Drag to draw lines on Canvas

Draw a line on mouse drag on a canvas

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
var canvasOffset = $("#canvas").offset();
var offsetX = canvasOffset.left;
var offsetY = canvasOffset.top;
var storedLines = [];
var startX = 0;
var startY = 0;
var isDown;

ctx.strokeStyle = "orange";
ctx.lineWidth = 3;

$("#canvas").mousedown(function(e) {
  handleMouseDown(e);
});
$("#canvas").mousemove(function(e) {
  handleMouseMove(e);
});
$("#canvas").mouseup(function(e) {
  handleMouseUp(e);
});
$("#clear").click(function() {
  storedLines.length = 0;
  redrawStoredLines();
});

function handleMouseDown(e) {
  var mouseX = parseInt(e.clientX - offsetX);
  var mouseY = parseInt(e.clientY - offsetY);
  isDown = true;
  startX = mouseX;
  startY = mouseY;
}

function handleMouseMove(e) {
  if (!isDown) {
    return;
  }
  redrawStoredLines();
  var mouseX = parseInt(e.clientX - offsetX);
  var mouseY = parseInt(e.clientY - offsetY);
  // draw the current line
  ctx.beginPath();
  ctx.moveTo(startX, startY);
  ctx.lineTo(mouseX, mouseY);
  ctx.stroke();
}


function handleMouseUp(e) {
  isDown = false;
  var mouseX = parseInt(e.clientX - offsetX);
  var mouseY = parseInt(e.clientY - offsetY);
  storedLines.push({
    x1: startX,
    y1: startY,
    x2: mouseX,
    y2: mouseY
  });
  redrawStoredLines();
}


function redrawStoredLines() {
  ctx.clearRect(0, 0, canvas.width, canvas.height);
  if (storedLines.length == 0) {
    return;
  }
  // redraw each stored line
  for (var i = 0; i < storedLines.length; i++) {
    ctx.beginPath();
    ctx.moveTo(storedLines[i].x1, storedLines[i].y1);
    ctx.lineTo(storedLines[i].x2, storedLines[i].y2);
    ctx.stroke();
  }
}

DEMO