Overriding prototype method and calling the original method

Question

I want to overrid the drawImage function of Html5 Canvas.Here is code.

var p = CanvasRenderingContext2D.prototype.drawImage;
CanvasRenderingContext2D.prototype.drawImage = function() {
    var len = arguments.length;
    var ig, sx, sy, swidth, sheight, x, y, width, height
    if (len === 3) {
        p(arguments[0], arguments[1] * 2, arguments[2] * 2, this);
    } else if (len === 5) {
        //Uncaught TypeError: Illegal invocation.
        p(arguments[0], arguments[1] * 2, arguments[2] * 2, arguments[3] * 2, arguments[4] * 2, this); 
    } else if (len === 9) {
        p(arguments[0], arguments[1] * 2, arguments[2] * 2, arguments[3] * 2, arguments[4] * 2, arguments[5] * 2, arguments[6] * 2, arguments[7] * 2, arguments[8] * 2, this);
    }
}

And call the function as the following.

var canvas = document.getElementById("canvas");
var ctx = canvas.getContext("2d");
ctx.drawImage(img, 0, 0);

But It throw an error in the annotations line. After overrided prototype method,how to call the original method.