JavaScript Canvasについて詳しい方回答お願いいたします

1 2<input type="button" value="消去" onclick="c.clearRect(0, 0, a.clientWidth, a.clientHeight);"> 3<canvas width="1000" height="1000"></canvas> 4 5<script> 6var f=0; 7var a = document.querySelector("canvas"), 8 c = a.getContext("2d"); 9 10c.lineWidth = 10; 11c.strokeStyle = "rgb(0, 0, 255)"; 12a.onmousedown = function (e) { 13 if(f==0){ 14 f=1; 15 c.moveTo(e.pageX, e.pageY); 16 }; 17}; 18 19a.onmous