crypto.js

const Promise = require('bluebird');
const crypto = Promise.promisifyAll(require('crypto'));
const argon = Promise.promisifyAll(require('argon2-ffi').argon2i);

const securePassword = (rawPassword) => {
    crypto.randomBytes(16, (err, salt) => {
        if (err) throw err;

        argon.hash(rawPassword, salt, (err, hash) => {
            
        })
    })
};
module.exports.securePassword = securePassword;

Closer to what you already have (with bluebird):

const Promise = require('bluebird');
const crypto = Promise.promisifyAll(require('crypto'));
const argon = Promise.promisifyAll(require('argon2-ffi').argon2i);

function securePassword(rawPassword) {
    return crypto.randomBytes(16).then(salt => {
        return argon.hash(rawPassword, salt);
    }).then(hash => {
        // do something with the hash
        return hash;
    });

    // add catch if needed
}
module.exports.securePassword = securePassword;

(although I'm a little confused why you are using a different random salt each time, unless your gonna store the salt too)