perf: eliminate conditional check in json strict mode hot path

Refactor JSON parser creation to return specialized parser functions
based on strict mode setting, removing the per-request `if (strict)`
conditional check in the parsing hot path.

Author: Phillip9587

lib/types/json.js

@@ -50,23 +50,61 @@ var JSON_SYNTAX_REGEXP = /#+/g
 function json (options) {
   const normalizedOptions = normalizeOptions(options, 'application/json')
 
-  var reviver = options?.reviver
-  var strict = options?.strict !== false
+  const parse = createJsonParser(options)
 
-  function parse (body) {
-    if (body.length === 0) {
-      // special-case empty json body, as it's a common client-side mistake
-      // TODO: maybe make this configurable or part of "strict" option
-      return {}
-    }
+  const readOptions = {
+    ...normalizedOptions,
+    // assert charset per RFC 7159 sec 8.1
+    isValidCharset: (charset) => charset.slice(0, 4) === 'utf-'
+  }
 
-    if (strict) {
-      var first = firstchar(body)
+  return function jsonParser (req, res, next) {
+    read(req, res, next, parse, debug, readOptions)
+  }
+}
+
+/**
+ * Create a JSON parse function
+ *
+ * @param {object} [options]
+ * @return {function}
+ * @private
+ */
+function createJsonParser (options) {
+  const reviver = options?.reviver
+  const strict = options?.strict !== false
+
+  if (strict) {
+    return function parse (body) {
+      if (body.length === 0) {
+        // special-case empty json body, as it's a common client-side mistake
+        // TODO: maybe make this configurable or part of "strict" option
+        return {}
+      }
 
+      const first = firstchar(body)
       if (first !== '{' && first !== '[') {
         debug('strict violation')
         throw createStrictSyntaxError(body, first)
       }
+
+      try {
+        debug('parse json')
+        return JSON.parse(body, reviver)
+      } catch (e) {
+        throw normalizeJsonSyntaxError(e, {
+          message: e.message,
+          stack: e.stack
+        })
+      }
+    }
+  }
+
+  return function parse (body) {
+    if (body.length === 0) {
+      // special-case empty json body, as it's a common client-side mistake
+      // TODO: maybe make this configurable or part of "strict" option
+      return {}
     }
 
     try {
@@ -79,16 +117,6 @@ function json (options) {
       })
     }
   }
-
-  const readOptions = {
-    ...normalizedOptions,
-    // assert charset per RFC 7159 sec 8.1
-    isValidCharset: (charset) => charset.slice(0, 4) === 'utf-'
-  }
-
-  return function jsonParser (req, res, next) {
-    read(req, res, next, parse, debug, readOptions)
-  }
 }
 
 /**
@@ -128,7 +156,7 @@ function createStrictSyntaxError (str, char) {
  * Get the first non-whitespace character in a string.
  *
  * @param {string} str
- * @return {function}
+ * @return {string | undefined}
  * @private
  */