diff --git a/codeflash/languages/javascript/instrument.py b/codeflash/languages/javascript/instrument.py
index ef5840cab..02d5239e3 100644
--- a/codeflash/languages/javascript/instrument.py
+++ b/codeflash/languages/javascript/instrument.py
@@ -119,7 +119,8 @@ def split_call_args(args_str: str) -> tuple[str, str]:
     s = args_str
     s_len = len(s)
 
-    for i in range(s_len):
+    i = 0
+    while i < s_len:
         char = s[i]
 
         if char in "\"'`" and (i == 0 or s[i - 1] != "\\"):
@@ -129,9 +130,11 @@ def split_call_args(args_str: str) -> tuple[str, str]:
             elif char == string_char:
                 in_string = False
                 string_char = None
+            i += 1
             continue
 
         if in_string:
+            i += 1
             continue
 
         if char in "([{":
@@ -141,6 +144,8 @@ def split_call_args(args_str: str) -> tuple[str, str]:
         elif char == "," and depth == 0:
             return s[:i].strip(), s[i + 1 :].strip()
 
+        i += 1
+
     return s.strip(), ""
 
 
@@ -599,6 +604,9 @@ def __init__(
             rf"(\s*)expect\s*\(\s*((?:\w+\.)*){re.escape(self.func_name)}\.call\s*\("
         )
 
+        # Cache whitespace characters for faster checking
+        self._whitespace = frozenset(" \t\n\r")
+
     def transform(self, code: str) -> str:
         """Transform all expect calls in the code."""
         result: list[str] = []
@@ -752,10 +760,11 @@ def _parse_expect_dot_call(self, code: str, match: re.Match[str]) -> ExpectCallM
             return None
 
         # Find closing ) of expect(
+        code_len = len(code)
         expect_close_pos = call_close_pos
-        while expect_close_pos < len(code) and code[expect_close_pos].isspace():
+        while expect_close_pos < code_len and code[expect_close_pos] in self._whitespace:
             expect_close_pos += 1
-        if expect_close_pos >= len(code) or code[expect_close_pos] != ")":
+        if expect_close_pos >= code_len or code[expect_close_pos] != ")":
             return None
         expect_close_pos += 1
 
@@ -764,7 +773,7 @@ def _parse_expect_dot_call(self, code: str, match: re.Match[str]) -> ExpectCallM
         if assertion_chain is None:
             return None
 
-        has_trailing_semicolon = chain_end_pos < len(code) and code[chain_end_pos] == ";"
+        has_trailing_semicolon = chain_end_pos < code_len and code[chain_end_pos] == ";"
         if has_trailing_semicolon:
             chain_end_pos += 1
 
@@ -790,7 +799,8 @@ def _find_balanced_parens(self, code: str, open_paren_pos: int) -> tuple[str | N
             Tuple of (content inside parens, position after closing paren) or (None, -1)
 
         """
-        if open_paren_pos >= len(code) or code[open_paren_pos] != "(":
+        code_len = len(code)
+        if open_paren_pos >= code_len or code[open_paren_pos] != "(":
             return None, -1
 
         depth = 1
@@ -798,7 +808,7 @@ def _find_balanced_parens(self, code: str, open_paren_pos: int) -> tuple[str | N
         in_string = False
         string_char = None
 
-        while pos < len(code) and depth > 0:
+        while pos < code_len and depth > 0:
             char = code[pos]
 
             # Handle string literals
@@ -841,32 +851,33 @@ def _parse_assertion_chain(self, code: str, start_pos: int) -> tuple[str | None,
         """
         pos = start_pos
         chain_parts: list[str] = []
+        code_len = len(code)
 
         # Skip any leading whitespace (for multi-line)
-        while pos < len(code) and code[pos] in " \t\n\r":
+        while pos < code_len and code[pos] in self._whitespace:
             pos += 1
 
         # Must start with a dot
-        if pos >= len(code) or code[pos] != ".":
+        if pos >= code_len or code[pos] != ".":
             return None, -1
 
-        while pos < len(code):
+        while pos < code_len:
             # Skip whitespace between chain elements
-            while pos < len(code) and code[pos] in " \t\n\r":
+            while pos < code_len and code[pos] in self._whitespace:
                 pos += 1
 
-            if pos >= len(code) or code[pos] != ".":
+            if pos >= code_len or code[pos] != ".":
                 break
 
             pos += 1  # Skip the dot
 
             # Skip whitespace after dot
-            while pos < len(code) and code[pos] in " \t\n\r":
+            while pos < code_len and code[pos] in self._whitespace:
                 pos += 1
 
             # Parse the method name
             method_start = pos
-            while pos < len(code) and (code[pos].isalnum() or code[pos] == "_"):
+            while pos < code_len and (code[pos].isalnum() or code[pos] == "_"):
                 pos += 1
 
             if pos == method_start:
@@ -875,11 +886,11 @@ def _parse_assertion_chain(self, code: str, start_pos: int) -> tuple[str | None,
             method_name = code[method_start:pos]
 
             # Skip whitespace before potential parens
-            while pos < len(code) and code[pos] in " \t\n\r":
+            while pos < code_len and code[pos] in self._whitespace:
                 pos += 1
 
             # Check for parentheses (method call)
-            if pos < len(code) and code[pos] == "(":
+            if pos < code_len and code[pos] == "(":
                 args_content, after_paren = self._find_balanced_parens(code, pos)
                 if args_content is None:
                     return None, -1