initial commit

skills/software-development/systematic-debugging/references/js-block-scope-crash.md

@@ -0,0 +1,56 @@
+# JavaScript Silent Crash: Block-Scoped `const` in `try`
+
+**Pattern:** A `const` (or `let`) declared inside a `try {}` block is referenced
+outside the block. JavaScript throws a `ReferenceError` that silently crashes
+async event handlers — no error visible in the UI, no console output if the
+handler is an `addEventListener` callback calling an `async function`.
+
+## Example
+
+```javascript
+async function handleFinancialCompletion() {
+    // ...
+    try {
+        const roData = getROData(roId);  // block-scoped to try
+        // ...
+    } catch (e) {}
+
+    // BUG: roData is undefined here — ReferenceError
+    if (roData) {
+        roData.statusHistory.push({ ... });
+    }
+    // ...
+}
+```
+
+## Why it's silent
+
+The `completeBtn.addEventListener('click', (e) => { handleFinancialCompletion(); })`
+does NOT catch the rejected promise from the async function. The ReferenceError
+propagates as an unhandled promise rejection — which browsers log but the UI
+shows nothing. The user sees "nothing happens."
+
+## Fix
+
+Declare the variable OUTSIDE the `try` block:
+
+```javascript
+const roData = getROData(roId);  // outside try
+try {
+    const servicesLines = roData?.services ? ...;
+    // ...
+} catch (e) {}
+// roData is accessible here
+if (roData) { ... }
+```
+
+## Detection
+
+Grep for `const.*=.*try` patterns or any variable accessed after a `} catch`
+that was declared inside the `try`:
+```bash
+grep -n 'const ' file.js | while read line; do
+    # Check if any const declared in try block is used after catch
+    ...
+done
+```