase()}`;
+  return 'U+' + code.toString(16).toUpperCase().padStart(4, '0');
 }
 
 /**