gulpfile.js

var gulp    = require('gulp');
var replace = require('gulp-replace-task');
var args    = require('yargs').argv;
var fs      = require('fs');

gulp.task('replace', function () {
  // Get the environment from the command line
  var env = args.env || process.env.ENV || 'dev';

  console.log("environment: " + env);

  // Read the settings from the right file
  var filename = env + '.json';
  var settings = JSON.parse(fs.readFileSync('./config/' + filename, 'utf8'));

// Replace each placeholder with the correct value for the variable.
gulp.src('./config/config.js')
  .pipe(replace({
    patterns: [
      {
        match: 'serverUrl',
        replacement: settings.serverUrl
      },
      {
        match: 'fbAppId',
        replacement: settings.fbAppId
      }
    ]
  }))
  .pipe(gulp.dest('client/js'))
  .pipe(gulp.dest('nibs-shell/www/js'));
});