Custom errors in JavaScript

JavaScript gives us a set of 8 error objects, which are raised in a try/catch expression depending on the error type. They are:

• Error
• EvalError
• RangeError
• ReferenceError
• SyntaxError
• TypeError
• URIError

I analyzed them all in the JavaScript errors tutorial.

Here I want to explain how to create your own custom errors by extending the base Error class:

class OutOfFuelError extends Error {}

class FlatTireError extends Error {}

Custom errors allow you to behave differently based on the specific error type, without resorting to use error messages to understand the kind of error.

try {
  //some code
} catch (err) {
  if (err instanceof OutOfFuelError) {
    //handle error
  } else if (err instanceof FlatTireError) {
    //handle error
  }
}

Before you can do so, of course the error must be explicitly thrown in your code:

try {
  const car = new Car() //imagine we have a Car object

  if (!car.fuel) {
    throw new OutOfFuelError('No fuel!')
  }
  if (car.flatTire) {
    throw new FlatTireError('Flat tire!')
  }
} catch (err) {
  if (err instanceof OutOfFuelError) {
    //handle error
  } else if (err instanceof FlatTireError) {
    //handle error
  }
}

During the error creation you can also customize anything related to the class, even customizing the parameters received by the constructor if you need:

class OutOfFuelError extends Error {
  constructor(message) {
    super(message)
    this.name = "OutOfFuelError"
  } 
}