Part 4.5Error handling in JavaScript

Modern programming languages have some method of handling errors when they happen. The term handling means having a code path to follow when the error occurs, for instance it might mean simply displaying that an error has occured or it might have a different solution to the problem.

JavaScript has the try and the catch keywords to handle errors. This is shown in the next code sample:

var list = [30, 40, 50];
try {
	alert(list[1]);
	//Undefined listf, so getting an array index will not work
	alert(listf[1]);
} catch(e){
	alert(e);
}

Why errors happen

JavaScript is a language that is compiled and immediately interpreted. This means that it is not checked until compile time. When this is the case, if a program has a typing error or a value that is not within the normal bounds the program will stop executing.

In a language such as C, Java or VB.NET an error could be identified at compile time. This is, understandably, called a compile time error. An error that occurs when the program is running is called a runtime error. In a language such as Java, a compile time error will cause the program to stop compiling.

In JavaScript, both compile and runtime errors can be handled by a try-catch statement.

Try-catch

The previous example demonstrated an error being captured. This works by attempting (or trying) to run the code within the try block. If this fails, the error is a handled (caught) and the catch block is executed.

Most programming language support passing the error to the catch block. In the previous example, the error is passed to a variable e.

Producing errors

Most programming languages also offer a method to create an error. This is called throwing an error. In JavaScript, the throw keyword is used to throw an error:

throw "Error occured!";
alert("Reached");

In the previous example, because an error occured or at least an error was thrown, the alert could not be reached.