Throw and Custom Errors

Throw and Custom Errors

You can create your own errors with throw and catch them with try...catch.

The throw statement

function divide(a, b) {
  if (b === 0) {
    throw new Error("Cannot divide by zero");
  }
  return a / b;
}

Throw types

You can throw anything, but Error objects are best because they carry useful info:

throw new Error("message");
throw new TypeError("expected a number");

Handling a custom error

try {
  divide(10, 0);
} catch (err) {
  console.log(err.message); // "Cannot divide by zero"
}

Why use throw?

• Validate inputs before using them.
• Fail loudly and clearly instead of silently producing wrong results.
• Give useful messages to anyone using your function.

Validation example

function setAge(age) {
  if (age < 0) {
    throw new RangeError("Age cannot be negative");
  }
  return age;
}

TL;DR

• throw raises an error.
• new Error, TypeError, RangeError create typed errors.
• try...catch catches thrown errors.
• Use throw for input validation and clear failures.