Comparing between class and factory function when creating object

Collection.js

// @flow
type ItteratorFunction = (key: string, value: mixed) => void;

type DataItem = {
  key: string;
  value: mixed;
};

type Data = {
  [key: string]: DataItem;
};

export class Collection {
  _data: Data;

  constructor() {
    this._data = {};
  }

  set(key: string, value: mixed) {
    this._data[key.toLowerCase()] = {key, value};
  }

  get(key: string) {
    let item = this._data[key.toLowerCase()];
    return item !== null ? item : null;
  }

  forEach(fn: ItteratorFunction) {
    for (let item of Object.keys(this._data)) {
      let {key, value} = this._data[item];
      fn(key, value);
    }
  }
}

createCollection.js

// @flow
type IteratorFunction = (key: string, value: mixed) => void;
type Data = {[key: string]: DataItem};
type DataItem = {
  key: string;
  value: mixed;
};


export function createcollection() {
  let data: Data = {};
  return {
    set(key: string, value: mixed) {
      data[key.toLowerCase()] = {key, value};
    },
    get(key: string): mixed {
      let item = data[key.toLowerCase()];
      return item !== null ? item : null;
    },
    forEach(fn: IteratorFunction) {
      for (let item of Object.keys(data)) {
        let {key, value} = data[item];
        fn(key, value);
      }
    },
  };
}