蓝鸟承诺链：结果'抓住'

Question

In order to make this question as useful to as many people as possible, I will exclude my specific implementation details beyond that fact that I am using the Bluebird promise library with Node + Express below.

So, let's say that I have the following chain (where P returns a promise, and res is the Express HTTP response object):

P().then(function(){
    // do nothing if all went well (for now)
    // we only care if there is an error
}).catch(function(error){
    res.status(500).send("An error occurred");
}).then(function(){
    return P();
}).then(function(pVal1){
    return [pVal1, P()];
}) // TODO: catch an error from P() here and log pVal1
.spread(function(pVal1, pVal2){
    if(pVal1 === pVal2) {
        console.log("Success!");
    } else {
        console.log("Failure");
    }
});

Where I have placed the TODO comment above is where I would like to catch an error that might occur from my call to P . If I do catch an error, I would like to log pVal1 and then send a 500 error, as is done in the first catch. However, I am not sure if this is possible with how I am structuring my chain.

I believe that I need to do some "branching," but I do not think that I understand this concept well enough to stop the asynchronous nature of JavaScript from getting the best of me! As such, any help is thoroughly appreciated.

Answer 1

Don't forget to catch errors in the end of the chain. That's also the place to send the response.

Catching errors in the middle of a chain is for intermittent error handling; the chain continues to run, so don't send a response just yet.

Here is something to try it out:

 // example middleware function handle(req, res, next) { log("----------------"); return async("p1", "foo").then(function (pVal1) { return pVal1; }).then(function (pVal1) { var p2a = async("p2a", "bar"), p2b = async("p2a", "bar").catch(function (error) { log("Logging: " + error + " (pVal1 " + pVal1 + ")"); }); return [p2a, p2b]; }).spread(function (pVal1, pVal2) { if (pVal1 === pVal2) { res.send("Success!"); } else { res.send("Failure"); } }).catch(function (error) { res.status(500).send("An error occurred"); log("Logging: " + error); }); } // --------------------------------------------------------------------- // mockup response object var res = { status: function (code) { log("Sending status: " + code); return this; }, send: function () { log("Sending response: " + [].join.call(arguments, " ")); return this; } }; // mockup promise generator function async(name, value) { return new P(function (resolve, reject) { if ( confirm("let " + name + " succeed?") ) { log(name + " succeeds..."); resolve(value); } else { log(name + " fails..."); reject(name + " has failed"); } }); } function log() { var msg = document.createElement("DIV"); msg.textContent = [].join.call(arguments, " "); document.getElementById("log").appendChild(msg) document.body.scrollTop = document.body.scrollHeight; } 

 button { position: fixed; top: 5px; } 

 <script src="http://cdnjs.cloudflare.com/ajax/libs/bluebird/2.9.33/bluebird.min.js"></script> <button onclick="handle(null, res, null)">Go</button> <div id="log"></div> 

Answer 2

This is possible if you use the explicit Promise.all instead of returning an array into .spread .

}).then(function(pVal1){
    // this becomes a `Promise.all` - the aggregation is explicit
    var all = Promise.all([pVal1, P()]);
    all.catch(function(e){  // "branching", we both return and `catch` the promise
        console.log("Error, pVal1 is", pVal1);
    });
    return all; // return it
}).spread(function(pVal1, pVal2){
       // ....
});