^^Count

Contare e' Accumulare. Variabile accumulatore.

See the effect of  an instruction eliminating it..
To eliminate an instruction: delete it, or make it a comment, writing before:  //   (double slash).
Making a comment is convenient, because you can restore the instruction deleting the // (double slash).

Try with:

    ctx.clearRect(0, 0, cv.width, cv.height);

 doing:

//    ctx.clearRect(0, 0, cv.width, cv.height);