How to move express-mailer config to separated file?

Using js config (what you want)

// project/config_dir/mail.js
module.exports = {
  from: 'no-reply@example.com',
  host: 'smtp.gmail.com', // hostname 
  secureConnection: true, // use SSL 
  port: 465, // port for secure SMTP 
  transportMethod: 'SMTP', // default is SMTP. Accepts anything that nodemailer accepts 
  auth: {
    user: 'gmail.user@gmail.com',
    pass: 'userpass'
  }
} 
// project/app.js 
var app = require('express')(),
    mail_config = require('./config_dir/mail');
    mailer = require('express-mailer');
mailer.extend(app, mail_config);

Using json config

// project/config_dir/mail.json
{
  from: "no-reply@example.com",
  host: "smtp.gmail.com",
  secureConnection: true,
  port: 465,
  transportMethod: "SMTP",
  auth: {
    user: "gmail.user@gmail.com",
    pass: "userpass"
  }
} 
// project/app.js 
var app = require('express')(),
    mail_config = require('./config_dir/mail.json');
    mailer = require('express-mailer');
mailer.extend(app, mail_config);