Here's an easy one straight from the text book I can't seem to find.
I have a javascript function. I want it to contain a private variable which remembers its value between invocations.
Can someone jog my memory please.
+5
A:
Create it using a closure:
function f() {
var x = 0;
return function() {return x++;};
}
Then use it as follows:
> g = f()
function () {return x++}
> g()
0
> g()
1
> g()
2
Max Shawabkeh
2010-04-18 08:07:51
@OP: This is the correct way to do this. Be aware, though, that there is a cost involved: Every call to `f` creates a *new* function, which can have memory implications. You can't rely on implementations reusing the underlying code and just creating a new environment for it every time (and in fact, I don't think most do, from my informal experiments). So there's the trade-off, you can get a truly private variable, but at a memory cost. Not important if there won't be many of these, but significant if there will.
T.J. Crowder
2010-04-18 08:46:11
A:
Here is a truly private implementation
(function() {
var privateVar = 0;
window.getPreviousValue = function(arg) {
var previousVal = privateVar;
privateVar = arg;
return previousVal;
}
})()
alert(getPreviousValue(1));
alert(getPreviousValue(2));
Cheers
Sky Sanders
2010-04-18 08:08:20
This won't create a *private* variable. It will be accessible from outside.
Max Shawabkeh
2010-04-18 08:09:05
While it's always a good idea to remind people that functions are objects, your first example isn't even "privileged", it's completely public. *Anything* can look at `foo.privateVar`.
T.J. Crowder
2010-04-18 08:55:18
@tj - Ok, you are right. I was incorrectly applying the priveledged concept to a property when it rightly relates to a method. thanks for that.
Sky Sanders
2010-04-18 09:30:39
+1
A:
var accumulator = (function() {
var accum = 0;
return function(increment) {
return accum += increment;
}
})();
alert(accumulator(10));
alert(accumulatot(15));
Displays 10 then 25.
AnthonyWJones
2010-04-18 08:08:52
A:
I am not sure if I understood correctly but maybe something like this would do the trick :
function Foo() {
var x = "some private data";
return {
getPrivateData : function(){
return x;
}
};
};
var xx = new Foo();
xx.getPrivateData();
miensol
2010-04-18 08:10:16