Custom errors, extending Error

When we develop something, we often need our own error classes to reflect specific things that may go wrong in our tasks. For errors in network operations we may need HttpError, for database operations DbError, for searching operations NotFoundError and so on.

Our errors should support basic error properties like message, name and, preferably, stack. But they also may have other properties of their own, e.g. HttpError objects may have a statusCode property with a value like 404 or 403 or 500.

JavaScript allows to use throw with any argument, so technically our custom error classes don’t need to inherit from Error. But if we inherit, then it becomes possible to use obj instanceof Error to identify error objects. So it’s better to inherit from it.

As the application grows, our own errors naturally form a hierarchy. For instance, HttpTimeoutError may inherit from HttpError, and so on.

Extending Error

As an example, let’s consider a function readUser(json) that should read JSON with user data.

Here’s an example of how a valid json may look:

let json = `{ "name": "John", "age": 30 }`;

Internally, we’ll use JSON.parse. If it receives malformed json, then it throws SyntaxError. But even if json is syntactically correct, that doesn’t mean that it’s a valid user, right? It may miss the necessary data. For instance, it may not have name and age properties that are essential for our users.

Our function readUser(json) will not only read JSON, but check (“validate”) the data. If there are no required fields, or the format is wrong, then that’s an error. And that’s not a SyntaxError, because the data is syntactically correct, but another kind of error. We’ll call it ValidationError and create a class for it. An error of that kind should also carry the information about the offending field.

Our ValidationError class should inherit from the Error class.

The Error class is built-in, but here’s its approximate code so we can understand what we’re extending:

// The "pseudocode" for the built-in Error class defined by JavaScript itself
class Error {
  constructor(message) {
    this.message = message;
    this.name = "Error"; // (different names for different built-in error classes)
    this.stack = <call stack>; // non-standard, but most environments support it
  }
}

Now let’s inherit ValidationError from it and try it in action:

class ValidationError extends Error {
  constructor(message) {
    super(message); // (1)
    this.name = "ValidationError"; // (2)
  }
}

function test() {
  throw new ValidationError("Whoops!");
}

try {
  test();
} catch(err) {
  alert(err.message); // Whoops!
  alert(err.name); // ValidationError
  alert(err.stack); // a list of nested calls with line numbers for each
}

Please note: in the line (1) we call the parent constructor. JavaScript requires us to call super in the child constructor, so that’s obligatory. The parent constructor sets the message property.

The parent constructor also sets the name property to "Error", so in the line (2) we reset it to the right value.

Let’s try to use it in readUser(json):

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

// Usage
function readUser(json) {
  let user = JSON.parse(json);

  if (!user.age) {
    throw new ValidationError("No field: age");
  }
  if (!user.name) {
    throw new ValidationError("No field: name");
  }

  return user;
}

// Working example with try..catch

try {
  let user = readUser('{ "age": 25 }');
} catch (err) {
  if (err instanceof ValidationError) {
    alert("Invalid data: " + err.message); // Invalid data: No field: name
  } else if (err instanceof SyntaxError) { // (*)
    alert("JSON Syntax Error: " + err.message);
  } else {
    throw err; // unknown error, rethrow it (**)
  }
}

The try..catch block in the code above handles both our ValidationError and the built-in SyntaxError from JSON.parse.

Please take a look at how we use instanceof to check for the specific error type in the line (*).

We could also look at err.name, like this:

// ...
// instead of (err instanceof SyntaxError)
} else if (err.name == "SyntaxError") { // (*)
// ...

The instanceof version is much better, because in the future we are going to extend ValidationError, make subtypes of it, like PropertyRequiredError. And instanceof check will continue to work for new inheriting classes. So that’s future-proof.

Also it’s important that if catch meets an unknown error, then it rethrows it in the line (**). The catch block only knows how to handle validation and syntax errors, other kinds (caused by a typo in the code or other unknown reasons) should fall through.

Further inheritance

The ValidationError class is very generic. Many things may go wrong. The property may be absent or it may be in a wrong format (like a string value for age instead of a number). Let’s make a more concrete class PropertyRequiredError, exactly for absent properties. It will carry additional information about the property that’s missing.

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

class PropertyRequiredError extends ValidationError {
  constructor(property) {
    super("No property: " + property);
    this.name = "PropertyRequiredError";
    this.property = property;
  }
}

// Usage
function readUser(json) {
  let user = JSON.parse(json);

  if (!user.age) {
    throw new PropertyRequiredError("age");
  }
  if (!user.name) {
    throw new PropertyRequiredError("name");
  }

  return user;
}

// Working example with try..catch

try {
  let user = readUser('{ "age": 25 }');
} catch (err) {
  if (err instanceof ValidationError) {
    alert("Invalid data: " + err.message); // Invalid data: No property: name
    alert(err.name); // PropertyRequiredError
    alert(err.property); // name
  } else if (err instanceof SyntaxError) {
    alert("JSON Syntax Error: " + err.message);
  } else {
    throw err; // unknown error, rethrow it
  }
}

The new class PropertyRequiredError is easy to use: we only need to pass the property name: new PropertyRequiredError(property). The human-readable message is generated by the constructor.

Please note that this.name in PropertyRequiredError constructor is again assigned manually. That may become a bit tedious – to assign this.name = <class name> in every custom error class. We can avoid it by making our own “basic error” class that assigns this.name = this.constructor.name. And then inherit all our custom errors from it.

Let’s call it MyError.

Here’s the code with MyError and other custom error classes, simplified:

class MyError extends Error {
  constructor(message) {
    super(message);
    this.name = this.constructor.name;
  }
}

class ValidationError extends MyError { }

class PropertyRequiredError extends ValidationError {
  constructor(property) {
    super("No property: " + property);
    this.property = property;
  }
}

// name is correct
alert( new PropertyRequiredError("field").name ); // PropertyRequiredError

Now custom errors are much shorter, especially ValidationError, as we got rid of the "this.name = ..." line in the constructor.

Wrapping exceptions

The purpose of the function readUser in the code above is “to read the user data”. There may occur different kinds of errors in the process. Right now we have SyntaxError and ValidationError, but in the future readUser function may grow and probably generate other kinds of errors.

The code which calls readUser should handle these errors. Right now it uses multiple ifs in the catch block, that check the class and handle known errors and rethrow the unknown ones.

The scheme is like this:

try {
  ...
  readUser()  // the potential error source
  ...
} catch (err) {
  if (err instanceof ValidationError) {
    // handle validation errors
  } else if (err instanceof SyntaxError) {
    // handle syntax errors
  } else {
    throw err; // unknown error, rethrow it
  }
}

In the code above we can see two types of errors, but there can be more.

If the readUser function generates several kinds of errors, then we should ask ourselves: do we really want to check for all error types one-by-one every time?

Often the answer is “No”: we’d like to be “one level above all that”. We just want to know if there was a “data reading error” – why exactly it happened is often irrelevant (the error message describes it). Or, even better, we’d like to have a way to get the error details, but only if we need to.

The technique that we describe here is called “wrapping exceptions”.

1. We’ll make a new class ReadError to represent a generic “data reading” error.
2. The function readUser will catch data reading errors that occur inside it, such as ValidationError and SyntaxError, and generate a ReadError instead.
3. The ReadError object will keep the reference to the original error in its cause property.

Then the code that calls readUser will only have to check for ReadError, not for every kind of data reading errors. And if it needs more details of an error, it can check its cause property.

Here’s the code that defines ReadError and demonstrates its use in readUser and try..catch:

class ReadError extends Error {
  constructor(message, cause) {
    super(message);
    this.cause = cause;
    this.name = 'ReadError';
  }
}

class ValidationError extends Error { /*...*/ }
class PropertyRequiredError extends ValidationError { /* ... */ }

function validateUser(user) {
  if (!user.age) {
    throw new PropertyRequiredError("age");
  }

  if (!user.name) {
    throw new PropertyRequiredError("name");
  }
}

function readUser(json) {
  let user;

  try {
    user = JSON.parse(json);
  } catch (err) {
    if (err instanceof SyntaxError) {
      throw new ReadError("Syntax Error", err);
    } else {
      throw err;
    }
  }

  try {
    validateUser(user);
  } catch (err) {
    if (err instanceof ValidationError) {
      throw new ReadError("Validation Error", err);
    } else {
      throw err;
    }
  }

}

try {
  readUser('{bad json}');
} catch (e) {
  if (e instanceof ReadError) {
    alert(e);
    // Original error: SyntaxError: Unexpected token b in JSON at position 1
    alert("Original error: " + e.cause);
  } else {
    throw e;
  }
}

In the code above, readUser works exactly as described – catches syntax and validation errors and throws ReadError errors instead (unknown errors are rethrown as usual).

So the outer code checks instanceof ReadError and that’s it. No need to list all possible error types.

The approach is called “wrapping exceptions”, because we take “low level” exceptions and “wrap” them into ReadError that is more abstract. It is widely used in object-oriented programming.

Summary

• We can inherit from Error and other built-in error classes normally. We just need to take care of the name property and don’t forget to call super.
• We can use instanceof to check for particular errors. It also works with inheritance. But sometimes we have an error object coming from a 3rd-party library and there’s no easy way to get its class. Then name property can be used for such checks.
• Wrapping exceptions is a widespread technique: a function handles low-level exceptions and creates higher-level errors instead of various low-level ones. Low-level exceptions sometimes become properties of that object like err.cause in the examples above, but that’s not strictly required.