perf(json): avoid first-char regex for direct JSON bodies (#744)

Skip the RFC 7159 whitespace scan when strict JSON bodies start directly with `{` or `[`. This avoids regex execution on the common valid path while preserving strict-mode handling for leading whitespace and primitive values.

Update the first non-whitespace regex to match end-of-string so whitespace-only bodies return `undefined` without relying on a failed match/backtracking path.

Author: Phillip9587

lib/types/json.js

@@ -23,7 +23,7 @@ const { normalizeOptions } = require('../utils')
 module.exports = json
 
 /**
- * RegExp to match the first non-space in a string.
+ * RegExp to match the first non-whitespace character in a string.
  *
  * Allowed whitespace is defined in RFC 7159:
  *
@@ -33,7 +33,7 @@ module.exports = json
  *            %x0A /              ; Line feed or New line
  *            %x0D )              ; Carriage return
  */
-const FIRST_CHAR_REGEXP = /^[\x20\x09\x0a\x0d]*([^\x20\x09\x0a\x0d])/ // eslint-disable-line no-control-regex
+const FIRST_NON_WHITESPACE_REGEXP = /^[\x20\x09\x0a\x0d]*([^\x20\x09\x0a\x0d]|$)/ // eslint-disable-line no-control-regex
 
 const JSON_SYNTAX_CHAR = '#'
 const JSON_SYNTAX_REGEXP = /#+/g
@@ -80,10 +80,12 @@ function createJsonParser (options) {
         return {}
       }
 
-      const first = firstchar(body)
-      if (first !== '{' && first !== '[') {
-        debug('strict violation')
-        throw createStrictSyntaxError(body, first)
+      if (body[0] !== '{' && body[0] !== '[') {
+        const first = firstNonWhitespaceChar(body)
+        if (first !== '{' && first !== '[') {
+          debug('strict violation')
+          throw createStrictSyntaxError(body, first)
+        }
       }
 
       try {
@@ -152,12 +154,10 @@ function createStrictSyntaxError (str, char) {
  * @returns {string|undefined}
  * @private
  */
-function firstchar (str) {
-  const match = FIRST_CHAR_REGEXP.exec(str)
+function firstNonWhitespaceChar (str) {
+  const match = FIRST_NON_WHITESPACE_REGEXP.exec(str)
 
-  return match
-    ? match[1]
-    : undefined
+  return match && match[1] ? match[1] : undefined
 }
 
 /**