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    I broke AJAX in Chrome 52 ?

    This strange bug from yesterday won’t let me sleep. Why does Chrome 52 sometimes behave like different instances of a class are the same object?

    Backbone’s fetch method triggers the bug. But maybe that’s not the real issue? I’d hate to submit a bug report to Chrome project only to be told “Fool, that’s a Backbone issue.”

    It’s terrifying to tell the Chrome team they made a mistake.

    This is what yesterday’s repro code looks like:

    var BugModel = Backbone.Model.extend({
    url: "bla.json",
    let bug = new BugModel();
    success: () => {
    console.log("fetch 1"); // prints
    function doWeirdness(bug) {
    let newBug = new BugModel({ id: 1 });
    console.log("about to re-fetch"); // prings
    success: () => console.log("fetch 2", newBug), // doesn't print
    error: () => console.log("error"),
    success: () => console.log("fetch 3"), // prints

    2 fetches, 1 callback

    If you press Cmd+R, the bug happens. If you press Cmd+Shift+R, it does not. That’s a new clue that points at either Chrome’s speed optimizations, or worse, the network stack. Can we call it a network stack? I guess Chrome is almost an operating system at this point …

    Adding console.log(newBug == bug) prints false, which implies that Chrome does not think both instances are the same object. This invalidates my original hypothesis. ?

    So what does Backbone’s Model.fetch method do?

    fetch: function(options) {
    options = _.extend({parse: true}, options);
    var model = this;
    var success = options.success;
    options.success = function(resp) {
    var serverAttrs = options.parse ? model.parse(resp, options) : resp;
    if (!model.set(serverAttrs, options)) return false;
    if (success) success.call(options.context, model, resp, options);
    model.trigger('sync', model, resp, options);
    wrapError(this, options);
    return this.sync('read', this, options);

    A lot of this stuff is unnecessary in ES6, but Backbone is from the before times.

    We start with a default value for options - {parse: true}, then use the var = this trick because we don’t have arrow functions. Then we copy the options.success callback to a variable and define our own. You can think of it as a wrapper.

    Inside the success wrapper, we parse data returned from the server and set new values on our model. Then we trigger a sync event. This could be where the bug happens.

    Outside the wrapper, we defer to sync to actually talk to the server.

    If I copy this method to my own model definition, we can inspect where it fails.

    Success wrapper doesn't fire

    The success wrapper doesn’t fire. ?

    Let’s see what happens inside sync … ugh, it’s a long function. I’m not pasting it here. It does some setup, then defers to $.ajax to perform an ajax request to the server.

    Can we make the same bug happen without Backbone, then?

    url: "bla.json",
    complete: () => {
    console.log("done 1st request");
    url: "bla.json",
    complete: () => {
    console.log("done 2nd request");

    Bug without Backbone

    ? It worked! 8 lines of code reproduce the bug. ?

    And yes, both requests happen without error.

    Network calls do happen

    It might be safe to say that jQuery is battle tested enough that this couldn’t be a jQuery bug. But let’s try superagent to make sure. It’s a great library for making requests and it’s implemented independently of jQuery.

    Does the bug still happen?

    request.get("bla.json").end(() => {
    console.log("1st success");
    request.get("bla.json").end(() => {
    console.log("2nd success");

    Superagent repros too


    Now you might think: “A-ha! Every even Ajax call to the same URL fails.” I tried that, too -> it doesn’t. If you make requests in a loop, they all work.

    The bug only happens, if you make the same AJAX request in the callback. You can extend a chain like this forever:

    request.get("bla.json").end(() => {
    console.log("1st success");
    request.get("bla.json").end(() => {
    console.log("2nd success");
    request.get("bla.json").end(() => {
    console.log("3rd success");
    request.get("bla.json").end(() => {
    console.log("4th success");

    And it only prints the odd numbered console.logs.

    Guess it’s time to submit my first bug report to a big open source project. Yay I’m helping!

    Did you enjoy this article?

    Published on August 11th, 2016 in Front End, Technical,

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