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

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

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:

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:

grep -n 'const ' file.js | while read line; do
    # Check if any const declared in try block is used after catch
    ...
done