Example for a possible alterItems implementation

alterItems.js

function alterItems(func) {
  if (!func) {
    func = () => { };
  }

  if (typeof func !== 'function') {
    throw new errors.BadRequest('Function required. (alter)');
  }

  return context => {
    let items = context;
    const isArray = Array.isArray(items);

    function getResult(result, item) {
      if (typeof result === 'object' && result !== null) {
        return result
      } else {
        return item
      }
    }

    const results = (isArray ? items : [items]).map(
      (item, index) => {
        let result = func(item, context);

        if (typeof result === 'object' && typeof result.then === 'function') {
          return result.then(result => getResult(result, item))
        }

        return getResult(result, item)
      }
    );

    hasPromises = results.some(result => typeof result === 'object' && typeof result.then === 'function')

    if (!hasPromises) return results

    return Promise.all(results)
      .then((items) => context = items)
      .then(() => context)
  };
};

console.log('first', alterItems(rec => { rec.test = '456' })([{ blub: '123' }, { test: '123' }]))

console.log('second', alterItems(rec => ({ new: '123' }))([{ blub: '123' }, { test: '123' }]))

alterItems(rec => new Promise(res => {
  rec.test = '456'
  res()
}))([{ blub: '123' }, { test: '123' }])
  .then((data) => console.log('third', data))

alterItems(rec => new Promise(res => {
  res({ new: '123' })
}))([{ blub: '123' }, { test: '123' }])
  .then((data) => console.log('fourth', data))

alterItems(rec => {
  if (rec.blub === '123') {
    return new Promise(res => {
      res({ new: '123' })
    })
  }
  rec.test = '456'
})([{ blub: '123' }, { test: '123' }])
  .then((data) => console.log('fifth', data))

Output:

​​​​​first [ { blub: '123', test: '456' }, { test: '456' } ]​​​​​
​​​​​​​​​​
​​​​​second [ { new: '123' }, { new: '123' } ]​​​​​
​​​​​​​​​​
​​​​​third [ { blub: '123', test: '456' }, { test: '456' } ]​​​​​
​​​​​​​​​​
​​​​​fourth [ { new: '123' }, { new: '123' } ]​​​​​
​​​​​​​​​​
​​​​​fifth [ { new: '123' }, { test: '456' } ]​​​​​