Pass variable to html template in nodemailer

What you can do is read the HTML file using fs module in node and then replace the elements that you want changed in the html string using handlebars

var nodemailer = require('nodemailer');
var smtpTransport = require('nodemailer-smtp-transport');
var handlebars = require('handlebars');
var fs = require('fs');

var readHTMLFile = function(path, callback) {
    fs.readFile(path, {encoding: 'utf-8'}, function (err, html) {
        if (err) {
           callback(err);                 
        }
        else {
            callback(null, html);
        }
    });
};

smtpTransport = nodemailer.createTransport(smtpTransport({
    host: mailConfig.host,
    secure: mailConfig.secure,
    port: mailConfig.port,
    auth: {
        user: mailConfig.auth.user,
        pass: mailConfig.auth.pass
    }
}));

readHTMLFile(__dirname + 'app/public/pages/emailWithPDF.html', function(err, html) {
    if (err) {
       console.log('error reading file', err);
       return;
    }
    var template = handlebars.compile(html);
    var replacements = {
         username: "John Doe"
    };
    var htmlToSend = template(replacements);
    var mailOptions = {
        from: 'my@email.com',
        to : 'some@email.com',
        subject : 'test subject',
        html : htmlToSend
     };
    smtpTransport.sendMail(mailOptions, function (error, response) {
        if (error) {
            console.log(error);
        }
    });
});