px2map, functions to calculate WGS84 longitude/latitude differences (deltas) per pixel (please notice it's not necessarily the actual screen pixel, but more like logic point for mobile devices).
// Initialization
const px2map = require('px2map');
// When you want to submit a promise
let deltaLng = px2map.deltaLngPerPx(<zoom level>[, <tile extent in pixel>]);
let deltaLat = px2map.deltaLatPerPx(<zoom level>, <latitude>[, <tile extent in pixel>]);
No third party dependency