Errors, try and catch

Errors stop a program

• When JavaScript cannot carry out a line, it throws an error, and the program stops there.
• The message names the kind of error and what went wrong. Read it before changing anything.
• ReferenceError means a name does not exist, often a typo. TypeError means a value is the wrong kind, such as calling something that is not a function.

console.log("before");
let total = 5;
console.log(totl);       // typo
console.log("after");    // never runs

Output:
before
ReferenceError: totl is not defined

try and catch

• Put the risky code inside try { … }.
• If it throws, the program jumps to catch (err) { … } instead of stopping.
• err.message holds the text of the error. Code after the try…catch runs either way.

try {
  const data = JSON.parse("{ broken");
  console.log(data);
} catch (err) {
  console.log("Could not read the data");
}
console.log("still running");
// → Could not read the data
// → still running

throw your own error

• throw new Error("message") stops the function and hands the error to whoever called it.
• Throw when a function gets a value it cannot sensibly use. Catch where you can do something about it.

Worked example: a safe age check

Question: checkAge(age) should throw for an age below 0, and otherwise return "ok". Call it with two ages and report each result.

function checkAge(age) {
  if (age < 0) {
    throw new Error("age cannot be negative");
  }
  return "ok";
}

for (const age of [12, -3]) {
  try {
    console.log(age, checkAge(age));
  } catch (err) {
    console.log(age, "rejected:", err.message);
  }
}
// → 12 ok
// → -3 rejected: age cannot be negative

The try…catch sits inside the loop, so one bad age is reported and the loop still goes on to the next one.

Watch out

• catch only catches errors thrown while the try block runs. A typo that breaks the whole program's syntax is never caught.
• Do not hide every error with an empty catch {}. Say what went wrong, or the bug becomes invisible.

Common mistakes

• throw "bad age" instead of throw new Error("bad age"). Then err.message is undefined.
• Putting the console.log that uses the result after the try, where the variable from inside try no longer exists.
• Catching an error and carrying on as if the value were fine. Decide what the program should do instead.

Now you try

• Throw an error for bad input, then catch it and print its message.
• Press Check answer to test it.