tweet-when-to-use-generators-iterators.js

// Builds array of everything ahead of time
function collectAllItems() {
  return [calculateFirst(), calculateSecond(), ...]
}

// This loop will end as soon as `isMatch(item)` is truthy.
// If the very first item in the array is a match, then we
// wasted all this time building the array in the first place.
for (let item of collectAllItems()) {
  if (isMatch(item)) {
    return true
  }
}

// Creates iterator that lazily computes values as requested
function* iterateAllItems() {
  yield calculateFirst()
  yield calculateSecond()
  // ...
}

// The loop will request each item as needed.
// If the very first item returned from the iterator is a match,
// then we don't waste any time calculating additional items.
for (let item of iterateAllItems()) {
  if (isMatch(item)) {
    return true
  }
}

// **Nuance:** Building an array is very fast in most cases.
// 
// These "generator functions" do have some overhead of their own,
// particularly if you still need to transpile them. Check out the
// browser support to see if you really need to be transpiling them.
// 
// You won't need generators/iterators in most cases, but when you
// do, they can be a huge boost.