test.js

let Eos = require('eosjs');

let httpEndpoint = "https://eos.greymass.com";
let eos = Eos({httpEndpoint});

// curl  https://eos.greymass.com/v1/chain/abi_json_to_bin -X POST -d '{"code":"eosdotaprod1", "action":"sign", "args":{"account":"atlantisland","timestamp":1553514689676000,"nonce":351544}}'

// curl  https://eos.greymass.com/v1/chain/abi_json_to_bin -X POST -d '{"code":"eosdotaprod1", "action":"sign", "args":{"account":"atlantisland","timestamp":"2019-03-25T12:25:39.492","nonce":351544}}'

(async() => {
    const acc = "eosdotaprod1"
    const act = "sign"
    let abi = (await eos.contract(acc)).fc

    const typeName = abi.abi.actions.find(x => x.name === act).type;

    let dataobj = {
        "account":"atlantisland",
        "timestamp":1553514689676000,
        "nonce":351544
    }

    let buf = abi.toBuffer(typeName, dataobj)

    console.log(buf.toString('hex'));

})()