Are Promises in JavaScript like Exception Handling in Java?

Question

Just like the try(), catch() and throw() in Java as an exception handling is like then() and catch() in JavaScript as a promise!? Just a beginner asking some questions. Please don't judge. xD

Answer 1

Not really.. In JavaScript we have try/catch block as well.

Main benefits of Promises in JavaScript are things like Chaining.

The equivalent of Promises in Java is CompletableFuture . read about :)

Answer 2

No, they're quite different. Exception handling handles exceptions. Promises are a way to return values and act on them in an asynchronous environment, which JavaScript is.

Suppose you want to customize the JS confirm box. You could set up a jQuery dialog box, and return yes or no, depending on which button the user clicked. Like this:

function msgBoxConfirm(msgText, e) {
    e.preventDefault();               // Cancel the default behavior
    e.stopPropagation();              // Stop any other events from firing down the line
    $('#myDialogDiv').html(msgText).dialog({
        modal: true,
        title: boxTitle,
        buttons: {
            'Yes': function() {
                $(this).dialog('close');
                return true;
            },
            'No': function() {
                $(this).dialog('close');
                return false;
            }
        }
    });
}

Now, you call your message box function:

var retval = msgBoxConfirm('Do you really want to do that?', e);
if (!retval) {
    //Do the no behavior
} else {
    //Do the yes behavior
}

What you will find is that this code will continue before it finds out the value of retval , so retval will have a value of undefined when you evaluate it with the if . This is what it means to be asynchronous; the call to your function doesn't wait until the function is done executing before it moves on, potentially causing all sorts of bad behavior.

Promises are a way of waiting until the value is returned before checking it. To see how to set up this example properly using promises, see this .

Answer 3

try/catch detect the error itself and transfer control to catch part. this syntax exists in js too.

but in then/catch , you must notify occurring an error yourself (with calling reject ). This means you can notify error when you recognize an error is occurred. that error can be a real or not real error (programming/logical error).

Answer 4

Promises and exception handling are different, they are not same. In javascript promises give us a away to use . then to know when the function has completed its execution , it helps to know when async functions have completed its execution,and javascript also provides try/catch support as you know as that of java.