var context=document.getElementById("canvas").getContext("2d");function drawDashedLine(context,x1,y1,x2,y2,dashlength){    dashlength=dashlength===undefined?5:dashlength;    var deltaX=x2-x1; //一條直角邊的長    var deltay=y2-y1; //另一條指教邊的長    var numDashes=Math.floor(        Math.sqrt(deltaX*deltaX+deltay*deltay)/dashlength  //Math.sqrt返回一個數的平方根  dashlength虛線每個點的長度    )    var everydashLength_x=deltaX/numDashes  //確定X軸每條虛線點的起始點    var everydashLength_y=deltay/numDashes  //確定Y軸每條虛線點的起始點    for(var i=0;i<numDashes;i++){        context[i%2===0?'moveTo':"lineTo"]        (x1+everydashLength_x*i,y1+everydashLength_y*i)    }    context.stroke()}context.lineWidth=3context.strokeStyle="blue"drawDashedLine(context,20,20,context.canvas.width-20,20,20)