fix: use line-based heuristic for auto-export detection (#39)

The naive .contains("export") check in register_handler() would
false-positive on string literals (e.g. '<config mode="export">'),
comments (e.g. // TODO: export data), and identifiers (e.g. exportPath),
skipping auto-export and causing handler registration to fail.

Replace with has_export_statement() which checks whether any source line
starts with 'export' (after leading whitespace), correctly ignoring
occurrences inside strings, comments, and variable names.

Closes #39

Signed-off-by: Simon Davies <simongdavies@users.noreply.github.com>

Author: simongdavies

src/hyperlight-js-runtime/src/lib.rs

@@ -155,10 +155,15 @@ impl JsRuntime {
         let handler_script = handler_script.into();
         let handler_pwd = handler_pwd.into();
 
-        // If the handler script doesn't already export the handler function, we export it for the user.
-        // This is a convenience for the common case where the handler script is just a single file that defines
-        // the handler function, without needing to explicitly export it.
-        let handler_script = if !handler_script.contains("export") {
+        // If the handler script doesn't already contain an ES export statement,
+        // append one for the user. This is a convenience for the common case where
+        // the handler script defines a handler function without explicitly exporting it.
+        //
+        // We check whether any line *starts* with `export` (after leading whitespace)
+        // rather than using a naive `.contains("export")`, which would false-positive
+        // on string literals (e.g. '<config mode="export">'), comments
+        // (e.g. // TODO: export data), or identifiers (e.g. exportPath).
+        let handler_script = if !has_export_statement(&handler_script) {
             format!("{}\nexport {{ handler }};", handler_script)
         } else {
             handler_script
@@ -315,6 +320,20 @@ fn make_handler_path(function_name: &str, handler_dir: &str) -> String {
     handler_path
 }
 
+/// Returns `true` if the script contains an actual ES `export` statement
+/// (as opposed to the word "export" inside a string literal, comment, or
+/// identifier like `exportPath`).
+///
+/// The heuristic checks whether any source line begins with `export` (after
+/// optional leading whitespace). This avoids the false positives from a
+/// naive `.contains("export")` while staying `no_std`-compatible.
+fn has_export_statement(script: &str) -> bool {
+    script.lines().any(|line| {
+        let trimmed = line.trim_start();
+        trimmed.starts_with("export ") || trimmed.starts_with("export{")
+    })
+}
+
 // RAII guard that flushes the output buffer of libc when dropped.
 // This is used to make sure we flush all output after running a handler, without needing to manually call it in every code path.
 struct FlushGuard;

src/hyperlight-js/tests/handlers.rs

@@ -126,3 +126,124 @@ fn handle_event_rejects_empty_name() {
         "Error should mention empty name, got: {err}"
     );
 }
+
+// ── Auto-export heuristic tests (issue #39) ──────────────────────────
+// The auto-export logic must only detect actual ES export statements,
+// not the word "export" inside string literals, comments, or identifiers.
+
+#[test]
+fn handler_with_export_in_string_literal() {
+    // "export" appears inside a string — auto-export should still fire
+    let handler = Script::from_content(
+        r#"
+        function handler(event) {
+            const xml = '<config mode="export">value</config>';
+            return { result: xml };
+        }
+        "#,
+    );
+
+    let proto = SandboxBuilder::new().build().unwrap();
+    let mut sandbox = proto.load_runtime().unwrap();
+    sandbox.add_handler("handler", handler).unwrap();
+    let mut loaded = sandbox.get_loaded_sandbox().unwrap();
+
+    let res = loaded
+        .handle_event("handler", "{}".to_string(), None)
+        .unwrap();
+    assert_eq!(
+        res,
+        r#"{"result":"<config mode=\"export\">value</config>"}"#
+    );
+}
+
+#[test]
+fn handler_with_export_in_comment() {
+    // "export" appears in a comment — auto-export should still fire
+    let handler = Script::from_content(
+        r#"
+        function handler(event) {
+            // TODO: export this data to CSV
+            return { result: 42 };
+        }
+        "#,
+    );
+
+    let proto = SandboxBuilder::new().build().unwrap();
+    let mut sandbox = proto.load_runtime().unwrap();
+    sandbox.add_handler("handler", handler).unwrap();
+    let mut loaded = sandbox.get_loaded_sandbox().unwrap();
+
+    let res = loaded
+        .handle_event("handler", "{}".to_string(), None)
+        .unwrap();
+    assert_eq!(res, r#"{"result":42}"#);
+}
+
+#[test]
+fn handler_with_export_in_identifier() {
+    // "export" is part of an identifier — auto-export should still fire
+    let handler = Script::from_content(
+        r#"
+        function handler(event) {
+            const exportPath = "/tmp/out.csv";
+            return { result: exportPath };
+        }
+        "#,
+    );
+
+    let proto = SandboxBuilder::new().build().unwrap();
+    let mut sandbox = proto.load_runtime().unwrap();
+    sandbox.add_handler("handler", handler).unwrap();
+    let mut loaded = sandbox.get_loaded_sandbox().unwrap();
+
+    let res = loaded
+        .handle_event("handler", "{}".to_string(), None)
+        .unwrap();
+    assert_eq!(res, r#"{"result":"/tmp/out.csv"}"#);
+}
+
+#[test]
+fn handler_with_explicit_export_is_not_doubled() {
+    // Script already has an export statement — auto-export should be skipped
+    let handler = Script::from_content(
+        r#"
+        function handler(event) {
+            return { result: "explicit" };
+        }
+        export { handler };
+        "#,
+    );
+
+    let proto = SandboxBuilder::new().build().unwrap();
+    let mut sandbox = proto.load_runtime().unwrap();
+    sandbox.add_handler("handler", handler).unwrap();
+    let mut loaded = sandbox.get_loaded_sandbox().unwrap();
+
+    let res = loaded
+        .handle_event("handler", "{}".to_string(), None)
+        .unwrap();
+    assert_eq!(res, r#"{"result":"explicit"}"#);
+}
+
+#[test]
+fn handler_with_export_default_function() {
+    // `export function` — auto-export should be skipped
+    let handler = Script::from_content(
+        r#"
+        export function handler(event) {
+            return { result: "inline-export" };
+        }
+        "#,
+    );
+
+    let proto = SandboxBuilder::new().build().unwrap();
+    let mut sandbox = proto.load_runtime().unwrap();
+    sandbox.add_handler("handler", handler).unwrap();
+    let mut loaded = sandbox.get_loaded_sandbox().unwrap();
+
+    let res = loaded
+        .handle_event("handler", "{}".to_string(), None)
+        .unwrap();
+    assert_eq!(res, r#"{"result":"inline-export"}"#);
+}