New Host Object Primer
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From: Thomas 'PointedEars' Lahn on 23 Mar 2010 18:11 David Mark wrote: > Garrett Smith wrote: >> Thomas 'PointedEars' Lahn wrote: >>> var reFeaturedMethod = new RegExp('^(function|object)$', 'i'); >> >> A Literal would be shorter and would stay cached: >> >> /^(?:func|obj)/; > > I fail to see how that is the same thing, but the non-capturing bit is a > good idea. Not if you want this to be backwards-compatible. The Matrix says: ES JavaScript JScript V8 JSCore Opera KJS /(?:…)/ : RegExp 3 1.5 5.5.6330 1.3 525.13 7.02 3.5.9 > As for caching, I don't see how it makes any difference as I create the > RegExp object once. The question is moot anyway since ES5 implementations instantiate a new object each time they encounter the literal. (This is different in ES3.) >>> Furthermore, AISB, >>> >>> return !!((reFeaturedMethod.test(t) && o[m]) || t == 'unknown'); >>> >>> becomes more efficient when writing >>> >>> return !!(t == 'unknown' || (reFeaturedMethod.test(t) && o[m])); >>> >>> The double negation to cast to boolean is a matter of taste; I do not >>> think it is necessary, because one possible result is a boolean >>> already and the other has been proven by this that it can be used in a >>> type-converting test. >> >> Double negation on a boolean is pointless. Yes. >> However, `o[m]` should not be a boolean; it should be a function or an >> object. I beg your pardon? > Right. return (t == 'unknown' || (reFeaturedMethod.test(t) && !!o[m])); But AISB the `!!' does not really save anything as in a boolean context the return value would be subject to type conversion anyway. >> Caveats: >> Object `o` could be callable and falsish, such as nonstandard callable >> "document.all". Is there a good reason for document.all(...) instead of document.all[...]? If not, that fact is largely irrelevant. >> Object `o` could be the `item` method, for which typeof will result >> "string" in IE. This would result in isHostMethod returning false. > > Yes, I should add both of those stipulations to the docs and this > example. That argument only makes sense if _o[m]_ refers to the item() method of NodeList or HTMLCollection implementations. Then again, is there a good reason to call o.item(i) instead of accessing o[i]? If not, that fact is largely irrelevant. PointedEars -- Danny Goodman's books are out of date and teach practices that are positively harmful for cross-browser scripting. -- Richard Cornford, cljs, <cife6q$253$1$8300dec7(a)news.demon.co.uk> (2004)
From: David Mark on 23 Mar 2010 18:17 Thomas 'PointedEars' Lahn wrote: > David Mark wrote: > >> Garrett Smith wrote: >>> Thomas 'PointedEars' Lahn wrote: >>>> var reFeaturedMethod = new RegExp('^(function|object)$', 'i'); >>> A Literal would be shorter and would stay cached: >>> >>> /^(?:func|obj)/; >> I fail to see how that is the same thing, but the non-capturing bit is a >> good idea. > > Not if you want this to be backwards-compatible. The Matrix says: > > ES JavaScript JScript V8 JSCore Opera KJS > /(?:…)/ : RegExp 3 1.5 5.5.6330 1.3 525.13 7.02 3.5.9 I wasn't sure about that, which is why I tend to stick with what I know for sure. Thanks for illuminating that. Verboten for My Library. > >> As for caching, I don't see how it makes any difference as I create the >> RegExp object once. > > The question is moot anyway since ES5 implementations instantiate a new > object each time they encounter the literal. (This is different in ES3.) Interesting. What possessed them to do that? > >>>> Furthermore, AISB, >>>> >>>> return !!((reFeaturedMethod.test(t) && o[m]) || t == 'unknown'); >>>> >>>> becomes more efficient when writing >>>> >>>> return !!(t == 'unknown' || (reFeaturedMethod.test(t) && o[m])); >>>> >>>> The double negation to cast to boolean is a matter of taste; I do not >>>> think it is necessary, because one possible result is a boolean >>>> already and the other has been proven by this that it can be used in a >>>> type-converting test. >>> Double negation on a boolean is pointless. > > Yes. > >>> However, `o[m]` should not be a boolean; it should be a function or an >>> object. > > I beg your pardon? o[m] will not normally be a boolean, hence the double negation. > >> Right. > > return (t == 'unknown' || (reFeaturedMethod.test(t) && !!o[m])); > > But AISB the `!!' does not really save anything as in a boolean context the > return value would be subject to type conversion anyway. That's true. But I prefer to have the function return booleans only. > >>> Caveats: >>> Object `o` could be callable and falsish, such as nonstandard callable >>> "document.all". > > Is there a good reason for document.all(...) instead of document.all[...]? > If not, that fact is largely irrelevant. I missed that that second part was mentioned. I already mentioned about the sometimes callable objects in the explanation and documentation. Don't request an opinion from isHostMethod on those. > >>> Object `o` could be the `item` method, for which typeof will result >>> "string" in IE. This would result in isHostMethod returning false. >> Yes, I should add both of those stipulations to the docs and this >> example. > > That argument only makes sense if _o[m]_ refers to the item() method of > NodeList or HTMLCollection implementations. Then again, is there a good > reason to call o.item(i) instead of accessing o[i]? If not, that fact is > largely irrelevant. > Right. It is odd that the one exception to an otherwise golden rule is something you would/should never need anyway. Still, it's an interesting caveat and I think I will mention it.
From: kangax on 23 Mar 2010 18:31 On 3/23/10 6:00 PM, David Mark wrote: > kangax wrote: >> On 3/23/10 12:52 PM, David Mark wrote: >>> I have posted a new primer related to host objects and feature >>> detection/testing. >>> >>> http://www.cinsoft.net/host.html >> >> Is there a reason `isHostObjectProperty` is not called `isHostProperty` >> (to be consistent with `isHostMethod`)? > > The "Object" goes with "Property", not the "Host" part. I understand that :) But what does "Property" clarify there? What's wrong with having `isHostMethod` and `isHostProperty` � where first one is for testing anything that's intended to be called (i.e. method), and latter � for anything that won't be called (i.e. property). [...] >> >> Finally, it might be worth mentioning that `isEventSupported` could (and >> _does_, as any other inference) return false positives; from those I >> know about � `window`'s "error" in Chrome (present but "defunct"), and >> "contextmenu" in Opera 10.50 (even when corresponding option is off in >> settings!). > > It is only meant to be used with elements (which I should stipulate of > course). As for "contextmenu", I never considered that a false > positive. The event is supported, but like many things in browsers, the > user has the ability to get in the way. But from your wording, it > sounds as if there is a bug in Opera 10.5 that should be noted (and > reported). Yeah, I should report it to them. The fact that Opera bug tracker is not open is annoying (I have no idea what's going on with the bugs I filed in the past). -- kangax
From: Garrett Smith on 23 Mar 2010 19:02 David Mark wrote: > Garrett Smith wrote: >> Thomas 'PointedEars' Lahn wrote: >>> David Mark wrote: >>> >>>> I have posted a new primer related to host objects and feature >>>> detection/testing. >> [...] >> >>> ISTM the RegExp is borken: >>> >>> var reFeaturedMethod = new RegExp('^function|object$', 'i'); >>> >>> It matches case-insensitive either "function" at the begin of input or >>> "object" at the end, when it should match case-insensitive an input >>> that is either "function" or "object": >>> >>> var reFeaturedMethod = new RegExp('^(function|object)$', 'i'); >>> >> A Literal would be shorter and would stay cached: >> >> /^(?:func|obj)/; > > I fail to see how that is the same thing, but the non-capturing bit is a > good idea. > It is not the same thing. Either would do the job as well as: /^(?:function|object)$/; Being case-insensitive is pointless, though. I'd ditch the 'i' flag either way. > As for caching, I don't see how it makes any difference as I create the > RegExp object once. > The difference would be when the object is created. Either at runtime (as with constructor) or during lexical scan for regexp literal. -- Garrett comp.lang.javascript FAQ: http://jibbering.com/faq/
From: Garrett Smith on 23 Mar 2010 19:13
David Mark wrote: > Garrett Smith wrote: >> David Mark wrote: >>> I have posted a new primer related to host objects and feature >>> detection/testing. >>> >>> http://www.cinsoft.net/host.html >>> >> | The isHostObjectProperty function tests if the specified host object >> | property references an object that is safe to evaluate. >> >> The term "evaluate" is non-standard terminology. What do you mean? > > Anything along the lines of type conversion, assigning a reference to a > variable, etc. What would you call it? I like to see the standard terminology to describe the problems. I mentioned a few of the problems with host objects here: http://jibbering.com/faq/notes/code-guidelines/#hostObjects Posted inline, for convenience: | Host Objects: | | * Operators: | o Do not use delete operator with host object (IE Errors) | o Do not add any expando properties (unselectable is safe) | o Host objects that error upon [[Get]] access are often ActiveX | objects. These include, but are not limited to: | + Disconnected nodes whose parentNode is not an element | (node.offsetParent) | + XMLHttpRequest methods (open, send, etc). | + filters: elem.filters.alpha, elem.style.filters.alpha, etc. | + document.styleSheets[99999] - Error from [[Get]] for a | nonexistent numeric property of a styleSheets collection. | + link.href for nntp: links in IE. | + NodeList in Safari 2 - do not attempt access a nonexistent | property (e.g. document.childNodes.slice). | | * Type conversion | [[ToString]] | Perform string conversion by starting concatenation with a string | value. See Newsgroup message explanation. <URL: http://groups.google.bg/group/comp.lang.javascript/msg/1528f612e31f09fe > -- Garrett comp.lang.javascript FAQ: http://jibbering.com/faq/ |