a node.js ldap service use IIFE implementation

ldap.js

const ldapjs = require('ldapjs');

const url = process.env.LDAP_URL;
const LDAP_BIND_DN = process.env.LDAP_BIND_DN;
const LDAP_BIND_PWD = process.env.LDAP_BIND_PWD;
const SEARCH_DN = process.env.SEARCH_DN;

module.exports = (() => {
  let client;

  const connect = () => {
    client = ldapjs.createClient({
      url,
      tlsOptions: {
        rejectUnauthorized: false
      },
      reconnect: true
    });

    client.on('error', err => {
      console.warn('LDAP connection failed, but fear not, it will reconnect OK', err);
    });
  }

  const bind = (username, password) => {
    return new Promise((resolve, reject) => {
      client.bind(username, password, (err, result) => {
        if (err) {
          console.error('error!!!!');
          return reject(err);
        }
        resolve(result);
        console.log('bind!!!!');
      });
    })
  };

  const bindAdPwd = () => {
    return new Promise((resolve, reject) => {
      client.bind(LDAP_BIND_DN, LDAP_BIND_PWD, (err, result) => {
        if (err) {
          console.error('error!!!!');
          return reject(err);
        }
        resolve(result);
        console.log('bind!!!!');
      });
    })
  };

  const unbind = () => {
    return new Promise((resolve, reject) => {
      client.unbind(err => {
        if (err) return reject(err);
        resolve('unbind!!');
        console.log('unbind!!!!');
      })
    })
  };

  const search = (username) => {
    const opts = {
      filter: `(&(objectClass=user)(sAMAccountName=${username}))`,
      scope: 'sub'
    };
    let result = [];

    return new Promise((resolve, reject) => {
      client.search(SEARCH_DN, opts, (err, search) => {
        if (err) reject(err);
        search.on('searchEntry', entry => {
          result.push(entry.object);
        });

        search.on('end', () => {
          resolve(result);
        });
    });
    })
  };

  const modifyPwd = (dn, newPassword) => {
    return new Promise((resolve, reject) => {
      client.modify(dn, [
        new ldapjs.Change({
          operation: 'replace',
          modification: {
            unicodePwd: Buffer.from('"' + newPassword + '"', 'utf16le').toString()
          }
        })
      ], (err, result) => {
        if (err) reject(err);
        resolve(result);
      });
    });
  };

  connect();

  return {
    bind,
    bindAdPwd,
    unbind,
    search,
    modifyPwd
  }
})();