function cropImage(image, croppingCoords) {
    var cc = croppingCoords;
    var workCan = document.createElement("canvas"); // create a canvas
    workCan.width = Math.floor(cc.width);  // set the canvas resolution to the cropped image size
    workCan.height = Math.floor(cc.height);
    var ctx = workCan.getContext("2d");    // get a 2D rendering interface
    ctx.drawImage(image, -Math.floor(cc.x), -Math.floor(cc.y)); // draw the image offset to place it correctly on the cropped region
    image.src = workCan.toDataURL();       // set the image source to the canvas as a data URL
    return image;
}

var image = new Image();
image.src = "image URL"; // load the image
image.onload = function () {  // when loaded
    cropImage(
        this, {
        x : this.width / 4,     // crop keeping the center
        y : this.height / 4,
        width : this.width / 2,
        height : this.height / 2,
    });
    document.body.appendChild(this);  // Add the image to the DOM
};