Flatten multi-dimensional array

flatten.js

const flatten = (arr)  => arr.reduce(function (flat, toFlatten) {
  return flat.concat(Array.isArray(toFlatten) ? flatten(toFlatten) : toFlatten);  
}, []);
 
const multiDimensionalArray = [[1,2,[3]],4];
const flattenedArray = flatten(multiDimensionalArray);

it('flattens an array of arbitrarily nested arrays of integers into a flat array of integers', () => {
  assert.equal(flatten([[1,2,[3]],4]), [1, 2, 3, 4])
})