Dragon Curve fractal in JavaScript for Khan Academy project

gistfile1.txt

/*

The dragon curve drawn using an L-system.
variables : X Y
constants : F + −
start  : FX
rules  : (X → X+YF+), (Y → -FX−Y)
angle  : 90°
Here, F means "draw forward", − means "turn left 90°", and + means "turn right 90°". X and Y do not correspond to any drawing action and are only used to control the evolution of the curve.


*/

var dragonCurve = function(depth){
    //base case
    if(depth === 0){return 'FX';}
    
    var turtle = dragonCurve(depth - 1);
    var newTurtle = '';
    for(var i = 0; i < turtle.length; i++){
        if(turtle.charAt(i) === 'X'){
            newTurtle += 'X+YF+';
        }
        else if (turtle.charAt(i) === 'Y'){
            newTurtle += '-FX-Y';
        }
        else {
            newTurtle += turtle.charAt(i);
        }
    }
    return newTurtle;
};

var turtleDraw = function(text, startx, starty, length){
    var direction = 0;
    var x = startx;
    var y = starty;
    
    for(var i = 0; i < text.length; i++){
         
        if(text.charAt(i) === 'F'){
            //'F', draw forward length in direction
            var endx = x + length * Math.cos(direction * 0.017453292519);
            var endy = y + length * Math.sin(direction * 0.017453292519);
            
            var red = x/400 * 255;
            var blue = y/400 * 255;
            var green = (x + y + 300)/400 * 255;
            stroke(red, green, blue);
            
            line(x,y,endx,endy);
            x = endx;
            y = endy;
        }
        else if(text.charAt(i) === '-'){
            //'-' add 90 degrees to direction
            direction += 90;
        }
        else if(text.charAt(i) === '+'){
            //'+' subtract 90 degrees from direction
            direction -= 90;
        }
    }
};

fill(0, 0, 0);
rect(0,0,width,height);
stroke(255, 0, 0);
//strokeWeight(3);
//line(0,0,400,400);
var dragon = dragonCurve(14);
turtleDraw(dragon,200,90,2);

/*High definition... runs kinda slow
var dragon = dragonCurve(16);
turtleDraw(dragon,90,150,1);
//*/