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// <canvas id="canvas" width="800" height="650"></canvas>

var canvas = document.getElementById('canvas'),
    ctx = canvas.getContext('2d'),
    rect = {},
    drag = false;

function init() {
  canvas.addEventListener('mousedown', mouseDown, false);
  canvas.addEventListener('mouseup', mouseUp, false);
  canvas.addEventListener('mousemove', mouseMove, false);
}

function mouseDown(e) {
  rect.startX = e.pageX - this.offsetLeft;
  rect.startY = e.pageY - this.offsetTop;
  drag = true;
}

function mouseUp() {
  drag = false;
    ctx.clearRect(0,0,canvas.width,canvas.height);
}
function mouseMove(e) {
  if (drag) {
    rect.w = (e.pageX - this.offsetLeft) - rect.startX;
    rect.h = (e.pageY - this.offsetTop) - rect.startY ;
    ctx.clearRect(0,0,canvas.width,canvas.height);
    draw();
  }
}

function draw() {
    ctx.setLineDash([6]);
  ctx.strokeRect(rect.startX, rect.startY, rect.w, rect.h);
}

init();