I played around with brush tools with the help with amazing tutorials made by Max Bittker. Here is his final code on sandbox.

His tutorials on interpolation of points and textured lines with randomness are so helpful!

I had several ideas before making the drawing tool. Here are some ideas I still wanna pursue in my future projects:

1. changing lineWidth based on time spent on drawing
2. changing color into rainbow - I succeeded in creating gradient in js for fillrect() but not for arc()