Flatten Array

flattenArray.js

 /**
  * Return a flattened array when given an array of arbitrary depth.
  */
  
  const flattenArray = array => array.reduce((accum, element) => {
    if (Array.isArray(element)) {
      const flattenedArray = flattenArray(element);

      accum.push.apply(accum, flattenedArray);
    } else {
      accum.push(element);
    }

    return accum;
  }, []);