为什么'新承诺（...）'返回'未定义'？

Question

It seems that if the 'resolve' function is not referenced in the function that is used to create a promise, then the promise is undefined. In the code below, ...

var count = 0;
var limit = 3;
//
var thePromise;
function timeout(_count) {
    thePromise.resolve(_count);
}
function sendMessage() {
    return new Promise(function(resolve, reject) {
        if (++count > limit) {
            reject({'Limit Exceeded': count});
        }
        else {
// With this line in place, the return value from this function
// (expected to be a promise) is undefined
            setTimeout(timeout.bind(null, count), 1000);
// If the line above is replaced by this, it works as expected
//            setTimeout(/*timeout.bind(null, count)*/function (_count) {
//                resolve(_count);
//            }.bind(null, count), 1000);
        }
    });
}

function sendAnother(_count) {
    console.log('Resolved with count %j', _count);
    return sendMessage();
}
function detectError(_error) {
    console.log('Rejected with %s', JSON.stringify(_error));
    process.exit(1);
}
thePromise = sendMessage();
thePromise = thePromise.then(function (_count) { return sendAnother(_count)}, function(_error) {detectError(_error)});

trying to do the resolve outside of the function that creates the promise, results in:

node-promises.js:6
    thePromise.resolve(_count);
               ^
TypeError: undefined is not a function
    at timeout (node-promises.js:6:16)
    at Timer.listOnTimeout (timers.js:110:15)

But if line 16 is commented out and lines 18-20 are uncommented, the output is:

Resolved with count 1

.. which is what I expected. What am I missing? This is using nodejs v0.12.2 on Windows 7, if that makes any difference.

Answer 1

It happens because of this line:

thePromise.resolve(_count);

there is no resolve function on that object. resolve comes from the function you have created when instantiating the new promise:

return new Promise(function(resolve, reject) {

By commenting out that line, and using the alternate function, you are calling the correct resolve() , which causes the desired output. One option to fix this could be to pass the resolve function into your timeout call, eg:

function timeout(resolve, _count) {
    resolve(_count);
}

Although I am not sure why you would want to do this.

---

Your title asks why new Promise is returning undefined, when the fact is that it isn't. It is indeed returning a valid promise. It is just that resolve is not a valid function on the promise object.