What is faster: try catch vs Promise

Question

I heard such an opinion that you should avoid usage of try/catch at all because it takes many resources. So could the promise error handling to be faster? Or it does not matter at all?

function f(somethingDangerous) {
  return new Promise((resolve, reject) => {
    // try {
    //   somethingDangerous();
    //   resolve();
    // } catch (err) {
    //   reject(err);
    // }

    // VS

    somethingDangerous();
    resolve();
  }).catch((err) => {
    console.error('Catched: ' + err);
  });
}

f(() => {throw 'DANGEROUS THING';});

UPD: I know that try/catch won't work with async code inside. I'm just wondering if there any reasons to avoiding of try/catch because of performance issues? And is there any difference between the two approaches above?

UPD2: Tried to race my horses :) https://jsperf.com/try-catch-vs-promise

Answer 1

You should use Promises only for asynchronous functions and nothing else. Do not abuse them as an error monad, that would be a waste of resources and their inherent asynchrony will make every­thing more cumbersome.

When you have synchronous code, use try/catch for exception handling.

/* Wrong */
return new Promise(function(resolve, reject) {
    resolve(x / y);
}).catch(err => NaN)

/* Right */
try {
    return x / y;
} catch(e) {
    return NaN;
}

If you already have promise code, you can avoid that in certain situations: when you want the exception to reject the promise. In those cases you should just let the builtin error handling of your promises do its job, and not complicate everything by an additional but pointless try/catch layer:

/* Wrong */
new Promise(function(resolve, reject) {
    try { // when used synchronous in the executor callback
        …
        resolve(somethingSynchronous());
    } catch (e) {
        reject(e);
    }
});

/* Right */
new Promise(function(resolve, reject) {
    …
    resolve(somethingExceptionally());
});

/* Wrong */
….then(function(res) {
    try {
        …
        return somethingExceptionally();
    } catch(e) {
        return Promise.reject(e);
    }
}).…

/* Right */
….then(function(res) {
    …
    return somethingExceptionally();
}).…

Answer 2

try/catch idiom works very well when you have fully synchronous code, but asynchronous operations render it useless, no errors will be caught. i.e., the function will begin its course while the outer stack runs through and gets to the last line without any errors. If an error occurs at some point in the future inside asynchronous function – nothing will be caught.

When we use Promises, “we’ve lost our error handling”, you might say. That’s right, we don’t need to do anything special here to propagate error because we return a promise and there’s built in support for error flow.