Fix JS line profiler empty output file causing JSONDecodeError

The profiler's save() was called every 100 hit() calls. With O(n²)
algorithms this produced hundreds of thousands of writeFileSync calls,
each truncating the file to 0 bytes before writing. If the subprocess
timed out (SIGKILL), the file was left at 0 bytes → JSONDecodeError.

Fixes:
- Move require('fs')/require('path') to module scope (not inside save())
- Reduce save-every-N from 100 → 10,000 hits (100x fewer syscalls)
- Pre-create output file with {} before running Jest (safety net)
- Handle empty files gracefully in parse_results
- Fix misleading "file not found" warning → "file empty or no timing data"

Author: KRRT7

codeflash/languages/javascript/function_optimizer.py

@@ -182,6 +182,11 @@ def line_profiler_step(
         try:
             line_profiler_output_path = get_run_tmp_file(Path("line_profiler_output.json"))
 
+            # Pre-create with valid empty JSON so the file is never 0 bytes
+            # even if the JS profiler save() is interrupted (e.g. SIGKILL on timeout)
+            line_profiler_output_path.parent.mkdir(parents=True, exist_ok=True)
+            line_profiler_output_path.write_text("{}", encoding="utf-8")
+
             success = self.language_support.instrument_source_for_line_profiler(
                 func_info=self.function_to_optimize, line_profiler_output_file=line_profiler_output_path
             )

codeflash/languages/javascript/line_profiler.py

@@ -86,9 +86,10 @@ def _generate_profiler_init(self) -> str:
         # Serialize line contents map for embedding in JavaScript
         line_contents_json = json.dumps(getattr(self, "line_contents", {}))
 
-        return f"""
+        return f"""// @ts-nocheck
 // Codeflash line profiler initialization
-// @ts-nocheck
+const __codeflash_fs__ = require('fs');
+const __codeflash_path__ = require('path');
 const {self.profiler_var} = {{
     stats: {{}},
     lineContents: {line_contents_json},
@@ -123,19 +124,18 @@ def _generate_profiler_init(self) -> str:
         this.lastLineTime = now;
 
         this.totalHits++;
-        // Save every 100 hits to ensure we capture results even with --forceExit
-        if (this.totalHits % 100 === 0) {{
+        // Save periodically to capture results even with --forceExit.
+        // Use 10000 (not 100) to avoid excessive I/O on hot loops.
+        if (this.totalHits % 10000 === 0) {{
             this.save();
         }}
     }},
 
     save: function() {{
-        const fs = require('fs');
-        const pathModule = require('path');
-        const outputDir = pathModule.dirname('{self.output_file.as_posix()}');
+        const outputDir = __codeflash_path__.dirname('{self.output_file.as_posix()}');
         try {{
-            if (!fs.existsSync(outputDir)) {{
-                fs.mkdirSync(outputDir, {{ recursive: true }});
+            if (!__codeflash_fs__.existsSync(outputDir)) {{
+                __codeflash_fs__.mkdirSync(outputDir, {{ recursive: true }});
             }}
             // Merge line contents into stats before saving
             const statsWithContent = {{}};
@@ -145,7 +145,7 @@ def _generate_profiler_init(self) -> str:
                     content: this.lineContents[key] || ''
                 }};
             }}
-            fs.writeFileSync(
+            __codeflash_fs__.writeFileSync(
                 '{self.output_file.as_posix()}',
                 JSON.stringify(statsWithContent, null, 2)
             );
@@ -298,8 +298,11 @@ def parse_results(profile_file: Path) -> dict:
             return {"timings": {}, "unit": 1e-9, "functions": {}}
 
         try:
-            with profile_file.open("r") as f:
-                data = json.load(f)
+            content = profile_file.read_text(encoding="utf-8").strip()
+            if not content:
+                logger.warning("Line profiler output file is empty: %s", profile_file)
+                return {"timings": {}, "unit": 1e-9, "functions": {}}
+            data = json.loads(content)
 
             # Group by file and function
             timings = {}

codeflash/languages/javascript/support.py

@@ -2497,13 +2497,17 @@ def instrument_source_for_line_profiler(
     def parse_line_profile_results(self, line_profiler_output_file: Path) -> dict:
         from codeflash.languages.javascript.line_profiler import JavaScriptLineProfiler
 
-        if line_profiler_output_file.exists():
-            parsed_results = JavaScriptLineProfiler.parse_results(line_profiler_output_file)
-            if parsed_results.get("timings"):
-                # Format output string for display
-                str_out = self._format_js_line_profile_output(parsed_results)
-                return {"timings": parsed_results.get("timings", {}), "unit": 1e-9, "str_out": str_out}
-        logger.warning("No line profiler output file found at %s", line_profiler_output_file)
+        if not line_profiler_output_file.exists():
+            logger.warning("Line profiler output file not found: %s", line_profiler_output_file)
+            return {"timings": {}, "unit": 0, "str_out": ""}
+
+        parsed_results = JavaScriptLineProfiler.parse_results(line_profiler_output_file)
+        if parsed_results.get("timings"):
+            # Format output string for display
+            str_out = self._format_js_line_profile_output(parsed_results)
+            return {"timings": parsed_results.get("timings", {}), "unit": 1e-9, "str_out": str_out}
+
+        logger.warning("Line profiler output file empty or contained no timing data: %s", line_profiler_output_file)
         return {"timings": {}, "unit": 0, "str_out": ""}
 
     def _format_js_line_profile_output(self, parsed_results: dict) -> str: