Firefox: Promise.then not called asynchronously

Question

I read the Promise/A+ specification and it says under 2.2.4:

onFulfilled or onRejected must not be called until the execution context stack contains only platform code

But in Firefox (I tested 38.2.1 ESR and 40.0.3) the following script executes the onFulfilled method synchronously:

var p = Promise.resolve("Second");
p.then(alert);
alert("First");

(It does not seem to run using alerts here, it can also be tried here: http://jsbin.com/yovemaweye/1/edit?js,output)

It works as expected in other browsers or when using the ES6Promise-Polyfill.

Did I miss something here? I always though that one of the points of the then-method is to ensure asynchronous execution.

Edit:

It works when using console.log, see answer by Benjamin Gruenbaum:

function output(sMessage) {
  console.log(sMessage);
}

var p = Promise.resolve("Second");
p.then(output);

output("First");

As he points out in the comments, this also happens when using synchronous requests, which is exactly why it happens in your test scenario. I created a minimal example of what happens in our Tests:

function request(bAsync) {
  return new Promise(function(resolve, reject) {
    var xhr = new XMLHttpRequest();
    xhr.addEventListener("readystatechange", function() {
      if (xhr.readyState === XMLHttpRequest.DONE) {
        resolve(xhr.responseText);
      }
    });
    xhr.open("GET", "https://sapui5.hana.ondemand.com/sdk/resources/sap-ui-core.js", !!bAsync);
    xhr.send();
  });
}

function output(sMessage, bError) {
  var oMessage = document.createElement("div");
  if (bError) {
    oMessage.style.color = "red";
  }
  oMessage.appendChild(document.createTextNode(sMessage));
  document.body.appendChild(oMessage);
}

var sSyncData = null;
var sAsyncData = null;

request(true).then(function(sData) {
  sAsyncData = sData;
  output("Async data received");
});

request(false).then(function(sData) {
  sSyncData = sData;
  output("Sync data received");
});


// Tests
if (sSyncData === null) {
  output("Sync data as expected");
} else {
  output("Unexpected sync data", true);
}
if (sAsyncData === null) {
  output("Async data as expected");
} else {
  output("Unexpected async data", true);
}

In Firefox this leads to:

Answer 1

This is because you're using alert

When you use alert here it blocks and all bets are off - the page has frozen, execution halted and things are at "platform level".

It might be considered a bug, and it's certainly not what I would expect - but at the core this is about the incompatibility between alert and JavaScript task/microtask semantics.

If you change that alert to a console.log or appending to document.innerHTML you'd get the result you expect.

var alert = function(arg) { // no longer a magical and blocking operation
  document.body.innerHTML += "<br/>" + arg;
}

// this code outputs "First, Second, Third" in all the browsers.

setTimeout(alert.bind(null, "Third"), 0);

var p = Promise.resolve("Second");
p.then(alert);

alert("First");
            
 

From what I can tell, this is actually permitted optional behavior:

Optionally, pause while waiting for the user to acknowledge the message.

(Emphasis mine)

Basically, what firefox does is this:

• Execute until it encounters the first alert.
• Run any microtasks to completion before pausing (tasks are paused, and not microtasks).
• The then is run as a microtask, so "Second" gets queued and precedes the alert.
• The Second alert gets run.
• The First alert gets run.

Confusing, but allowed from what I can tell.