Script error in shedule.js in IE9/Win7

[KarolBuchta]

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Please answer the questions the best you can:

1. What version of bluebird is the issue happening on?
   3.4.6

2. What platform and version? (For example Node.js 0.12 or Google Chrome 32)
   Internet Explorer 9, Windows 7 SP1 (Modern IE VM)

3. Did this issue happen with earlier version of bluebird?
   3.4.1

First: I am running your code in an environment where a third party js stack is running (Atlassian).

My issue is that in IE9 this line leads to a script error:
https://github.com/petkaantonov/bluebird/blob/master/src/schedule.js#L50

Unable to get value of the property 'toggle': object is null or undefined.

htmlElement.classList is not defined in IE9:
http://www.w3schools.com/jsref/prop_element_classlist.asp

[benjamingr]

@KarolBuchta do you have a mutationobserver polyfill used?

I think we can just opt out of that scheduler if classList isn't there too. Should be a simple enough fix.

[benjamingr]

Ping @KarolBuchta - please comment if you have additional info

I can't reproduce this without using a faulty polyfill. I recommend you try including bluebird first and any polyfills second.

[mqudsi]

I ran into this when loading a MutationObserver polyfill prior to loading bluebird. The reason for manually attempting to load a MutationObserver polyfill was this error communicated to our servers:

{
    Browser: "Mozilla/5.0 (Linux; U; Android 4.2.2; en-us; R2001 Build/JDQ39) AppleWebKit/534.30 (KHTML, like Gecko) Version/4.0 Mobile Safari/534.30 UCBrowser/11.5.2.1188 (UCMini) Mobile",
    Message: "Uncaught ReferenceError: MutationObserver is not defined",
    Line: 1,
    Column: 0,
    File: null,
    Stacktrace: null,
    _typeTag: "ClientError"
}

Without a stack trace (it wasn't included in the client-side event by the browser) it's hard to know if this error is coming from bluebird attempting to polyfill Promise or a client trying to use it.

Loading bluebird first stopped the toggle error from occurring on IE9.