microtask.md

Promise spec: https://promisesaplus.com/#point-34 2.2.4: onFulfilled or onRejected must not be called until the execution context stack contains only platform code. [3.1]. This can be implemented with either a “macro-task” mechanism such as setTimeout or setImmediate, or with a “micro-task” mechanism such as MutationObserver or process.nextTick. Since the promise implementation is considered platform code, it may itself contain a task-scheduling queue or “trampoline” in which the handlers are called. ECMAScript promise spec: https://tc39.github.io/ecma262/#sec-promise-objects

Promise

https://html.spec.whatwg.org/#microtask-queue

Each event loop has a microtask queue.

2 kinds of microtasks: solitary callback, compound microtasks. This spec only covers solitary.

Microtask can be moved to a regular task queue if it spins the event loop.

Algorithms:

perform a microtask checkpoint:

let MicrotaskQueue = function(eventLoop) {
    this.eventLoop = eventLoop;
}

MicrotaskQueue.prototype = {

    microtasks: [],

    queueTask: task => {
        this.microtasks.push(task);
    },

    getOldestTask: () => {
        return this.microtasks.shift();
    },

    performCheckpoint: () => {
        if (this.performing)
            return;
        this.performing = this.true;

        while (let task = this.getOldestTask()) {

            this.eventLoop.currentlyRunningTask = task;

            task.run();

            this.eventLoop.currentlyRunningTask = null;
        }

        // For each environment settings object whose responsible event loop
        // is this event loop, notify about rejected promises on that
        // environment settings object.
    }

}

When is MicrotaskQueue.performCheckpoint() called?

1. https://html.spec.whatwg.org/#clean-up-after-running-script If the JavaScript execution context stack is now empty, perform a microtask checkpoint.
2. https://html.spec.whatwg.org/#event-loop-processing-model Runs after task, before rendering Step ... execute task Step 6: Microtasks: Perform a microtask checkpoint. Step 7: Update the rendering
3. https://heycam.github.io/webidl/#es-invoking-callback-functions Invoking callback functions: Invoking callback performs "Clean up after running script with relevant settings"

Microtasks are queued using: https://html.spec.whatwg.org/#enqueuejob(queuename,-job,-arguments):queue-a-microtask

8.1.3.7.1 EnqueueJob(queueName, job, arguments)

ECMAScript promise spec: https://tc39.github.io/ecma262/#sec-promise-objects

ECMAScript defines EnqueueJob method for executing promise callbacks.

EnqueueJob is defined in https://html.spec.whatwg.org/#enqueuejob(queuename,-job,-arguments):queue-a-microtask

Firefox:

XPCJSRuntime::BeforeProcessTask Promise::PerformMicroTaskCheckpoint

Bugs:

• PerformMicroTaskCheckpoint called from XPCJSRuntime::BeforeProcessTask it should happen after task

FF run & build ./mach build ./mach run

FF microtask bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1193394

1. Promise runs immediately after a callback rAF Promise.resolve().then( var p = new Promise( function(fulfill, reject) { fulfill(); // log("promise A fulfill"); }) promise.resolve() assert
2. Promise resolved inside a promise is run immediately
3. Inside event loop, promise runs after a task, and before rAF
4. What happens with alerts

Firefox rAF schedule: nsGlobalWindow::RequestAnimationFrame mDoc->ScheduleFrameRequestCallback nsRefreshDriver::ScheduleFrameRequestCallbacks mFrameRequestCallbackDocs.AppendElement(aDocument); dispatch: nsRefreshDriver::Tick nsRefreshDriver::RunFrameRequestCallbacks

Firefox promise dispatch dispatches from: Promise::PerformMicroTaskCheckpoint called from: nsGlobalWindow::RunTimeoutHandler XPCJSRuntime::BeforeProcessTask