How to use the gherkin.Lexer function in gherkin

function GherkinLexer(content, gherkinListener) {
  // Strip BOM
  content = content.replace(/^\ufeff/g, '');

  var language = 'en';
  var lines = content.split('\n');
  var len = lines.length;
  for (var i = 0; i < len; i++) {
    var line = lines[i];
    if(!/^\s*#/.exec(line)) break;
    var languageMatch = /^\s*#\s*language:\s*([a-zA-Z-_]+)\s*$/m.exec(line);
    language = languageMatch === null ? 'en' : languageMatch[1].toLowerCase();
  }
  language = language.replace('-', '_');
  var Lexer = gherkin.Lexer(language);
  var lexer = new Lexer(gherkinListener);

  this.scan = function () {
    lexer.scan(content);
  };
}

var fs = require('fs'),
  path = require('path'),
  util = require('util'),
  vm = require('vm'),
  stream = require('stream'),
  Lexer = require('gherkin').Lexer('en'),
  template = require('../template'),
  sandbox = require('./sandbox');

//
// Basic gherkin feature -> mocha bdd test suite
//
//    fs.createReadStream(something).pipe(new Parser).pipe(process.stdout);
//
// Basic gherkin to mocha mapping:
//
// - feature      -> top-level describe()
// - scenario(s)  -> inner describe(), might have one or more
// - step(s)      -> it()
//

module.exports = Parser;

function lexFeature (contents, name) {
    var tokens;
    try {
        var formatter = new Formatter();
        /* eslint-disable new-cap */
        var EnLexer = gherkin.Lexer('en');
        /* eslint-enable new-cap */
        var enLexer = new EnLexer(formatter);
        enLexer.scan(contents);
        tokens = formatter.done();
    } catch (e) {
        throw new LexFeatureError('Lexing "' + name + '" failed.');
    }
    return tokens;
}

function lex (file) {
    try {
        var formatter = new Formatter();
        /* eslint-disable new-cap */
        var EnLexer = gherkin.Lexer('en');
        /* eslint-enable new-cap */
        var enLexer = new EnLexer(formatter);
        enLexer.scan(file.content);
        file.tokens = formatter.done();
        delete file.content;
    } catch (e) {
        throw new LexFeatureError('Lexing "' + file.name + '" failed.');
    }
}

.then(() => {
            let formatter = new FeatureLexerFormatter();
            /* eslint-disable new-cap */
            let EnLexer = gherkin.Lexer('en');
            /* eslint-enable new-cap */
            let enLexer = new EnLexer(formatter);
            enLexer.scan(this.content);
            this.tokens = formatter.done();
        })
        .catch((error) => {

featureFiles.forEach(function (file) {
        try {
            var formatter = new Formatter();
            /* eslint-disable new-cap */
            var EnLexer = gherkin.Lexer('en');
            /* eslint-enable new-cap */
            var enLexer = new EnLexer(formatter);
            enLexer.scan(file.content);
            file.tokens = formatter.done();
        } catch (e) {
            throw new LexFeatureError('Lexing "' + file.name + '" failed.');
        }
    });
}

function setTokens (file, content) {
    let formatter = new FeatureLexerFormatter();
    let EnLexer = gherkin.Lexer('en');
    let enLexer = new EnLexer(formatter);
    enLexer.scan(content);
    file.tokens = formatter.done();
}