Browserify CMD vs GULP

file_a.js

module.exports = req_a;

function req_a() {
  
}

gulpfile.js

const gulp      = require('gulp');
const nodemon   = require('nodemon');
const b         = require('browserify');
const source    = require('vinyl-source-stream');
const buffer    = require('vinyl-buffer');
const rename    = require('gulp-rename');
const child     = require('child_process');

var _src_a = `${__dirname}/src/index_funcs.js`;
var _dst_a = `${__dirname}/dst/funcs.js`;

// "browserify src/index_paint > dst/funcs.js"

gulp.task('browserify_cmd', (done) => {

  var cmp_a = child.spawn('browserify', [ _src_a, '-o', _dst_a ], { stdio: 'inherit'});

  cmp_a.on('exit', (exitCode) => {
    done()
  });
});

var browserifier = function (done) { 

    b({
        entries: _src_a,
        debug: false,
        fullpath: false
    }).bundle()
      .pipe(source('./src/index_funcs.js'))
      .pipe(buffer())
      .pipe(rename('funcs.js',{dirname: ''}))
      .pipe(gulp.dest('./dst'));

    if (done) {
      done()
    }

};

gulp.task('browserify', browserifier);

gulp.task('watch', function () {
  nodemon({
    script: 'server.js',
    env: {
     'PORT': 6006
    },
    //args: ['--bundle'],
    watch: ['gulpfile.js', 'src/**/*.*'],
    stdout: false
  })
  .on('restart', function() {
    browserifier(function(){
    });
  })
});

gulp.task('default', gulp.series(browserifier, 'watch'));

index_funcs.js

var req_a = require('./app/file_a'); // in src/app/file_.js

window.yourFunc = function () {
  req_a()
}