es6-obj-class-conversion-final-result.js

// Final config.js
export default class {
  init() {
    this.config = // ... load the object ...
    
    // For posterity, but we don't need to return it at the moment. 
    return this.config;
  },

  settingExists( key ) {
    let found = Object.keys( this.config ).indexOf( key )
    return found;
  }

  static printFormatInstructions() {
     console.log('Your config file must be in json format, and contain the following keys: foo, bar, baz.');
  }
}

// Final usage

import Config from 'config.js';

Config.init();

// Later
if ( ! Config.settingExists( 'foo' ) ) {
  Config.printFormatInstructions();
}