Random walker using Processing.JS on Khan Academy, with new walkers generated on mouse click
I would put noStroke in a different location.

You are trying to make this code more efficient. You call noStroke each time one of these guys is called. You really only need to call it once right? So change the state once in your draw function:

draw = function() {
    noStroke();
    for(var i = 0; i &lt; walkers.length; i++) {
        walkers[i].walk().display();
    }
};


And remove it from your Walker.display function:

Walker.prototype.display = function() {
    fill(this.r, this.g, this.b, 20);
    ellipse(this.x, this.y, this.radius, this.radius);
    return this;
};