Cannot read property 'width' of undefined

var dom=document.getElementById('clock');

var ctx=dom.getContext('2d');

var width=ctx.canves.width;

var height=ctx.canves.height;

var r=width/2;

function drawbackground() {

ctx.translate(r,r);

ctx.beginPath();

ctx.lineWidth=10;

ctx.arc(0,0,r-5,Math.PI,false)

ctx.stroke();

ctx.closePath();

// body...

}

drawbackground();

为什么会报这个的错误，想不明白