An array filter function which does not create a new array if no items are removed.

maybeFilter.js

function maybeFilter(array, predicate) {
  var newArray;
  array.forEach((item, i) => {
    if (predicate(item)) {
      if (newArray) {
        newArray.push(item)
      }
    } else if (!newArray) {
      newArray = array.slice(0, i)
    }
  })
  return newArray || array;
}

🆒

Note that this is only safe if you treat the result of this function as if it were immutable. Mutating the result may in some cases be mutating the original input as well which may be surprising. If you are not mutating the output, then this function may help reduce memory footprint if a filter rarely removes items, and more importantly allows for input === maybeFilter(input, myFunction) to very cheaply know if the filter did or did not remove anything.