How to draw using canvas

canvas_drawing.js

var mousePressed = false;
var lastX, lastY;
var ctx;
var li = [];

function Draw(x, y, isDown) {
    if (isDown) {
        ctx.beginPath();
        ctx.strokeStyle = $('#selColor').val();
        ctx.lineWidth = $('#selWidth').val();
        ctx.lineJoin = "round";
        ctx.moveTo(lastX, lastY);
        ctx.lineTo(x, y);
        ctx.closePath();
        ctx.stroke();
    }
    var t = [x,y];
//     li.push(t);
//     console.log(t);
    lastX = x; lastY = y;
}

ctx = document.getElementById('myCanvas').getContext("2d");
console.log(ctx);

$('#myCanvas').mousedown(function (e) {
  mousePressed = true;
  Draw(e.pageX - $(this).offset().left, e.pageY - $(this).offset().top, false);
});

$('#myCanvas').mousemove(function (e) {
  if (mousePressed) {
    Draw(e.pageX - $(this).offset().left, e.pageY - $(this).offset().top, true);
  }
});

$('#myCanvas').mouseup(function (e) {
  mousePressed = false;
});
$('#myCanvas').mouseleave(function (e) {
  mousePressed = false;
});

index.html

<canvas id="myCanvas" width="28" height="28" style="border:2px solid black">