es6-iterator.js

Object.prototype[Symbol.iterator] = function() {
  let self = this;
  let keys = null;
  let length = 0;
  let index = -1;
  
  return {
    next() {
      if (keys === null) {
        keys = Object.keys(self);
        length = keys.length;
      }
      
      let value = {};
      value.key = keys[++index];
      value.value = self[value.key];

      return {
        value: value,
        done: index >= length
      };
    }
  }
};

let obj = {
  a: 1,
  b: 2,
  c: 3
};

for (let entry of obj) {
  console.log(`key: ${entry.key}, value=${entry.value}`);
}

for (let entry of Object.entries(obj)) {
  console.log(entry);
}