Description

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Intermediate · 12 min

Validate input and handle errors

Reject invalid input deliberately and recover at an appropriate boundary.

Signal a problem

throw stops the current path and sends an error to the nearest matching catch. Validate values before calculating with them.

function quantity(value) {
  if (!Number.isInteger(value) || value < 0) throw new Error("Invalid quantity");
  return value;
}
try { console.log(quantity(-1)); } catch (error) { console.log(error.message); }

Output

Invalid quantity

Handle expected failures

Catch a failure where you can respond usefully. Avoid silently replacing every failure with a success value. finally runs when leaving the try/catch, including after an error.

try {
  JSON.parse("invalid");
} catch {
  console.log("Invalid JSON");
} finally {
  console.log("Finished");
}

Output

Invalid JSON
Finished

Put it into practice

  1. Predict each result before running the example.
  2. Complete the coding task, then explain why your solution works.

Try it yourself

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Reject a negative quantity

Make validate throw an Error with the message "Invalid quantity" when value is negative.

Practice