in JavaScript, how to wrap a promise in timeout?

Question

It\'s a common pattern to implement timeout of some asynchronous function, using deffered/promise:

// Create a Deferred and return its Promise
function timeout(f         


        

Answer 1

You should always promsiify at the lowest level possible. Let's start from the basics.

I'll use jQuery promises here, but this should really be done with a stronger library like Bluebird Let's start simple, by creating our delay as:

function delay(ms){
    var d = $.Deferred();
    setTimeout(function(){ d.resolve(); }, ms);
    return d.promise();
}

Note delay doesn't do anything surprising, all our delay function does is cause a delay of ms milliseconds.

Now, for your library, we want to create a version of doSomething that works with promises:

 superImportantLibrary.doSomethingAsync = function(){
     var d = $.Deferred();
     superImportantLibrary.doSomething(function(data){ d.resolve(data); });
     return d.promise();
 };

Note both our delay and doSomethingAsync functions both do just one thing. Now the fun begins.

function timeout(promise,ms){
    var timeout = delay(ms); // your timeout
    var d = $.Deferred();
    timeout.then(function(){ d.reject(new Error("Timed Out")); });
    promise.then(function(data){ d.resolve(data); });
    return d.promise();
}

timeout(superImportantLibrary.doSomethingAsync(),1000).then(function(data){
     // handle success of call
}, function(err){
     // handle timeout or API failure.
});

Now in Bluebird, this whole code would have been:

superImportantLibrary.doSomethingAsync().timeout(1000).then(function(){
    // complete and did not time out.
});