JavaScript instanceof: can't define what instance of function's argument is
Question
I'm still learning JavaScript,reading books,utilizing FireBug,experimenting.
I'm amazed and stuck on thing below.
Have function declaration:
var t = function (args){
...
}
It's assumed,that it's varargs.
I call it like:
<body onload="t({to:100,from:0})">
It's possible to get from argument value by calling:
args.from
The result of typeof args.from is number
That looks sane.
pay attention: number is in lower case
I am interested of what instance args.from is.
Actually, can't get its instance value.
tried:
args.from instanceof Number
args.from instanceof String
args.from instanceof Object
args.from instanceof Boolean
It is not Number - very strange
It's not Object - quite strange
It's not String -that's OK
It 's not Boolean -that's OK
It's neither null nor 'undefined' - looks OK.
What is it?
2 answers
solution1
5 ACCPTED 2011-12-23 14:29:26
That's a primitive numeric value.
It isn't an instance of any class.
instanceof can only return true on objects (for which typeof returns "object" or "function" ).
This has nothing to do with the args object ; you can get the same falsity from 4 instanceof Number .
By contrast, new Number(4) is an object wrapping the primitive 4 , so typeof new Number(4) === "object and new Number(4) instanceof Number === true
solution2
2 2011-12-23 14:33:44
As passed, from.to is a primitive type, so you can't call instanceof on it.
If you really want to use instanceof instead of typeof , wrap it in an Object :
var from = Object(args.from);
alert(from instanceof Number); // true!
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