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=3
context.strokeStyle="blue"
drawDashedLine(context,20,20,context.canvas.width-20,20,20)