The ES5 for-in requirements expressed as an ES6 generator function

forin.js

function *forin(obj) {
  let processed = new Set();
  while (obj!==null) {
    let here = Object.getOwnPropertyNames(obj);
    for (let i=0; i<here.length; i++) {
      let name = here[i];
      if (processed.has(name)) continue;
      processed.add(name);
      let desc = Object.getOwnPropertyDescriptor(obj,name);
      if (desc && desc.enumerable) yield name;
    }
    obj = Object.getPrototypeOf(obj);
  }
}