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browser-asap.js

"use strict";

// rawAsap provides everything we need except exception management.
var rawAsap = require("./raw");
// RawTasks are recycled to reduce GC churn.
var freeTasks = [];
// We queue errors to ensure they are thrown in right order (FIFO).
// Array-as-queue is good enough here, since we are just dealing with exceptions.
var pendingErrors = [];
var requestErrorThrow = rawAsap.makeRequestCallFromTimer(throwFirstError);

function throwFirstError() {
    if (pendingErrors.length) {
        throw pendingErrors.shift();
    }
}

/**
 * Calls a task as soon as possible after returning, in its own event, with priority
 * over other events like animation, reflow, and repaint. An error thrown from an
 * event will not interrupt, nor even substantially slow down the processing of
 * other events, but will be rather postponed to a lower priority event.
 * @param {{call}} task A callable object, typically a function that takes no
 * arguments.
 */
module.exports = asap;
function asap(task) {
    var rawTask;
    if (freeTasks.length) {
        rawTask = freeTasks.pop();
    } else {
        rawTask = new RawTask();
    }
    rawTask.task = task;
    rawAsap(rawTask);
}

// We wrap tasks with recyclable task objects.  A task object implements
// `call`, just like a function.
function RawTask() {
    this.task = null;
}

// The sole purpose of wrapping the task is to catch the exception and recycle
// the task object after its single use.
RawTask.prototype.call = function () {
    try {
        this.task.call();
    } catch (error) {
        if (asap.onerror) {
            // This hook exists purely for testing purposes.
            // Its name will be periodically randomized to break any code that
            // depends on its existence.
            asap.onerror(error);
        } else {
            // In a web browser, exceptions are not fatal. However, to avoid
            // slowing down the queue of pending tasks, we rethrow the error in a
            // lower priority turn.
            pendingErrors.push(error);
            requestErrorThrow();
        }
    } finally {
        this.task = null;
        freeTasks[freeTasks.length] = this;
    }
};

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