You-dont-know-js: Error handling with Promises using setTimeout (async & performance - Chapter 3)

[Konrud]

There is an interesting issue with handling Exception with Promises, using setTimeout, that I can't wrap my head around.

Example:

function boo() {
    return new Promise(function (resolve, reject) {
        setTimeout(function () {
            throw new Error("Error !!!!!!!!!!!!!!!!!!!!!!!!!!!;");
            resolve("You're my hero !");
        }, 2 * 1000);
    });
};

The code above will throw an error after 2 seconds, so according to the Chapter 3 from async & performance we should be able to catch that error in catch method or in rejection handler from the then method of the Promise.

boo()
.then(function (val) {
    debugger;
    console.log("THEN FULLFILLED");
},
function (rejVal) {
    debugger; // <--- Should catch the Error here
    console.log("THEN REJECTION");
})
.catch(function (er) {
    debugger; // <--- or here
    console.log("CATCH");
});

The interesting thing is that exception is thrown in global context and never gets into catch or rejection handler from then method.

But, if we remove setTimeout from the example,

i.e.

function boo() {
    return new Promise(function (resolve, reject) {
        //setTimeout(function () {
            throw new Error("Error !!!!!!!!!!!!!!!!!!!!!!!!!!!;");
            //resolve("You're my hero !");
        //}, 2 * 1000);
    });
};

This will work as expacted and reject handler from the then method is called.

[ghost]

I have commented the usage of throwing error and called reject with error message.

1. I assumed that you are using setTimeout to simulate an ajax call.
2. Using throw directly on the setTimeout will take preference and will throw error globally
   and will not simulate that the ajax call has returned with a failure.
3. When an ajax call is made, it could succeed or fail for various reasons.
4. So I think if you are simulating for the ajax call to reject with failure then a simple
   reject call with an error message will achieve what you want.

function boo() { return new Promise(function (resolve, reject) { setTimeout(function () { /*throw new Error("Error !!!!!!!!!!!!!!!!!!!!!!!!!!!;"); resolve("You're my hero !");*/ reject("Error"); }, 2 * 100); }); };

Please let me know if I have understood your intention correctly.

[Konrud]

@deeparaveendran thank you for your time and message. What I was trying to achieve is to simulate an async request, this is not necessarily an Ajax request, just any async operation that could take some time.
I was wondering what would happen if I throw an Error there.
I was sure that the catch block should, well catch it but it didn't happen. That is why I opened that issue hopping that @getify could help me with this.

Regarding your statement that: "_Using throw directly on the setTimeout will take preference and will throw error globally...._" could you please elaborate more, I couldn't find any information about it.

[getify]

When you use the setTimeout, the promise constructor has already completed, synchronously, before you throw the exception... so it obviously cannot catch it after the fact. More precisely, exceptions are synchronous to their execution stack, and you've moved the exception to a separate execution stack by wrapping it in the timeout.

[ghost]

@Konrud Kyle Simpson has explained very clearly and that is what I meant too. Thank you @getify

[getify]

(clearing out issues for second edition work)