Module trouble:drawing board, could not draw

[Ryroro]

Hey guys, I am developing a simple module of a drawing board. The board can show on the mirror, but the mouse input cannot draw anything on it. 0_1471672694931_canvas.js

[Ryroro]

I am fairly new to javascript. The original code of the drawing board run smoothly on the webrowser I test with on my mac, but after I re-edit to fit the mirror, action of drawing does not seem to be working