This depends if you are using a specific app for JavaScript, (such as Grasshopper) but I think this would work.
var rectWidth = getWidth() / 3;
var rectHeight = getHeight();
var blueRect = new Rectangle(rectWidth, rectHeight);
blueRect.setPosition(0, 0);
blueRect.setColor(Color.blue);
add(blueRect);
var redRect = new Rectangle(rectWidth, rectHeight);
redRect.setPosition(rectWidth * 2, 0);
redRect.setColor(Color.red);
add(redRect);