Array flatten

flattenArray.js

// flatten Array of any deep
const flattenArray = arr => {
  return arr.reduce((prev,next)=> (
     Array.isArray(next)
         ? prev.concat(flattenArray(next))
         : prev.concat(next)
  ),[]);
};

module.exports = flattenArray;

// Post Data 
// ---------
// I believe there is now a proposal for array.protoype.flat()
// but as long it is a proposal and this is a test I try to make my own implementation using
// recursive function and a reducer

flattenArray.test.js

// Test file using Jest

const flattenArray = require('./flattenArray');

test('flattenArray empty', () => {
  expect(flattenArray([])).toEqual([]);
});

test('flattenArray of 0 nested array', () => {
  expect(flattenArray([1,2,3,4])).toEqual([1,2,3,4]);
});

test('flattenArray of 1 nested array', () => {
  expect(flattenArray([1,2,[5,6],3,4])).toEqual([1,2,5,6,3,4]);
});

test('flattenArray of "n" nested arrays', () => {
  expect(flattenArray([[1,2],3,[4,5,[6,[7,8]],[9,10]],11,12])).toEqual([1,2,3,4,5,6,7,8,9,10,11,12]);
});