Throwing errors with throw

Throwing your own errors

Sometimes you want to trigger an error because you have detected an invalid situation. For that you use throw:

function squareRoot(n) {
  if (n < 0) {
    throw new Error("Cannot take the square root of a negative number");
  }
  return Math.sqrt(n);
}

When throw runs, the function is interrupted immediately and control rises looking for a catch to handle it.

Throw Error objects, not strings

Although throw "text" is valid, always throw an Error object (or a subclass). Error objects carry a message, a name and a stack trace (stack) that help with debugging:

throw new Error("Descriptive message");

Validating inputs: a good habit

Throwing errors early when arguments are not valid makes your code more predictable. It is better to fail fast with a clear message than to silently produce an incorrect result.

Examples

Validate arguments by throwing an error

function divide(a, b) {
  if (b === 0) {
    throw new Error("Cannot divide by zero");
  }
  return a / b;
}
try {
  console.log(divide(10, 2));
  console.log(divide(10, 0));
} catch (e) {
  console.log("Caught:", e.message);
}