flatten-fail.js

// This function fails, results are at the bottom.

var flatten = function (array) {
  var result = [],
      currentElem;

  for (var i=0, len=array.length; i<len; i++){
    currentElem = array[i];
    if (Array.isArray(currentElem)) {
      // The result of flatten(array) is an array, why isn't is combined with the
      // previous array by the concat method ?
      result.concat(flatten(currentElem));
    } else {
      result.push(currentElem);
    }
  }
  return result;
};

// Results
console.log(flatten([1,2,3,[4,5]])); //  => [1,2,3]
console.log(flatten([[1,2,3],["a","b","c"],[1,2,3]]));  // => []
console.log(flatten([[[1,2,3]]])); // => []

flatten.js

// This function works, results are at the bottom

var flatten = function (array) {
  var result = [],
      currentElem;

  for (var i=0, len=array.length; i<len; i++){
    currentElem = array[i];
    if (Array.isArray(currentElem)) {
      // This is the part that is different
      (flatten(currentElem)).forEach(function (v) {
          result.push(v);
        });
    } else {
      result.push(currentElem);
    }
  }
  return result;
};

// Results
console.log(flatten([1,2,3,[4,5]])); // [1,2,3,4,5]
console.log(flatten([[1,2,3],["a","b","c"],[1,2,3]]));  // [1,2,3,"a","b","c",1,2,3]
console.log(flatten([[[1,2,3]]])); // [[1,2,3]]

ProblemDescription.md

I tried to write a function that would flatten nested arrays but my solution did not work when I used the Array.prototype.concat method. As a work around, I used the Array.prototype.forEach method and it worked as expected. Both implimentations are below.
Does you know why .concat() doesn't work?