Speaking JavaScript

Chapter 14. Exception Handling

This chapter describes how JavaScript’s exception handling works. It begins with a general explanation of what exception handling is.

What Is Exception Handling?

In exception handling, you often group statements that are tightly coupled. If, while you are executing those statements, one of them causes an error, then it makes no sense to continue with the remaining statements. Instead, you try to recover from the error as gracefully as you can. This is loosely reminiscent of transactions (but without the atomicity).

Let’s look at code without exception handling:

function processFiles() {
    var fileNames = collectFileNames();
    var entries = extractAllEntries(fileNames);
    processEntries(entries);
}
function extractAllEntries(fileNames) {
    var allEntries = new Entries();
    fileNames.forEach(function (fileName) {
        var entry = extractOneEntry(fileName);
        allEntries.add(entry);  // (1)
    });
}
function extractOneEntry(fileName) {
    var file = openFile(fileName);  // (2)
    ...
}
...

What is the best way to react to an error in openFile() at (2)? Clearly, the statement (1) should not be executed anymore. But we wouldn’t want to abort extractAllEntries(), either. Instead, it is enough to skip the current file and continue with the next one. To do that, we add exception handling to the previous code:

function extractAllEntries(fileNames) {
    var allEntries = new Entries();
    fileNames.forEach(function (fileName) {
        try {
            var entry = extractOneEntry(fileName);
            allEntries.add(entry);
        }