meet it

raises the alarm; decides what happens next.

So far an error stopped your whole program. Professionals plan for errors. throw new Error("message") stops the current function on purpose and passes the error upwards, through every caller, until something catches it.

Wrap risky code in try...catch. If anything inside try throws, JavaScript jumps to catch (error), where error.name and error.message tell you what happened. A block runs afterwards either way, perfect for tidying up.

function parseAge(text) {
  const age = Number(text);
  if (Number.isNaN(age)) { throw new Error(`"${text}" is not a number`); }
  return age;
}
for (const input of ["42", "forty"]) {
  try {
    console.log(`Age ${parseAge(input)}`);
  } catch (error) {
    console.log(`${error.name}: ${error.message}`);
  } finally {
    console.log(`Checked ${input}`);
  }
}

Number("forty") gives NaN, short for Not a Number, and Number.isNaN spots it.

guess first

An error three calls deep. What will this show?

Guess before you run it. Wrong guesses are how you learn, and nothing is marked.

function step3() { throw new Error("disk full"); }
function step2() { step3(); console.log("step 2 done"); }
function step1() {
  try {
    step2();
  } catch (error) {
    console.log(`Caught: ${error.message}`);
  }
  console.log("step 1 carries on");
}
step1();

your turn

Your turn. Refuse a silly volume.

Pick a good volume from 0 to 10 for the first blank, then a bad one, like 11 or -3, for the second. Watch catch turn the crash into a friendly message.

Your code

press run to see what happens

fix it

The default settings never load. Can you fix it?

Broken code to fix

press run to see what happens

Which line throws the SyntaxError? Is that line inside the try block?
JSON.parse fails on line 4, before the try even starts, so nothing catches it.
Move the const settings = ... line inside the try block, above the console.log.

nailed it

you can catch trouble before it spreads. :D

throw stops a function and sends an error upwards.
try...catch catches it, so the program keeps going.
finally always runs, error or not.

new words in your collection

throw
try...catch
finally

This lesson isn’t saved. A free account remembers every lesson you finish.

throw raises the alarm; try...catch decides what happens next.

An error three calls deep. What will this show?

Your turn. Refuse a silly volume.

The default settings never load. Can you fix it?

you can catch trouble before it spreads. :D

raises the alarm; decides what happens next.