const gk = require('gauss-krueger')

let wgs84 = gk.toWGS({x: 4591270, y: 5819620}) // {longitude: 13.34255019849783, latitude: 52.50210050984162}

let gk = gk.toGK({longitude: 13.4, latitude: 52.5}) // {x: 4595175.488530577, y: 5819460.402152777}

// The script guesses the GK zone based on the input coordinate longitude.
// You can also supply a zone directly via the zone parameter:
let gk5 = gk.toGK({longitude: 13.4, latitude: 52.5}, 5) // {x: 5391482.283752493, y: 5819737.58836849}