Uglifying javascript code with grunt-contrib-uglify

I have the following file: package.json

{
  "name": "uglify",
  "devDependencies": {
    "grunt": "^1.0.1",
    "grunt-contrib-uglify": "^3.0.0"
  }
}

also the following file: Gruntfile.js

module.exports = function(grunt) {
    grunt.initConfig({
        uglify: {
            options: {
                beautify: true,
                mangle: {
                    properties: true
                }
            },
            log_sum_9: {
                src: 'log_sum_9.js',
                dest: 'log_sum_9.min.js'
            }
        }
    });
    grunt.loadNpmTasks('grunt-contrib-uglify');

    grunt.registerTask('log_sum_9', ['uglify:log_sum_9']);
}

also the following file: log_sum_9.js

(function() {
    var
        sum = "2+3+4",
        calc = function(operation) {
            return eval(operation);
        }
    ;
    console.log(calc(sum));
})();

Then I do:

to install the required NodeJS modules:

$ npm install

to uglify log_sum_9.js:

$ grunt log_sum_9

then I get the uglified file: log_sum_9.min.js:

!function() {
    var sum = "2+3+4", calc = function(operation) {
        return eval(operation);
    };
    console.log(calc("2+3+4"));
}();

Both scripts works properly:

$ node log_sum_9.js
9

$ node log_sum_9.min.js
9

My problem is that the uglified file log_sum_9.min.js didn't change the names of the variables: sum, calc, operation (all these variables are local variables).

If the content of the file log_sum_9.js is introduced on the following online obfuscator: https://www.javascriptobfuscator.com/Javascript-Obfuscator.aspx

then you get the following code:

var _0x257f = ["\x32\x2B\x33\x2B\x34", "\x6C\x6F\x67"];
(function() {
    var _0xb897x1 = _0x257f[0],
        _0xb897x2 = function(_0xb897x3) {
            return eval(_0xb897x3)
        };
    console[_0x257f[1]](_0xb897x2(_0xb897x1))
})()

where as you can see the previous 3 variables: sum, calc, operation has changed their names.

If you put the above code on the file: log_sum_9.online.js, then you can do:

$ node log_sum_9.online.js
9

(it works properly too)

My question is:

How do I have to configure the file: Gruntfile.js to get the previous 3 variables been obfuscated?

via davidesp