This is a neat idea I had for having variables that act like primitive values but are actually functions. Useful for having complex variables that automagically update.

R.js

// This calls the function whenever a primitive is needed
function R(fn){
    return {
        valueOf:fn,
        toString:function(){
            return fn()+"";
        }
    };
}

var t = 0;
var x = R(function(){
    return Math.sin(t);
});

// You can combine them by using R each time
var y = R(function(){
    return x / 2;
});

log(x) // 0

// Changes in t affect x right away
t = Math.PI/2;
log(x); // 1

// Works as expected and calls x/2
log(y); // 0.5

// Values are passed by reference
var z = y;
log(z); // 0.5
t = 0;
log(z); // 0

Demo JS fiddle: http://jsfiddle.net/R5SBK/2/