try-catch in javascript… isn't it a good practice?

Question

There is a provision for try-catch block in javascript. While in java or any other language it is mandatory to have error handling, I don't see anybody using them in javascript for greater extent. Isn't it a good practice or just we don't need them in javascript?

Answer 1

One should avoid throw errors as the way to pass error conditions around in applications.

The throw statement should only be used "For this should never happen, crash and burn. Do not recover elegantly in any way"

try catch however is used in situation where host objects or ECMAScript may throw errors.

Example:

var json
try {
    json = JSON.parse(input)
} catch (e) {
    // invalid json input, set to null
    json = null
}

Recommendations in the node.js community is that you pass errors around in callbacks (Because errors only occur for asynchronous operations) as the first argument

fs.readFile(uri, function (err, fileData) {
    if (err) {
        // handle
        // A. give the error to someone else
        return callback(err)
        // B. recover logic
        return recoverElegantly(err)
        // C. Crash and burn
        throw err
    }
    // success case, handle nicely
})

There are also other issues like try / catch is really expensive and it's ugly and it simply doesn't work with asynchronous operations.

So since synchronous operations should not throw an error and it doesn't work with asynchronous operations, no-one uses try catch except for errors thrown by host objects or ECMAScript

Answer 2

Try/catch in Javascript is not as bullet-proof as in other languages, due to Javascript's asynchronous nature. Consider this snippet:

try {
    setTimeout(function() {
        do_something_that_throws();
    }, 1000);
}
catch (e) {
    alert("You won't see this!");
}

The problem is that the control flow leaves the try block before do_something_that_throws() gets executed, so the error thrown inside the callback never gets catched.

So try/catch is basically inappropriate in many cases, and it's not always obvious whether something executes code asynchronously or not. Fortunately, javascript with its peculiar single-threaded-asynchronous-callback idiom and its support for actual closures provides an elegant alternative: continuation-passing style error handling. Just pass the proper response to any error as a function, e.g.:

setTimeout(function () {
    do_something_that_calls_err(function(err) {
        alert("Something went wrong, namely this: " + err);
    }),
    1000);

Answer 3

I believe that much of the reason that try..catch is rare in JavaScript is because the language has a pretty high tolerance for error. The vast majority of situations can be handled by using code checks, good defaults, and asynchronous events. In some cases, simply using a pattern will prevent issues:

function Foo() {
    //this may or may not be called as a constructor!!
    //could accidentally overwrite properties on window
}

function Bar() {
    if (!(this instanceof Bar)) {
        return new Bar();
    }
    //this will only work on Bar objects, and wont impact window
}

Some of the major issues in other languages that cause exceptions to occur simply don't exist in JS. Type casting isn't needed the vast majority of the time. Instead, the preferred method is typically to feature check (enforcing a particular interface):

function doFoo(arg) {
    if (arg.foo) {
        arg.foo();
    } else {
        Bar.prototype.foo.call(arg);
    }
}