Javascript local static variable

Not sure I completely understand answers to similar questions that I found here, so trying to be absolutely sure:

I would like to have a local variable in a function, initialized only once (similar to static variables in strongly-typed languages such as C, C++, etc).

Of course, I could declare it globally, but it seems better practice to have it within the scope of that function, since it is not used anywhere else.

Now, here is what I do:

function func(data) {
    func.PARAMS = [
        {"name": "from", "size": 160, "indexed": true},
        {"name": "input", "size": 256, "indexed": false},
        {"name": "output", "size": 256, "indexed": false},
    ];
    ...
}

And my question is, will func.PARAMS indeed be initialized only once, or will it be initialized every time the function is called?

According to some of the answers that I found (this one for example), I need to precede the initialization with something like:

if (typeof func.PARAMS == 'undefined')

This "supplemental" would be irrelevant in strongly-typed languages of course, so I just want to be sure that it is absolutely necessary in order to ensure "static behavior" (i.e., one-time initialization).

Solution

In addition to using properties of the function object, as you do in your example, there are 3 other ways to emulate function-local static variables in Javascript.

All of them rely on a closure, but using different syntax.

Method 1 (supported in old browsers):

var someFunc1 = (function(){
    var staticVar = 0 ;
    return function(){
        alert(++staticVar) ;
    }
})() ;

someFunc1() ; //prints 1
someFunc1() ; //prints 2
someFunc1() ; //prints 3

Method 2 (also supported in old browsers):

var someFunc2 ;
with({staticVar:0})
    var someFunc2 = function(){
        alert(++staticVar) ;
    } ;

someFunc2() ; //prints 1
someFunc2() ; //prints 2
someFunc2() ; //prints 3

Method 3 (requires support for EcmaScript 2015):

{
    let staticVar = 0 ;
    function someFunc3(){
        alert(++staticVar) ;
    }
}

someFunc3() ; //prints 1
someFunc3() ; //prints 2
someFunc3() ; //prints 3

Method 3 for strict mode:

'use strict'
{
    let staticVar = 0 ;
    var someFunc3 = function(){
        alert(++staticVar) ;
    } ;
}

someFunc3() ; //prints 1
someFunc3() ; //prints 2
someFunc3() ; //prints 3