var context=document.getElementById("canvas").getContext("2d");
function drawDashedLine(context,x1,y1,x2,y2,dashlength){
  dashlength=dashlength===undefined?5:dashlength;
  var deltaX=x2-x1; //一条直角边的长
  var deltay=y2-y1; //另一条指教边的长

  var numDashes=Math.floor(
    Math.sqrt(deltaX*deltaX+deltay*deltay)/dashlength //Math.sqrt返回一个数的平方根 dashlength虚线每个点的长度
  )

  var everydashLength_x=deltaX/numDashes //确定X轴每条虚线点的起始点
  var everydashLength_y=deltay/numDashes //确定Y轴每条虚线点的起始点

  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)