What is error propagation and when should you re-throw an error?

Error propagation means letting errors bubble up the call stack until something can meaningfully handle them.

// ❌ Anti-pattern: silently swallowing errors
try {
  await doSomething();
} catch (err) {} // hides all bugs — never do this!

// ❌ Anti-pattern: catching all errors at every level
async function fetchUser(id) {
  try { return await fetch(...); }
  catch (err) {
    console.log('error!'); // useless — caller doesn't know what happened
  }
}

// ✅ Correct pattern: handle what you can, re-throw the rest
async function fetchUser(id) {
  try {
    const res = await fetch(`/api/users/${id}`);
    if (!res.ok) throw new NetworkError('Not found', res.status);
    return await res.json();
  } catch (err) {
    if (err instanceof NetworkError && err.statusCode === 404) {
      return null; // 404 is expected — handle it
    }
    throw err; // unexpected error — propagate up
  }
}

// ✅ Top-level handler
async function main() {
  try {
    const user = await fetchUser(1);
    render(user);
  } catch (err) {
    logToErrorService(err); // catch everything remaining
    showErrorMessage();
  }
}