Converting X and Y coordinates from Mercator world map (square) image to Latitude and Longitude

Question

I'm using the following JS to transform Latitude and Longitude to X and Y coordinates but now I need the contrary, transforming Latitude and Longitude to X and Y.

export const latLon2xy = (lat, lon, width, height) => {
    const x = Math.round((width * (180 + lon) / 360) % (1.5 * width))
    const rad = lat * Math.PI / 180
    const merc = 0.5 * Math.log((1 + Math.sin(rad)) / (1 - Math.sin(rad)))
    const y = Math.round((width * merc / (2 * Math.PI)))
    return {x: x, y: y}
}

Answer 1

I think you will find that the proj4js library at http://proj4js.org/ does everything you need.