CODEWITHSUNDEEP
javascriptnode.jspromiseq
sakekasi
sakekasi

Reputation: 105

q difference between then with 2 functions and then followed by catch

Just a quick clarification question:

is there any difference between

promiseGeneratingFunction.then(successHandler, errorHandler).done();

and

promiseGeneratingFunction.then(successHandler).catch(errorHandler).done();

?

Thanks

Upvotes: 0

Views: 159

Answers (2)

loganfsmyth
loganfsmyth

Reputation: 161457

Yes, those two examples are different.

The other answers are not considering the effects of chaining. The docs say:

A sugar method, equivalent to promise.then(undefined, onRejected).

So yes, .catch is equivalent to .then. But that isn't what your examples show.

You compare

promiseGeneratingFunction.then(successHandler, errorHandler).done();

// VS

promiseGeneratingFunction.then(successHandler).catch(errorHandler).done();

If we break the second example down using the documented comparison, it is actually:

promiseGeneratingFunction.then(successHandler, errorHandler).done();

// VS

promiseGeneratingFunction.then(successHandler).then(undefined, errorHandler).done();

Those two behave differently in how they handle errors. Both examples will call successHandler if the generated promise succeeds, and call errorHandler if the promise fails, so they are very similar. The difference comes from how they handle errors in successHandler.

// this is a fulfilled promise since the thrown error was handled
Promise.resolve().then(function(){ throw new Error(); }).catch(function(){});

// this is a rejected promise
Promise.resolve().then(function() { throw new Error() }, function(){});

If your successHandler function were to throw an exception (or return a rejected promise), your first example has no error handler attached (errorHandler is only attached to promiseGeneratingFunction, so your promise will throw an uncaught exception from the .done(). On the second example, the error thrown from successHandler would also be caught in errorHandler since it is bound farther down the promise chain.

The equivalent synchronous code is something like this:

var error = null;
var result;

try {
    result = func();
} catch (err){
    error = err;
}

if (error){
    errorHandler(error);
} else {
    successHandler(result);
}

// VS

try {
    var result = func();

    successHandler(result);
} catch (err){
    errorHandler(err);
}

Upvotes: 7

monkeyinsight
monkeyinsight

Reputation: 4859

Q promises provide a fail shorthand for then when you are only interested in handling the error

As said documentation, it's only shorthand

Upvotes: -2

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