DeepFlattenArray.js

function flatten(array) {
  return array.reduce((acc, current) => {
    // if current element is an array call flatten method recursively 
    if (Array.isArray(current)) {
      return acc.concat(flatten(current));
    }
    // otherwise just accumulate the current array
    return acc.concat(current);
  }, []);
};

// Test
const deepArray = [[1], [2, 3, [4]], 5];

console.assert(JSON.stringify(flatten(deepArray)) === JSON.stringify([1, 2, 3, 4, 5]));