JS background-size: cover

DOM

function resizeToCover(el, imageWidth, imageHeight, sceneWidth, sceneHeight) {
    if (imageWidth/imageHeight < sceneWidth/sceneHeight) {
        el.style.width = sceneWidth + 'px';
        var newHeight = imageHeight*(sceneWidth/imageWidth);
        el.style.height = newHeight + 'px';
        el.style.top = -((newHeight - sceneHeight) / 2) + "px";
        el.style.left = 0;
    } 
    else {
        el.style.height = sceneHeight + 'px';
        var newWidth = imageWidth*(sceneHeight/imageHeight);
        el.style.width = newWidth + 'px';
        el.style.top = 0;
        el.style.left = -((newWidth - sceneWidth) / 2) + "px";
    }
}

Canvas

function drawCoverImage(ctx, img, w, h) {
    var image_w = img.width,
        image_h = img.height,
        scene_w = w,
        scene_h = h;
    if (scene_w <= scene_h) {
        var drawn_w = image_w*(scene_h/image_h),
            x_offset = ((drawn_w - scene_w) / 2)* -1;
        ctx.drawImage(img, x_offset, 0, drawn_w, scene_h);
    } else {
        var drawn_h = image_h*(scene_w/image_w),
            y_offset = ((drawn_h - scene_h) / 2)* -1;
        ctx.drawImage(img, 0, y_offset, scene_w, drawn_h);
    }
}