Error handling

2 min read 4 runnable examples Quiz included

try, catch, finally

JavaScript

try {
  const data = JSON.parse("{not valid json}");
  console.log(data);
} catch (err) {
  console.log("Could not parse:", err.name, "-", err.message);
} finally {
  console.log("finally always runs");
}
console.log("The program keeps going.");

Code in try runs until something throws.
catch receives the error object.
finally runs whether or not an error happened.

Common built-in errors

Error	Typical cause
`ReferenceError`	using a variable that doesn't exist
`TypeError`	calling something that isn't a function, or reading a property of `undefined`
`SyntaxError`	invalid code, or invalid JSON in `JSON.parse`
`RangeError`	a number outside an allowed range

JavaScript

const user = undefined;
try {
  console.log(user.name);
} catch (err) {
  console.log(err.constructor.name + ":", err.message);
}
console.log(user?.name ?? "no user");      // optional chaining avoids the error

Throwing your own errors

JavaScript

function withdraw(balance, amount) {
  if (amount <= 0) throw new Error("Amount must be positive");
  if (amount > balance) throw new RangeError(`Insufficient funds: balance is ${balance}`);
  return balance - amount;
}

for (const amount of [50, -5, 500]) {
  try {
    console.log("New balance:", withdraw(200, amount));
  } catch (err) {
    console.log(`Refused (${err.name}): ${err.message}`);
  }
}

Always throw Error objects (not strings), so you get a name, a message and a stack trace.

Custom error classes

JavaScript

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";
}

try {
  register({ email: "not-an-email" });
} catch (err) {
  if (err instanceof ValidationError) {
    console.log(`Fix the ${err.field} field: ${err.message}`);
  } else {
    throw err;     // not ours: let it go up
  }
}

Checking instanceof lets you handle the errors you expect and re-throw everything else.

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