Javascript run a function inside a loop every x iterations

Question

I have a variable of unknown value, it will be an integer. For this sake, lets say var a = 3;

I have a function that is called continuously:

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Answer 1

Here's an approach that encapsulates the counter, but uses a global variable for "a":

var a = 3;

function anim(){
    // Run the usual code here
    // ...

    if (++anim.counter % a === 0) {
        // Run the special code here
        // ...
    }
}
// Initialize static properties.
anim.counter = 0;

And here's an approach that encapsulates the "a" variable as well, referring to it as "frequency":

function anim(){
    // Run the usual code here
    // ...

    if (++anim.counter % anim.frequency === 0) {
        // Run the special code here
        // ...
    }
}
// Initialize static properties.
anim.counter = 0;
anim.frequency = 1;

Then set the desired frequency value before calling anim() for the first time:

anim.frequency = 3;