canvas问题 为什么矩形可以画出来 圆却不可以

$(document).ready(function (){

    width = window.innerWidth;
    height = window.innerHeight;

    canvas = document.getElementById('myCanv');        
    canvas.width = width;
    canvas.height = height;

    var c=document.getElementById("myCanv");
    var canv=c.getContext("2d");
    canv.fillStyle="#FF0000";

    canv.beginPath();
    var h = 20;


    var y = 0;
    
    function draw() {
        if (y < canvas.height/2) {
            canv.fillRect(170,y,15,1);
            canv.clearRect(170,y-15,15,1);
            y++;
        };