Best test for undefined?
in [JavaScript]
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From: Garrett Smith on 10 May 2010 19:08 nick wrote: > On May 10, 3:21 am, Garrett Smith <dhtmlkitc...(a)gmail.com> wrote: >> nick wrote: >>> What's the "best" way to determine if a variable is defined? > >> Depends. If conditional assignment results in an object value >> [...] then why not use boolean conversion? > >> if(x) { [...] > > True, I guess I should have mentioned it for the sake of completeness. > I'm really more concerned with undefined vs. not undefined ATM, but > this would be good to include in a FAQ entry or something, along with > 'in'. Sure. However there is some catchall behavior that does not provide hooks for "has". Some catchall implementations exist with host objects such as any of the dom collections or navigator.plugins. javascript: alert( ["Shockwave Flash" in navigator.plugins , navigator.plugins["Shockwave Flash"] ]); The example code shows in Firefox 3.6 "false, [object Plugin]". The `in` operator indicates that the plugin property does not exists yet [[Get]] results in the desired plugin object. > >> Identifiers that are declared with var get DontDelete, so don't worry >> about the case where they were deleted -- they can't be. > > Interesting. So deleting them just leaves their value as-is, huh? > Check result of calling delete. var x; alert(delete x); // false. > The console in Chromium tricked me. Look at this: > [...] Chrome console likely uses eval. For eval code: | Variable instantiation is performed using the calling context's | variable object and using empty property attributes. That means there is no DontDelete. See also: http://perfectionkills.com/understanding-delete/#firebug_confusion http://groups.google.am/group/comp.lang.javascript/browse_thread/thread/8604c3c08794dc34#msg_cb73090b3da02eda >> For other values, it is useful to make different checks, depending on >> what the code needs ot know. [...] > > Agreed. > >>> function isDefined (v) { return (v !== void 0) } > >> That tells if a value is not undefined. The isDefined function in the >> general sense does not provide any advantage over an inline check. > > That's true; If there's going to be a function call the function could > just be a no-op (returning undefined). > an isXXX function should return a boolean value. >> It does not say, in the general sense, if `v` has been declared, > > What do you mean? 'v' is declared as an argument to isDefined, right? > That was sort of the point of this example. > The only way for isDefined to if `v` was passed in is to check arguments.length. >> does not say if has been assigned a value (undefined is a value), does not >> say if `v` is a variable, a property of something, the result of trying >> to get a property that was not available. > > Ok, I see what you're getting at. > What I am getting at is that the function removed from context tells less than can be deduced in context. It does not differentiate any of the cases: var o = {}; isDefined(o.blah); o.blah = Function.prototype(); isDefined(undefined); var x; isDefined(x); x = undefined; isDefined(x); >> Consider that, given function scope: > >> (function(){ >> var undef, myVar; >> //... >> var isDef = isDefined(myVar); >> var isDef2 = myVar !== undef; >> })(); > > Hmm, I'm not totally sold. If we're going to do it like that why not > just shadow the global undefined and use that? > OK. >> For the cost of a function call, plus organizing and managing that >> function, it is not worth it. > > You are probably right. That function was not really the ultimate > point of the OP, it was just an example of a way to check for things > having a value of undefined (inline or otherwise) using the syntax > that I liked the most of the three examples above. > > Still, I sort of like the idea of encapsulating it in a function. What for? -- Garrett comp.lang.javascript FAQ: http://jibbering.com/faq/
From: Lasse Reichstein Nielsen on 11 May 2010 00:59 nick <nick___(a)fastmail.fm> writes: > Interesting. So deleting them just leaves their value as-is, huh? > > The console in Chromium tricked me. Look at this: > > var x=5; alert(x); delete x; alert(x); > // alerts "5" > // throws ReferenceError: x is not defined > > I figured I had successfully deleted the var. But this line acts > totally different: > > (function(){var x=5; alert(x); delete x; alert(x);})() > // alerts "5" twice > // function returns undefined as normal > > Looks like there's some serious black magic being performed by the > console in the first example. No black magic necessary, it probably just uses a direct call to "eval" to evaluate the input. The expected behavior of variables created in an eval code is to make them configurable (deletable), wheras variables in program or function code are not configurable. (See ECMAScript 5, section 10.5). /L 'Know thy spec'. -- Lasse Reichstein Holst Nielsen 'Javascript frameworks is a disruptive technology'
From: Bart Lateur on 11 May 2010 03:21 David Mark wrote: >Bart Lateur wrote: >> David Mark wrote: >> >>> If x will be either be undefined or a "truthy" value (e.g. won't be >>> null), use boolean type conversion:- >>> >>> if (x) { >>> doStuff(x); >>> } >> >> Carefull: x = 0 and x = "" both produce a false, too. >> > >Example given; it wasn't meant to be a complete reference of "falsey" >values. And which did you leave out? :) Well... I'm wondering why in the original question if (x == null) { doStuff(x); } won't do. -- Bart Lateur bart.lateur(a)telenet.be
From: Thomas 'PointedEars' Lahn on 11 May 2010 13:33 Bart Lateur wrote: > David Mark wrote: >> Bart Lateur wrote: >>> David Mark wrote: >>>> If x will be either be undefined or a "truthy" value (e.g. won't be >>>> null), use boolean type conversion:- >>>> >>>> if (x) { >>>> doStuff(x); >>>> } >>> >>> Carefull: x = 0 and x = "" both produce a false, too. >> >> Example given; it wasn't meant to be a complete reference of "falsey" >> values. And which did you leave out? :) > > Well... > > I'm wondering why in the original question > > if (x == null) { > doStuff(x); > } > > won't do. Keep in mind that the original question was “What's the 'best' way to determine if a variable is defined?” If "defined" is understood as "does not throw a ReferenceError exception with message '... is not defined', then the following applies: If `x' was not declared a variable (and so that variable was not defined), evaluation of your comparison would throw a ReferenceError. Not so with `typeof', but that comes at a price: With `typeof' you cannot determine whether the property referred to by the /Identifier/ was declared a variable (a property of a Variable Object with the attribute DontDelete). If `x' was declared and implicitly initialized with `undefined' -- var x; -- or `null' -- var x = null; -- or assigned `undefined', e.g. var x = void 0; -- doStuff() would be called anyway with your code as `undefined == null' (it is a type-converting comparison). ISTM the only way to determine whether an identifier is available is to try and catch possible ReferenceError exceptions. And without static code analysis there appears to be no way of determining if an identifier has been declared a variable but that variable has not been initialized (as it may have been explicitly initialized with `undefined' or assigned that value after initialization, but before the test.) 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: Dr J R Stockton on 11 May 2010 13:34
In comp.lang.javascript message <ab37b500-1b97-450c-9498-badd643ac361(a)j3 5g2000yqm.googlegroups.com>, Sun, 9 May 2010 20:02:32, nick <nick___(a)fastmail.fm> posted: >What's the "best" way to determine if a variable is defined? I don't recall any problem in testing X for the value undefined by using var U // ... if (X==U) ... although one must remember not to define U. That should continue to work even if ECMA 6 redefines the value of an undefined variable. But, of course, for that the variable must have been defined; only its value is, by definition, the value undefined - until it is defined. It was a design fault to give the language an undefined type and an undefined value, since it is then unnecessarily difficult to talk about it in English. Instead, non-words should have been used - "taghairm" would nearly serve; "trygdyl" would do. All who know English reasonably well will understand that those are not English words. One cannot test directly whether the variable 'akwidukt' has been declared, since a simple reference to it will give an error. But 'window.akwidukt' is merely 'undefined' if 'akwidukt' has not been defined. -- (c) John Stockton, nr London, UK. ?@merlyn.demon.co.uk Turnpike v6.05 IE 7. Web <URL:http://www.merlyn.demon.co.uk/> - FAQish topics, acronyms, & links. MiniTrue is good for viewing/searching/altering files, at a DOS / CMD prompt; free, DOS/Win/UNIX, new via <URL:http://www.merlyn.demon.co.uk/pc-links.htm>. |