Is this the only way to unit-test an events abstraction?

Discussion in 'Javascript' started by storm.mark, Dec 20, 2011.

  1. storm.mark

    storm.mark Guest


    I have two questions:

    1.) How best to unit-test an event listener?
    2.) Have I properly understood isHostMethod?

    I'm trying to unit-test an event listener abstraction but the only way I can think to test it is against the window 'load' event.

    I've a code example here:

    Also, I'm using David Mark's (@cinsoft) isHostMethod to detect the relevanthost object support but was wondering if I had properly understood the concepts or not? So for example, am I right in thinking that a host object/method should be available under the following conditions (although I understand that these 'conditions' are unreliable):

    - if typeof returns 'function' (for majority browsers)
    - if typeof returns 'unknown' (for IE < 9 where its implementation used ActiveX objects for native Functions)
    - if typeof returns 'object' and the value isn't 'null' (because ES3 specs have allowed null to return 'object' which is incorrect behaviour)

    If any of these conditions are true then that (again, unreliably) *should* mean that the specified host method is available to use.

    But even still the host object/method might be implemented differently to how the specification dictates it should be and so it would be more accurateto do full 'feature detection' where I create the object and see if an event is triggered (or some similar test)?

    Thanks for any advice regarding both these enquiries.

    storm.mark, Dec 20, 2011
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  2. wrote:
    Please put your real name there. In Google Groups, if you absolutely must
    use it (see below), by "subscribing" to the newsgroup.
    First of all, contrary what Google Groups might suggest to you, this is a
    newsgroup, not a Web forum. Primarily it is _not_ on the Web, but in a
    separate, plain-text based medium and a network separate from the Internet,
    called Network News (a subset of which is Usenet to which the `comp'
    hierarchy belongs). So you should post the code that you are asking about,
    not just a URL to some Web site (even if it is a code repository).

    You should also eschew Google Groups in the first place (except for
    research); it is *the* source of newsgroup spam, technically non-compliant
    postings, and therefore heavily filtered.

    That said, your approach is based on the common misconception that
    addEventListener(), attachEvent() and proprietary event-handler properties
    would be semantically identical; they are not:

    - *Several* event listeners can be added with element.addEventListener() in
    order of later execution. In the listener context, `this' refers to
    the object that handled the event (the same as `element'). The listener
    is passed an event object reference. The event object implements the
    standard attributes of the Event interface, like `target' and
    `relatedTarget', as properties, and methods like `preventDefault'.

    - *Several* event listeners can be added with attachEvent(). Order of
    later execution is _not_ guaranteed. `this' refers to the same object
    as `window', _not_ to the object that handled the event. The listener
    is _not_ passed an event object reference; that has be be acquired from
    window.event. The event object does _not_ implement standard attributes,
    but has MSHTML-proprietary properties like `srcElement', `fromElement',
    `toElement', `returnValue' and `cancelBubble', respectively, instead.

    - Only *one* (the primary) event listener can be added with proprietary
    event-handler properties. The existing property value will be
    *overwritten*. The listener is passed an event object reference unless
    the implementation is MSHTML-based (IE). The event object will have
    an implementation-dependent set of properties and methods. Depending
    on the implementation, the return value of the listener will define
    whether the default action for the event is prevented (the event is
    canceled) or not.

    Bottom line: Eschew attachEvent().


    Reference/DOM_Events> (I am only partially responsible for the current
    content and have not reviewed it yet; but it should give you a fair idea
    about what is going on).


    Further, you should _not_ assume that because the object referred to by
    `this' (the global object) exposes the method, all objects must expose it,
    and that it must be working for all event types. The same applies for the
    assumption of the opposite. The more reliable approach, if slightly less
    efficient, is to use the approach that appears to be working when the event
    was created.

    For an event-handling script that implements all of this and has
    (therefore?) survived bad storms (no pun intended), see
    That is partially correct. `typeof' is an *operator*, it does not *return*
    anything. "unknown" may be the *result* of a `typeof' operation in *all*
    DOM implementations; there is no limit as to in which implementations it may
    be the result, or up to which MSHTML version it can be returned.
    ActiveX/COM has little to do with it; the point is those are *host* objects,
    and their `typeof' operation is allowed, by Specification, to result in any
    string (loosely speaking). See ECMA-262 Ed. 3 and 5.1, sections 11.4.3, for

    [JFTR: I am partially responsible for the way David's isHostMethod() looks
    today, for IIRC I have started this is…Method() business with isMethod()
    years ago (search the archives). Curiously enough, I have recently
    (JSX@234) found it useful to "split" jsx.object.isMethod() so that there
    would be jsx.object.isNativeMethod() as well, as David did before, to tell
    user-defined event listeners from unsuitable objects apart in event.js.]
    Yes, it should. Unfortunately, it is only so most of the time. A notable
    exception is the postMessage() method discussed recently, which apparently
    existed before in MSHTML but had a very different implementation (not as
    specified in the HTML5 draft).
    If you mean feature-tests at runtime: No. For you would need to create an
    event object as well, which firstly cannot be done everywhere, secondly
    would only create an event of one of the numerous event types if it worked,
    and thirdly you would require feature detection for that as well. So that
    can only reduce your chances of getting a useful event library (0.5³ =


    Thomas 'PointedEars' Lahn, Dec 20, 2011
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  3. storm.mark

    storm.mark Guest

    Sorry about that, I had put my name at the bottom so people knew who they were talking to.
    Understood. Will do in future.
    I was aware of the differences (e.g. addEventListener includes capturing phase and attachEvent only bubbling, global window object for attachEvent compared to event object for addEventListener, only a single event allowed forproprietary event-handlers).

    That being said there were a couple of things I wasn't aware of though: theorder of events with attachEvents not being guaranteed & 'this' refers to the same object as 'window', _not_ to the object that handled the event.
    So the reason why (in your provided example script) you do not utilise attachEvent and fall back to proprietary event-handler is because of the lack of event execution order (in your fallback I believe you're storing up the listener functions and execute them in the relevant order and pass through anormalised event object)
    So (as I can see in your example code) instead of doing what I am, which isto check for addEventListener on the window object and then rewrite the adding function, you suggest doing the check every time for whatever object the adding of an event is called on (even though it's slightly less efficient).
    Thanks for the link, I will be going over this thoroughly to make sure I understand everything that's going on.
    OK, thanks for clarifying the 'terminology' of return and result (looking at it now I can see how my wording is confusing).
    I'll also be wary of relying on 'unknown' as a way of detecting a host object/method
    I'll search the archives here to see if I can locate your isMethod and review how that works
    I'll review your provided script and see how you've gone about implementingyour events library then so I make sure I'm understanding all the conceptscorrectly.

    storm.mark, Dec 21, 2011
  4. ^^^^^ ^^^^^
    It is customary to put your *name* in the *sender* information (From header
    field, with exceptions) as well (you may sign your postings at the bottom in
    any way you wish). Internet is about cables; Usenet is about people.
    Emphasis on "loosely speaking" here. While in theory it may be any value,
    for the relevant objects there has not been observed a non-specified value
    other than "unknown" (CMIIW).
    The original isMethod() function is only of historical value now. You
    should look into the current jsx.object.isMethod() and
    jsx.object.isNativeMethod() methods in JSX:eek:bject.js instead (same SVN
    repository, in /trunk/).
    Always have more than one source of information.

    Your postings become easier readable if you add an empty line after
    quotations (and do not use Google Groups for posting, but a good local
    newsreader application, like Thunderbird/Icedove or KNode, instead).

    Not with javascript. Nonsense propagates like wildfire in this field.
    -- Richard Cornford, comp.lang.javascript, 2011-11-14
    Thomas 'PointedEars' Lahn, Dec 21, 2011
  5. <snip>

    If you have a copy of Alice's Adventures in Wonderland burn it
    IMMEDIATELY! Lewis Carroll was not the author's real name!!!

    In other words, Ears is being silly.

    John G Harris, Dec 21, 2011
  6. I will leave it to the dedicated reader to decide from the content of our
    postings who of the both of us acts sillier here.

    Thomas 'PointedEars' Lahn, Dec 21, 2011
  7. Anyone who objects strongly to pen names is very silly.

    John G Harris, Dec 22, 2011
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