Flatten Array

flatten-function.js

var flatten = function(array) {
  return array.reduce(function(a,b) {
    return a.concat(Array.isArray(b) 
     ? flatten(b) 
     : [b] );
  },[]);
};

flatten-imperative.js

var flatten = function(array) {
  var flatArray = [];
  array.forEach(function(element) {
     Array.isArray(element) 
     ? flatArray = flatArray.concat(flatten(element))
     : flatArray.push(element);
  });
  return flatArray;
};

flatten.spec.js

describe('flatten',function(){
  it('should flatten the array',function(){
     expect(flatten([1,2,3,4])).to.be.an.array().and.to.equal([1,2,3,4]);
     expect(flatten([[1,2],[3,4]])).to.be.an.array().and.to.equal([1,2,3,4]);
     expect(flatten([1,[2,3],4])).to.be.an.array().and.to.equal([1,2,3,4]);
     expect(flatten([1,[2,[3,4],5,6,7]])).to.be.an.array().and.to.equal([1,2,3,4,5,6,7]);
  });
});

const flatten = arr => arr.reduce((a,e) => a.concat(Array.isArray(e) ? flatten(e) : e), [])