What is java script of french flag​

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);