JavaScript Error Handling

Things go wrong: bad input, failed requests, typos. Robust code expects failure and handles it deliberately.

try / catch / finally

try {
  const data = JSON.parse('{"name": "Alice"}');
  console.log(data.name);
} catch (error) {
  console.error("Parse failed:", error.message);
} finally {
  console.log("always runs — cleanup goes here");
}

Since ES2019 the binding is optional if you don't need the error details:

try { risky(); } catch { fallback(); }

See it end-to-end with a custom error type:

Know the built-in types

Custom classes extending Error carry domain meaning and work with instanceof — catch specifically, and re-throw what you can't handle (swallowing unknown errors hides bugs).

Async errors need async catches

An exception thrown inside a promise won't hit a surrounding sync try/catch — use await inside try, or .catch():

try {
  const data = await loadDashboard();
} catch (err) {
  showRetryDialog(err);
}

fetchData().catch(err => log(err)).finally(() => hideSpinner());

Debugging toolkit

console.table(rows);              // array of objects as a table
console.time("render"); ... console.timeEnd("render");
console.group("details"); ... console.groupEnd();

debugger;                          // pauses DevTools execution right here
console.log(error.stack);          // exact file:line trail

Breakpoints in DevTools (Sources panel) beat scattered console.logs — you can inspect every variable at the pause point.

Course complete

You've gone from console.log("Hello") to closures, promises, DOM events and custom errors — the core of practical JavaScript. Where next?

• Build something small, today. A todo list, a quiz, a unit converter. Chapter 6's task-list editor is a great starting point.
• TypeScript — JavaScript plus static types; the industry default for medium+ projects.
• A framework — React, Vue or Svelte for UIs; Node.js + Express for servers.
• Deepen the fundamentals — prototypes, this, and the event loop reward a second pass more than any framework does.

Happy hacking!