takeWhile

Removes elements in an array until the passed function returns false. Returns the removed elements.

• Loop through the array, using a for...of loop over Array.prototype.entries() until the returned value from the function is falsy.
• Return the removed elements, using Array.prototype.slice().
• The callback function, fn, accepts a single argument which is the value of the element.

const takeWhile = (arr, fn) => {
  for (const [i, val] of arr.entries()) if (!fn(val)) return arr.slice(0, i);
  return arr;
};

takeWhile([1, 2, 3, 4], n => n < 3); // [1, 2]