Canvas mouse drawing for a canvas #sketch plus button #clear.

drawing.js

(function(window){
  // Define the Pen object, which handles drawing on the canvas
  var Pen = function(context){
    // Whether the user is currently drawing
    var drawing;

    // Reset the pen's drawing style
    this.reset = function(){
      context.beginPath();
      context.lineCap = 'round';
      context.lineWidth = 5;
      context.strokeStyle = "#333333";
    };

    // Start drawing when the user clicks the mouse
    this.mousedown = function(e){
      drawing = true;
      context.moveTo(e.x, e.y);
    };

    // Continue drawing as the user moves the mouse
    this.mousemove = function(e){
      if (drawing){
        context.lineTo(e.x, e.y);
        context.stroke();
        context.moveTo(e.x, e.y);
      }
    };

    // Stop drawing when the user releases the mouse
    this.mouseup = function(e){
      drawing = false;
      context.lineTo(e.x, e.y);
      context.stroke();
    };

  };

  // Draw grid on canvas background
  drawGrid = function(context) {
    context.beginPath();
    for (var x = 0.5; x < 500; x += 10) {
      context.moveTo(x, 0);
      context.lineTo(x, 500);
    }
    for (var y = 0.5; y < 500; y += 10) {
      context.moveTo(0, y);
      context.lineTo(500, y);
    }

    context.lineWidth = 1;
    context.lineCap = "square";
    context.strokeStyle = "#EFEFEF";
    context.stroke();
  };

  // Once the page loads
  window.onload = function(){
    // Get the canvas element and its context
    var canvas = document.getElementById('sketch');
    var context = canvas.getContext('2d');

    // Draw the grid on the canvas
    drawGrid(context);

    // Create a new Pen object and reset its drawing style
    var pen = new Pen(context);
    pen.reset();

    // Set up the clear button to clear the canvas
    var clearBtn = document.getElementById('clear');
    clearBtn.addEventListener('click', function(e){
      e.preventDefault();
      context.clearRect(0,0,500,500);
      drawGrid(context);
      pen.reset();
    });

    // Set up the mouse event listeners for drawing
    canvas.addEventListener('mousedown', pen.mousedown, false);
    canvas.addEventListener('mousemove', pen.mousemove, false);
    canvas.addEventListener('mouseup', pen.mouseup, false);
  }
}(window));