citrusbyte - flatten // tested on https://repl.it/languages/jest

flatten.js

const reducer = (result, item) => {
  const flattened = Array.isArray(item) ? flatten(item) : item;
  return result.concat(flattened);
}

const flatten = (array) => array.reduce(reducer, []);

module.exports = flatten;

flatten.test.js

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

describe('flatten', () => {
  it('should not modify flat arrays', () => {
    expect(flatten([])).toEqual([]);
    expect(flatten([1,2,3])).toEqual([1,2,3]);
  });
  
  it('should not mutate existing array', () => {
    const originalArray = [1,[2,3],4];
    const copy = originalArray.slice(0);
    
    flatten(originalArray);
    
    expect(originalArray).toEqual(copy);
  });
  
  it('should flatten nested arrays', () => {
    expect(flatten([[3]])).toEqual([3]);
    expect(flatten([1,[2,3],4])).toEqual([1,2,3,4]);
  });
  
  it('should flatten deeply nested arrays', () => {
    expect(flatten([[[[3]]]])).toEqual([3]);
    expect(flatten([1,[[2,3]],4,[5,[6,7],8],9])).toEqual([1,2,3,4,5,6,7,8,9]);  
  });
});