在画布中拖动点并获取坐标

<canvas id="myCanvas" width="500" height="500"></canvas>

var canvas = document.getElementById("myCanvas");
var ctx = canvas.getContext("2d");

var isDragging = false;
var offsetX = 0;
var offsetY = 0;

canvas.addEventListener("mousedown", function(e) {
  isDragging = true;
  offsetX = e.offsetX;
  offsetY = e.offsetY;
});

canvas.addEventListener("mousemove", function(e) {
  if (isDragging) {
    var x = e.offsetX - offsetX;
    var y = e.offsetY - offsetY;
    
    // 在这里可以根据需要进行一些绘制操作，比如绘制一个点
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    ctx.beginPath();
    ctx.arc(x, y, 5, 0, 2 * Math.PI);
    ctx.fillStyle = "red";
    ctx.fill();
    ctx.closePath();
  }
});

canvas.addEventListener("mouseup", function(e) {
  isDragging = false;
});