GateShala
Playground

Error Handling

Catch and throw errors with try, catch and finally.

When something goes wrong at run time, JavaScript throws an error. If nothing catches it, the script stops. Wrap risky code in try and handle the problem in catch.

JavaScript
Open in playground
const input = '{"name": "Asha", "age": 21';   // missing closing brace

try {
  const data = JSON.parse(input);
  console.log(data.name);
} catch (err) {
  console.log("Could not read the data:", err.name);
}
console.log("Still running");
Output
Could not read the data: SyntaxError
Still running

The error object

The caught value usually has a name (the kind of error) and a message (what happened).

JavaScript
Open in playground
try {
  const user = null;
  console.log(user.name);
} catch (err) {
  console.log(err.name);
  console.log(err instanceof TypeError);
}
Output
TypeError
true

Throwing your own errors

Use throw new Error("...") when your function receives something it cannot handle. finally runs whether or not an error happened.

JavaScript
Open in playground
function withdraw(balance, amount) {
  if (amount > balance) {
    throw new RangeError(`Cannot withdraw ${amount}, balance is ${balance}`);
  }
  return balance - amount;
}

for (const amount of [200, 900]) {
  try {
    console.log("New balance:", withdraw(500, amount));
  } catch (err) {
    console.log(`${err.name}: ${err.message}`);
  } finally {
    console.log("  attempted", amount);
  }
}
Output
New balance: 300
  attempted 200
RangeError: Cannot withdraw 900, balance is 500
  attempted 900

Custom error classes

Extending Error lets you tell your own errors apart from others.

JavaScript
Open in playground
class ValidationError extends Error {
  constructor(field, message) {
    super(message);
    this.name = "ValidationError";
    this.field = field;
  }
}

function register(user) {
  if (!user.email.includes("@")) {
    throw new ValidationError("email", "Email looks invalid");
  }
  return "Registered " + user.email;
}

try {
  console.log(register({ email: "dev@example.com" }));
  console.log(register({ email: "dev.example.com" }));
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(`Problem with ${err.field}: ${err.message}`);
  } else {
    throw err;   // not ours: let it continue upwards
  }
}
Output
Registered dev@example.com
Problem with email: Email looks invalid