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Webmaster Forum / HTML, CSS, Scripts / JavaScript / September 2005for/in loop Array gotcha
I just wanted to let folks know of one more reason not to use for/in loops on Arrays. In the following code: var u = [1,1] for (var i in u) print([i, i+2, i-0+2].join()); print(typeof(i) + ' ' + typeof(i-0)); 0,02,2 1,12,3 string number (i+1) does not obtain the expected results because i is a string! Always use C-like loops when iterating arrays. JavaScript is not Python! Ian var u = [1,1] for (var i in u) print([i, 2+i, -0+2+parseInt(i)].join()); print(typeof(i) + ' ' + typeof(i-0)); output left for readers to guess [snip] > for (var i in u) [snip] > (i+1) does not obtain the expected results because i is a string! Of course, because property names are /always/ strings. [snip] Mike  SignatureMichael Winter Prefix subject with [News] before replying by e-mail. > I just wanted to let folks know of one more reason not to use for/in > loops on Arrays. [quoted text clipped - 13 lines] > Always use C-like loops when iterating arrays. JavaScript is not > Python! Or, you could read the ecmascript spec, in order to find out what the "for .. in" syntax actually does. Your "advice" sounds more like a warning than anything else, but I'm actually glad the "for .. in" syntax exists. > Ian Arnaud [snip] > ... I'm actually glad the "for .. in" syntax exists. I didn't know of it before; thanks. "The for...in statement is used to iterate a declared variable over every property in a specified object. The code in the body of the for ... in loop is executed once for each property." http://www.devguru.com/Technologies/ecmascript/quickref/for...in.html Note also the following odd usage to test for the existence of a property:- var a={}; a.myProp=1; var exists=(myProp in a); > Note also the following odd usage to test for the existence of a > property:- [quoted text clipped - 3 lines] > > var exists=(myProp in a); Is that proper JS? When I execute in SpiderMonkey: ----8<---- aundro@paddy:~$ js js> var a={}; js> a.myProp=1; 1 js> var exists=(myProp in a); 3: ReferenceError: myProp is not defined ----8<---- *sob* Regards, Arnaud > Note also the following odd usage to test for the existence of a > property:- [quoted text clipped - 3 lines] > > var exists=(myProp in a); That snippet will give you an error, you need var exists = "myProp" in a; There is nothing "odd" about that in my view only that the 'in' operator is not implemented in older browsers respectively the script engines they come with, e.g. in Netscape 4 the 'in' operator use will give a syntax error, the same for IE 5/Mac if I remember a recent discussion here correctly. SignatureMartin Honnen http://JavaScript.FAQTs.com/ >>That snippet will give you an error, you need >>var exists = "myProp" in a; Oops, thank you for that. > [snip] > [quoted text clipped - 5 lines] > property in a specified object. The code in the body of the for ... in loop > is executed once for each property." Ok, here comes a little pedantry: Some properties won't be iterated over (yet remain accessible) if they have the 'DontEnum' attribute. How you can reach/set/clear those attributes however, I absolutely have no idea.. Anybody has any idea? (see P.65 of www.ecma-international.org/publications/standards/Ecma-262.htm) Basically: you don't care about such properties, but heh, it's just nice to know there can be 'hidden' properties. > http://www.devguru.com/Technologies/ecmascript/quickref/for...in.html Arnaud > Some properties won't be iterated over (yet remain accessible) if they > have the 'DontEnum' attribute. How you can reach/set/clear those attributes > however, Script code itself can't read or set that attribute, it is internal to an implementation and if you have an application hosting a script engine then you usually have access to set such attributes on the properties of the host objects the application creates. SignatureMartin Honnen http://JavaScript.FAQTs.com/ >> Some properties won't be iterated over (yet remain accessible) if they >> have the 'DontEnum' attribute. How you can reach/set/clear those attributes [quoted text clipped - 4 lines] > engine then you usually have access to set such attributes on the > properties of the host objects the application creates. That's indeed more or less what I was conceiving, but thanks a lot for the clarification :) Arnaud [property attributes like DontEnum] > Script code itself can't read or set that attribute, Almost correct. There is Object.prototype.propertyIsEnumerable that allows you to read one of the three attributes (the other two being ReadOnly and DontDelete, which can't be read, but might be discoverable by trying to do it and catching the exception :) /L SignatureLasse Reichstein Nielsen - lrn@hotpop.com DHTML Death Colors: <URL:http://www.infimum.dk/HTML/rasterTriangleDOM.html> 'Faith without judgement merely degrades the spirit divine.' [snip] > (the other two being ReadOnly and DontDelete, which can't be read, > but might be discoverable by trying to do it and catching the > exception :) No exceptions should be thrown if the delete or assignment operators are used on DontDelete or ReadOnly properties, respectively. The operation should silently fail (though the delete operator should evaluate to false). Mike SignatureMichael Winter Prefix subject with [News] before replying by e-mail. > [snip] > [quoted text clipped - 7 lines] > > http://www.devguru.com/Technologies/ecmascript/quickref/for...in.html Nice reference. I note that this reference also mentions the use of for..in on arrays, but without mentioning that the variable comes back as a string instead of a number. Ian |
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