How to crop screenshot or image png in nodejs

Question

1. Consider a image file .png
2. Consider you have the xy-cords of some element like x:400, y:500
3. Consider you have the size of image to be cropped: width: 50, height: 20

I've included a piece from the easyimage nodejs package, as well as ImageMagick. When I execute the code below, it simply passes, but I am unable to crop the image.

easyimage.crop({
 src: 'F:/screenshot.png', //contains fullscreen image
 dst: 'F:/screenshot.png', //expect for a new image with cropped name
 x: 400,
 y: 500,
 cropwidth: 50,
 cropheight:20,
 gravity: 'North-West'
 },
 function(err, stdout, stderr) {
 if (err) throw err;
 });