Can I catch an error from async without using awai

问题:

Can errors from a non-awaited async call be caught, sent to an original encapsulating try/catch, or raise an uncaught exception?

Here's an example of what I mean:

async function fn1() {
    console.log('executing fn1');
}

async function fn2() {
    console.log('executing fn2');
    throw new Error('from fn2');
}

async function test() {
    try {
        await fn1();
        fn2();
    }
    catch(e) {
        console.log('caught error inside test:', e);
    }
}

test();

In this scenario, the error thrown from fn2 will be swallowed silently, and definitely not caught by the original try/catch. I believe this is expected behavior, since fn2 is most likely being shoved off to the event loop to finish at some point in the future, and test doesn't care when it finishes (which is intentional).

Is there any way to ensure that errors are not accidentally swallowed by a structure like this, short of putting a try/catch internal to fn2 and doing something like emitting an error? I would even settle for an uncaught error without knowing how to catch it, I think -- I don't expect thrown errors to be typical program flow with what I am writing, but swallowing errors makes it relatively annoying to debug.

Side note, I'm using Babel to transpile the code using the babel-runtime transform, and executing it with node.

回答1:

Dealing with unhandled rejected native promises (and async/await uses native promises) is a feature supported now in V8. It's used in the latest Chrome to output debugging information when a rejected promise is unhandled; try the following at the Babel REPL:

async function executor() {
  console.log("execute");
}

async function doStuff() {
  console.log("do stuff");
  throw new Error("omg");
}

function handleException() {
  console.error("Exception handled");
}

(async function() {
  try {
      await executor();
      doStuff();
  } catch(e) {
      handleException();
  }
})()

You see that, even though the exception from doStuff() is lost (because we're not using await when we call it), Chrome logs that a rejected promise was unhandled to the console:

This is also available in Node.js 4.0+, though it requires listening to a special unhandledRejection event:

process.on('unhandledRejection', function(reason, p) {
    console.log("Unhandled Rejection at: Promise ", p, " reason: ", reason);
    // application specific logging, throwing an error, or other logic here
});

回答2:

If you are familiar with promises, use them. If not, you can try this example to make you code more asynchronous :)

function fn1(callback) {
    console.log('executing fn1');
    callback({status: true});
}

function fn2(callback) {
    console.log('executing fn2');
    callback({status: false});
}

function test() {
    fn1(function(result) {
        console.log('fn1 executed with status ' + result.status);
    });

    fn2(function(result) {
        console.log('fn2 executed with status ' + result.status);
        if (result.status == false) {
            console.log('error in fn2');
        }
    });
}

test();