为什么clearRect没有作业

var dom = document.getElementById('clock');
var ctx = dom.getContext('2d');
var width = ctx.canvas.width;
var height = ctx.canvas.heigeht;
var r = width/2;
var rem = width / 200;

function drawBackground(){
ctx.save();
ctx.translate(r,r);//将原点移动到圆中心

//开始画圆
ctx.beginPath();
ctx.lineWidth = 10 * rem;
ctx.arc(0,0,r-ctx.lineWidth / 2,0,2*Math.PI,false);
ctx.stroke();

var hourNumbers = [3,4,5,6,7,8,9,10,11,12,1,2];
ctx.font = 18 * rem + 'px Arial';
ctx.textAlign = 'center';
ctx.textBaseline = 'middle';
hourNumbers.forEach(function(number,i){
var rad = 2 * Math.PI / 12 * i;
var x = Math.cos(rad) * (r-30 * rem);
var y = Math.sin(rad) * (r-30 * rem);
ctx.fillText(number,x,y);
});

for(var i = 0;i < 60; i++){
var rad = 2 * Math.PI / 60 * i;
var x = Math.cos(rad) * (r-18 * rem);
var y = Math.sin(rad) * (r-18 * rem);
ctx.beginPath();
if(i % 5 === 0 ){
ctx.fillStyle = '#000';
ctx.arc(x,y,2 * rem,0,2*Math.PI,false);
}else{
ctx.fillStyle = '#ccc';
ctx.arc(x,y,2 * rem,0,2*Math.PI,false);
}
ctx.fill();
}
}

function drawHour(hour,minute){
ctx.save();
ctx.beginPath();
var rad = 2*Math.PI/12*hour;
var mrad = 2*Math.PI/12/60*minute;
ctx.rotate(rad + mrad);
ctx.lineWidth = 6 * rem;
ctx.lineCap = 'round';
ctx.moveTo(0,10 * rem);
ctx.lineTo(0,-r/2);
ctx.stroke();
ctx.restore();
}

function drawMinute(minute){
ctx.save();
ctx.beginPath();
var rad = 2*Math.PI/60*minute;
ctx.rotate(rad);
ctx.lineWidth = 3 * rem;
ctx.lineCap = 'round';
ctx.moveTo(0,10 * rem);
ctx.lineTo(0,-r+30 * rem);
ctx.stroke();
ctx.restore();
}

function drawSecond(second){
ctx.save();
ctx.beginPath();
ctx.fillStyle = 'red';
var rad = 2*Math.PI/60*second;
ctx.rotate(rad);
ctx.moveTo(-2 * rem,20 * rem);
ctx.lineTo(2 * rem,20 * rem);
ctx.lineTo(1,-r+18 * rem);
ctx.lineTo(-1,-r+18 * rem);
ctx.fill();
ctx.restore();
}

function drawDot(){
ctx.beginPath();
ctx.fillStyle = 'white';
ctx.arc(0,0,3 * rem,0,2*Math.PI,false);
ctx.fill();
}

function draw(){
ctx.clearRect(0,0,width,height);
var now = new Date();
var hour = now.getHours();
var minute = now.getMinutes();
var second = now.getSeconds();
drawBackground();
drawHour(hour,minute);
drawMinute(minute);
drawSecond(second);
drawDot();
ctx.restore();
}

draw();
setInterval(draw,1000);