Function that flattens a nested array. You can see the result of the tests in this jsbin: http://jsbin.com/suvocovuzi/edit?html,js,output

Flatten

describe('Flatten', function() {

  it("Should put scalar values inside an array", function() {
  
    expect(flatten(1)).toEqual([1]);
    
  });

  it('Should leave arrays as they are', function() {
    
    expect(flatten([1, 2, 3, 4])).toEqual([1, 2, 3, 4]);
  
  });

  it('Should flatten nested arrays', function() {
  
  	var arrayToFlatten = [[1, 2, [3]], 4];
    
    expect(flatten(arrayToFlatten)).toEqual([1, 2, 3, 4]);
  
  });

});

function flatten(toFlatten) {

  if (!Array.isArray(toFlatten)) {

    return [toFlatten];

  }

  return toFlatten.reduce(function(flat, nextElem) {

    return flat.concat(flatten(nextElem));

  }, []);

}