javascript - Dividing an array by filter function

Question

I have a Javascript array that I would like to split into two based on whether a function called on each element returns true or false. Essentially, this is an array.filter, but I'd like to also have on hand the elements that were filtered out.

Currently, my plan is to use array.forEach and call the predicate function on each element. Depending on whether this is true or false, I will push the current element onto one of the two new arrays. Is there a more elegant or otherwise better way to do this? An array.filter where the will push the element onto another array before it returns false, for instance?

Answer 1

With ES6 you can make use of the spread syntax with reduce:

function partition(array, isValid) {
  return array.reduce(([pass, fail], elem) => {
    return isValid(elem) ? [[...pass, elem], fail] : [pass, [...fail, elem]];
  }, [[], []]);
}

const [pass, fail] = partition(myArray, (e) => e > 5);

Or on a single line:

const [pass, fail] = a.reduce(([p, f], e) => (e > 5 ? [[...p, e], f] : [p, [...f, e]]), [[], []]);