Stoplight timer example using millis()

stoplight_millis.js

/* Timer example using millis() */

var timer;
var light_state = 0;

function setup() {
  createCanvas(600, 600);
  timer = millis();
  noStroke();
}

function draw() {
  background(255);
  fill(100);
  rect(width/2-50, 50, 100, 200);
  //if time since last timer reset is greater than 1000
  if(millis() - timer > 1000) {
    //change the light state (0, 1, 2, then back to 0, 1, 2...)
    light_state = (light_state + 1) % 3;
    //reset timer
    timer = millis();
  }
  
  //draw the stop light
  stoplight(light_state);
}

function stoplight(s) {
  var y;
  switch(s) {
    case 0:
      fill(0, 255, 0);
      y = 100;
      break;
    case 1:
      fill(255, 255, 0);
      y = 150;
      break;
    case 2:
      fill(255, 0, 0);
      y = 200;
      break;
    default:
      break;
  }
  
  ellipse(width/2, y, 50, 50);
}