Get mouse position relative to canvas in JS

mouseposition.js

BitmapData.prototype.getContext = function() {
    return this.context;
};

BitmapData.prototype.getCanvas = function() {
    return this.canvas;
};

BitmapData.prototype.getCanvasPos = function(el) {
    var canvas = document.getElementById(el) || this.getCanvas();
    var _x = canvas.offsetLeft;
    var _y = canvas.offsetTop;

    while(canvas = canvas.offsetParent) {
        _x += canvas.offsetLeft - canvas.scrollLeft;
        _y += canvas.offsetTop - canvas.scrollTop;
    }

    return {
        left : _x,
        top : _y
    }
};

BitmapData.prototype.mousePos = function(e) {
    var mouseX = e.clientX - this.getCanvasPos(e.target).left + window.pageXOffset;
    var mouseY = e.clientY - this.getCanvasPos(e.target).top + window.pageYOffset;
    return {
        x : mouseX,
        y : mouseY
    };
};

BitmapData.prototype.touchPos = function(e) {
    if (e.touches !== undefined && e.touches.length == 1) {
        var touch = e.touches[0];
        var touchX = touch.pageX - this.getCanvasPos().left + window.pageXOffset;
        var touchY = touch.pageY - this.getCanvasPos().top + window.pageYOffset;
    }
    return {
        x : touchX,
        y : touchY
    }
};