enum.js

const POSITION = (function() {
  var inner = {};
  Object.defineProperty(inner, 'left', {
    writable: false,
    value: 1
  })
  
  Object.defineProperty(inner, 'right', {
    writable: false,
    value: 2
  })
  
  Object.defineProperty(inner, 'middle', {
    writable: false,
    value: 3
  })
  return inner;
})();

console.log(POSITION.left);

enum_maker.js

function ENUM(values) {
  var inner = {};
  for(var i=0; i < values.length; i++) {
     Object.defineProperty(inner, values[i], {
        writable: false,
        value: i + 1
      })
  }
  return inner;
}

POSITION = ENUM(['left', 'right', 'middle']);
console.log(POSITION.left);