In an object such as:
var a = {"a":"value1", "b": "value2", "c":"value3"};
How do I get the value of the second element in the object without knowing the element's name? Same for variable n3 and so on...
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3 Answers
Object properties do not have a defined order, as per the object specification.
If you're looking to come back and fix this project some day, you could do something stupid like...
var theThirdForAnyValueOfThird = a[Object.keys(a)[2]];
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According to the language spec (see sections 4.3.3 and 12.6.4), the properties of an object are not ordered, so there is no "second element". This is different from the numeric properties (subscripts) of an array.
The best approach, if you want to associate a particular order to the keys, is to store the keys themselves in an array and access them by subscript (not using for...in).
All major browser implementations will iterate the properties in the order in which they were added. While this is ill-advised, you might be able to get away with:
a[Object.keys(a)[1]]
to get the "second element" value. Just be aware that this relies on an accident of implementation and is not guaranteed to work everywhere. Basically, it's an accident waiting to happen if you do this.
6 Comments
Ted Hopp
@pst - See the section "for loop order" in John Resig's article JavaScript in Chrome.
Chris O'Kelly
I found this first in google: code.google.com/p/v8/issues/detail?id=164, which points out that this is NOT the case where a key can be parsed into a 32 bit integer
RobG
BTW, there is no order to Array properties either (Arrays are objects). Order is created by accessing them using an index. Using
for..in will return them out of order in some browsers in some cases. |
You can't and would need to switch to an array. As workaround, I have been doing weird things like this:
var a = [{"a":"value1"}, {"b": "value2"}, {"c":"value3"}];
