try/catch/finally, throw, custom errors
Use try/catch/finally, throw simple errors, and define a tiny custom Error for clearer messages.
try/catch/finally, throw, custom errors is a free JavaScript Academy lesson on CoddyKit — lesson 1 of 2. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the JavaScript Academy learning path, one of 2 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Intro to error handling
Goal: Catch errors safely and keep programs stable.
- try/catch basics
- throw with a clear message
- finally for cleanup
- tiny custom Error
try/catch basics
Wrap risky operations with try. In catch, show a short, helpful message.
// try/catch around risky code
try {
const data = JSON.parse("{\"name\":\"Ayla\"}");
console.log("Parsed:", data.name);
} catch (e) {
// e.message is short and useful on mobile
console.log("Failed to parse:", e.message);
}
throw basics
Use throw new Error("message") to stop and explain what went wrong.
// Throw when inputs are invalid
function doubleNumber(n) {
if (typeof n !== "number") {
throw new Error("Number required");
}
return n * 2;
}
try {
console.log(doubleNumber(4));
console.log(doubleNumber("4")); // will throw
} catch (e) {
console.log("Error:", e.message);
}
finally for cleanup
finally runs whether try succeeded or failed — useful for closing resources.
// finally runs after try/catch — great for cleanup
let opened = false;
try {
opened = true;
console.log("Opened:", opened);
// Force an error to see finally still run
JSON.parse("{ bad }");
} catch (e) {
console.log("Caught:", e.message);
} finally {
opened = false;
console.log("Closed:", opened);
}
Custom Error (light)
Create a tiny InputError for input issues. Use instanceof to handle it specifically.
// Make a small custom error
class InputError extends Error {
constructor(message) {
super(message);
this.name = "InputError";
}
}
function positive(n) {
if (typeof n !== "number") {
throw new InputError("Number required");
}
if (n <= 0) {
throw new InputError("Must be positive");
}
return n;
}
try {
console.log("OK:", positive(3));
console.log("Fail:", positive(0));
} catch (e) {
if (e instanceof InputError) {
console.log("Input problem:", e.message);
} else {
// Unknown error: rethrow or log differently
console.log("Unexpected:", e.message);
}
}
Tips & checklist
Tips:
- Validate inputs early; throw short messages.
- Use finally for cleanup (close flags, timers, etc.).
- Do not swallow errors: log briefly or rethrow.
finally quiz
Quick check: finally behavior.
Recap
Recap: Wrap risky code with try/catch, throw short helpful errors, use finally for cleanup, and define tiny custom errors for clearer handling.
Frequently asked questions
Is the “try/catch/finally, throw, custom errors” lesson free?
Yes — the full text of “try/catch/finally, throw, custom errors” is free to read here on the web, and the JavaScript Academy course includes 2 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the JavaScript Academy course, upgrade to CoddyKit PRO.
What will I learn in “try/catch/finally, throw, custom errors”?
Use try/catch/finally, throw simple errors, and define a tiny custom Error for clearer messages. You practise JavaScript Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start JavaScript Academy?
No prior experience is required. JavaScript Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 2, so you can start here or from the beginning and move at your own pace.
How long does the “try/catch/finally, throw, custom errors” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this JavaScript Academy lesson?
Yes. Every JavaScript Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- try/catch/finally, throw, custom errors
- Defensive Programming & Input Checks