Chapter 10: Error handling

10.1 Error handling, "try..catch"

The “try…catch” syntax

try {

  // code...

} catch (err) {

  // error handling

}

try..catch only works for runtime errors, not syntax errors.

try..catch works synchronously, which means that it cannot catch the errors in the scheduled function such as setTimeout, unless the try..catch is inside that function.

Error object

• name: Example: ReferenceError

• message: Textual message about error details.

• stack: A string with information about the sequence of nested calls that led to the error.

Using “try…catch”

If we don’t need error details, catch may omit it.

Throwing our own errors

JavaScript has many built-in constructors for standard errors: Error, SyntaxError, ReferenceError, TypeError and others.

throw <error object>

throw new SyntaxError("Incomplete data: no name");

Rethrowing

Catch should only process errors that it knows and “rethrow” all others.

if (e instanceof SyntaxError) {
    alert( "JSON Error: " + e.message );
} else {
    throw e; // rethrow (*)
}

try…catch…finally

If finally exists, it runs in all cases:

• After try, if there were no errors.

• After catch, if there were errors.

The finally clause is often used when we start doing something and want to finalize it in any case of outcome.

Variables are local inside 1try..catch..finally`.

Before the try returns the code, finally is executed just before it.

The try..finally construct, without catch clause, is also useful.

Global catch

window.onerror = function(message, url, line, col, error) {
  // ...
};

• message: Error message.

• url: URL of the script where error happened.

• line, col: Line and column numbers where error happened.

• error: Error object.

10.2 Custom errors, extending Error

Extending Error

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

Further inheritance

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