Question

I am trying to put a variable name within a variable passed to a function, and perform an operation on it.

Here is my code (simplified):

function animloop(object, increment, max, direction) {
  if (direction == 1) {
    object += increment;

    if (object >= max) {
      clearInterval( pingpong.animloop );
    }
  }

  if (direction == -1) {
    object -= increment;

    if (object <= max) {
      clearInterval( pingpong.animloop );
    }
  }

}

If I first set the variable xyz = 10; and then call pingpong.animloop = setInterval( animloop( xyz, 0.1, 40, 1 ) ), 1000 / 60 );, it won’t work.

What I learned is that when I call object += increment;, it isn’t actually modifying the value of whatever object is. Basically the value of xyz isn’t changing.

Hopefully you understand what I’m trying to do. :/

Answer 1

Primitive data types are passed by-value in JS, which means you’re not accessing the xyz you defined outside of the function call. You’re accessing some OTHER variable that has been given a copy of the value xyz had at the time of the function call.

There’s a few ways around it.

global vars:

var xyz = 123;
animLoop(inc,max,dir); // note, no xyz argument
function animLoop(i,m,d) {
   xyz++; // global scope xyz is now 124
}

return value:

xyz = 123;
xyz = animLoop(xyz, max, dir);
function animLoop(obj, max, dir) {
    obj++;
    return(obj);
}

or use an object. Objects are passed by reference, so this would work:

xyz = { value: 123; }
animLoop(xyz, max, dir);
function animLoop(obj, max, dir) {
    obj.value++; // xyz.value is now 124
}