Using onerror

Let’s look at a few more constructs not available in PHP. Using either the onerror event or a combination of the try and catch keywords, you can catch JavaScript errors and deal with them yourself.

Events are actions that can be detected by JavaScript. Every element on a web page has certain events that can trigger JavaScript functions. For example, the onclick event of a button element can be set to call a function and make it run whenever a user clicks on the button.

Example 14-11 illustrates how to use the onerror event.

Example 14-11. A script employing the onerror event

<script>
onerror = errorHandler
document.writ("Welcome to this website") // Deliberate error

function errorHandler(message, url, line)
{
    out  = "Sorry, an error was encountered.\n\n";
    out += "Error: " + message + "\n";
    out += "URL: "   + url + "\n";
    out += "Line: "  + line + "\n\n";
    out += "Click OK to continue.\n\n";
    alert(out);
    return true;
}
</script>

The first line of this script tells the error event to use the new errorHandler function in the future. This function takes three parameters—a message, a url, and a line number—and it’s a simple matter to display all these in an alert pop-up.

Then, to test the new function, a syntax error is deliberately placed in the code with a call to document.writ instead of document.write (the final e is missing). Figure 14-1 shows the result of running this script in a browser. Using onerror this way can also be quite useful during the debugging process.

Figure 14-1. Using the onerror event with an alert method pop-up