javascript - Promise - is it possible to force cancel a promise -


i use es6 promises manage of network data retrieval , there situations need force cancel them.

basically scenario such have type-ahead search on ui request delegated backend has carry out search based on partial input. while network request (#1) may take little bit of time, user continues type triggers backend call (#2)

here #2 naturally takes precedence on #1 cancel promise wrapping request #1. have cache of promises in data layer can theoretically retrieve attempting submit promise #2.

but how cancel promise #1 once retrieve cache?

could suggest approach?

no. can't yet.

es6 promises not support cancellation yet. it's on way, , design lot of people worked hard on. sound cancellation semantics hard right , work in progress. there interesting debates on "fetch" repo, on esdiscuss , on several other repos on gh i'd patient if you.

but, but, but.. cancellation important!

it is, reality of matter cancellation really important scenario in client-side programming. cases describe aborting web requests important , they're everywhere.

so... language screwed me!

yeah, sorry that. promises had in first before further things specified - went in without useful stuff .finally , .cancel - it's on way though, spec through dom. cancellation not afterthought it's time constraint , more iterative approach api design.

so can do?

you have several alternatives:

  • use third party library bluebird can move lot faster spec , have cancellation bunch of other goodies - large companies whatsapp do.
  • pass cancellation token.

using third party library pretty obvious. token, can make method take function in , call it, such:

function getwithcancel(url, token) { // token cancellation    var xhr = new xmlhttprequest;    xhr.open("get", url);    return new promise(function(resolve, reject) {       xhr.onload = function() { resolve(xhr.responsetext); });       token.cancel = function() {  // specify cancellation           xhr.abort(); // abort request           reject(new error("cancelled")); // reject promise       };       xhr.onerror = reject;    }); };

which let do:

var token = {}; var promise = getwithcancel("/someurl", token);  // later want abort promise: token.cancel();

your actual use case - last

this isn't hard token approach:

function last(fn) {     var lasttoken = { cancel: function(){} }; // start no op     return function() {         lasttoken.cancel();         var args = array.prototype.slice.call(arguments);         args.push(lasttoken);         return fn.apply(this, args);     }; }

which let do:

var synced = last(getwithcancel); synced("/url1?q=a"); // canceled  synced("/url1?q=ab"); // canceled synced("/url1?q=abc");  // canceled synced("/url1?q=abcd").then(function() {     // run });

and no, libraries bacon , rx don't "shine" here because they're observable libraries, have same advantage user level promise libraries have not being spec bound. guess we'll wait have , see in es2016 when observables go native. are nifty typeahead though.


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