30-seconds-of-code/snippets/bifurcateBy.md

bifurcateBy

Splits values into two groups according to a predicate function, which specifies which group an element in the input collection belongs to. If the predicate function returns a truthy value, the collection element belongs to the first group; otherwise, it belongs to the second group.

Use Array.reduce() and Array.push() to add elements to groups, based on the value returned by fn for each element.

const bifurcateBy = (arr, fn) =>
  arr.reduce((acc, val, i) => (acc[fn(val, i) ? 0 : 1].push(val), acc), [
    [],
    [],
  ]);

bifurcateBy([ 'beep', 'boop', 'foo', 'bar' ], x => x[0] === 'b'); // [ ['beep', 'boop', 'bar'], ['foo'] ]