• If an exception happens in “scheduled” code, like in setTimeout, then try..catch won’t catch it.
  • That’s because try..catch actually wraps the setTimeout call that schedules the function.
  • But the function itself is executed later, when the engine has already left the try..catch construct.
  • To catch an exception inside a scheduled function, try..catch must be inside that function.

setTimeout(function() {
  try {
    noSuchVariable; // try..catch handles the error!
  } catch (error) {
    console.log(error);
  } finally {
		// Execute always ...
	}
}, 1000);

• The try..finally construct, without catch clause, is also useful.
  • We apply it when we don’t want to handle errors right here, but want to be sure that processes that we started are finalized.

Error Object

• When an error occurs, JavaScript generates an object containing the details about it.
• For all built-in errors, the error object inside catch block has two main properties:
  • name: error name.
    • For an undefined variable that’s "ReferenceError".
  • message: textual message about error details.
• There are other non-standard properties available in most environments.
  • One of most widely used and supported is stack.

Throwing Custom Errors

• The throw operator generates an error.
• Technically, we can use anything as an error object.
  • That may be even a primitive, like a number or a string.
  • It’s better to use objects, preferably with name and message properties.
• JavaScript has many built-in constructors for standard errors: Error, SyntaxError, ReferenceError, TypeError and others.

let error = new Error(message);
let error = new SyntaxError(message);
let error = new ReferenceError(message);

throw error;
throw new Error(message);

• For built-in errors (not for any objects, just for errors), the name property is exactly the name of the constructor.
  • Message is taken from the argument.

Extending Error

class Error {
  constructor(message) {
    this.message = message;
		// (different names for different built-in error classes)
    this.name = "Error";
  }
}

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