grunt-contrib-uglify v5.0.0 still unable to support ES6 Template Literals?

Question

UPDATE, Jan 09, 2012:

At first, I thought it was due to the single quotes within the template literal:

replyTo: `'MTN Support' <${functions.config().supportgmail.email}>`

But now, it's confirmed that even a normal template literal is not supported by grunt-contrib-uglify:

throw new Error(`Unknown sender email address: ${mailOptions.from.address}.`);

Same Error:

Warning: Uglification failed.
Unexpected token: name «sender», expected: punc «,». 
Line 90 in functions/app-functions.js,functions/index.js,functions/viewer-functions.js,functions/site-functions.js
 Use --force to continue.

Note that even the uglify-js alone could not parse this. I'm also aware of the harmony or uglify-js-es6 version on GitHub, but as this is for production deployment purpose, I dare not use it because it has not been updated since 4 years ago.

So is it confirmed that grunt-contrib-uglify v5.0.0 is still not supporting ES6 Template Literals? Any other workarounds or alternative without changing the template literals to the traditional + string concatenation? Thanks!

---

Original Question:

I have the following Gruntfile.js:

module.exports = function (grunt) {
    grunt.initConfig({
        // define source files and their destinations
        uglify: {
            files: {
                src: 'functions/*.js',                  // source files mask
                dest: '../project-staging/functions/'  // destination folder
            }
        }
    });

    // load plugins
    grunt.loadNpmTasks('grunt-contrib-uglify');

    // register at least this one task
    grunt.registerTask('default', ['uglify']);

};

When I run grunt at the command line, I hit this error:

Warning: Uglification failed.
Unexpected token: string «MTN Support», expected: punc «,».
Line 44 in functions/app-functions.js,functions/index.js,functions/viewer-functions.js,functions/site-functions.js
 Use --force to continue.

Aborted due to warnings.

Below is the code that causes the error:

var mailOptions = {
    from: functions.config().supportgmail.email,
    to: null,
    cc: null,
    bcc: null,
    // This is the line that causes the error:
    replyTo: `'MTN Support' <${functions.config().supportgmail.email}>`,
    subject: null,
    text: null,
    html: null
};

I have no idea what the error means as I don't have any error running my code except when I try to uglify it with Grunt. I also heard that the latest version of grunt-contrib-uglify already support ES6, so what else could it be? Below is the version of my Grunt and plugin, which I just installed today:

  "devDependencies": {
    "grunt": "^1.3.0",
    "grunt-contrib-uglify": "^5.0.0"
  }

Anyone knows what's wrong with my code? I don't think it make sense to change my code to the old way of string concatenation using the + as the template string is everywhere in my entire project. Thanks in advance!

Microprint

Try using grunt-terser which utilizes terser. – Marvamarve

@Marvamarve : >> SyntaxError: "var-name" is redeclared due to same variable declaration on different JS files. Same error reported here and GitHub. It's ok, I'll figure out something else, thanks. – Microprint

I can confirm that ^5.0.0 uglify version works well with ES6 – Pulverize

U can confirm it works despite the fact all of us are trying to use it and it wont take simple things like type setting with ":" in function params – Weyermann