Dear diary, all I did today was fix 1 bug. I don't know why it happens, but I know how to make it not happen.#engineering
— Swizec Teller (@Swizec) August 6, 2016
That was on Friday. I still don’t know why the bug happens, but I can reproduce it. Evidence says it’s a Chrome 52 bug, not a general JavaScript bug. Neither Safari nor Firefox make it happen.
Sometimes when you call .fetch()
on a Backbone model, callbacks don’t fire. There’s no JavaScript error, the REST call works, the result is vanished by the browser. Joy of joys.
You need:
- Webpack
- Babel 6
- Backbone (maybe Backbone is doing it wrong?)
- A server that can serve JSON files
- 14 lines of code
I used create-react-app
to set up a basic environment, ejected with npm run eject
and added some config to Webpack to allow jQuery. Backbone depends on it. This part is not that relevant.
To show the bug itself, we need a Backbone model:
// A basic Backbone modelclass BugModel extends Backbone.Model {constructor() {super();this.url = "bla.json";}}
Now I know what you’re thinking, that’s an ES6 object and Backbone was built before ES6. Official docs never mention the new class
syntax. The traditional Backbone approach works too:
var BugModel = Backbone.Model.extend({url: "bla.json",});
The same bug happens. I tried.
To cause the bug, we have to call .fetch()
on two instances of BugModel
that share the same id
. The code looks like this:
let bug = new BugModel();bug.fetch({success: () => {console.log("fetch 1"); // this happensdoWeirdness(bug);},});function doWeirdness(bug) {let newBug = new BugModel({ id: bug.id });console.log("about to re-fetch");newBug.fetch({success: () => console.log("fetch 2", newBug), // don't get hereerror: () => console.log("error"),});}
We create an instance of BugModel
in a variable called bug
, then call .fetch()
. This makes an API call to /bla.json
, which returns a JSON file that looks like this: {"id": 1, "hai": "hello"}
.
Backbone automagically parses this JSON string and sets an id
and hai
attribute. This part works like a charm.
Then we call doWeirdness
and pass our bug
object as an attribute. Inside doWeirdness
, we make a new instance of BugModel
called newBug
and give it the same id
as bug
had. This is a crucial step.
When we call fetch
on this new instance, the API call happens, but the callbacks do not. 'fetch 2'
never prints.
This whole sequence of events might sound redundant, but it made sense in my real codebase. Even if re-fetching objects doesn’t make sense, it should work. It works in both Safari and Firefox.
Clues about why
This smells like a bug in memory optimization. Chrome thinks bug
and newBug
are the same object, even though they are not.
If you call fetch()
twice on the same instance of BugModel
, the same bug happens. (clue, part 1)
If both instances have a different id
, both fetch
calls fire callbacks. (clue, part 2)
Curiously, if you add a third fetch()
call, that works.
newBug.fetch({success: () => console.log("fetch 2", newBug), // don't get hereerror: () => console.log("error"),});newBug.fetch({success: () => console.log("fetch 3"), // prints});
The bug didn’t happen in Chrome 51.
I am confused. ?
Learned something new?
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