How to draw in "real time" on HTML5 canvas?

Question

Canvas

I want to indulge on Canvas, just to indulge.
How to draw on it "in real time"? That is, now he first draws, and then shows. How to make him seem to show how he draws. I don't even know how to explain.
For example, I want to draw random lines on the canvas, but in such a way that I would see how each line is drawn on the canvas.
How to do it? What libraries are there?