Array.flattening.js

// Array flatten Technique
var oneDepth = [[1], [2], [3], [4], [6], 7, 8];

var multiDepth = [[1, [2, 3], [4]], [5], [3, [4, 5]], [6], [7], 7, [[[[[8]]]]]];

// flatten one level
// function flattenOneLevel(array) {
//   var results = [];
//   for (var i = 0; i < array.length; i++) {
//     if (Array.isArray(array[i])) {
//       results = results.concat(array[i]);
//     } else {
//       results.push(array[i]);
//     }
//   }
//   return results;
// }

function flattenOneLevel(array) {
  return [].concat.apply([], array);
}

//console.log(flattenOneLevel(oneDepth));

// flatten queue technique
// [1] + [[2], [3]] => [1, [2], [3]]
function flattenQueue(array) {
  var results = [];
  var queue = array;
  while(queue.length !== 0) {
    var nextElement = queue.shift();
    if (Array.isArray(nextElement)) {
      queue = [].concat(nextElement, queue);
    } else {
      results.push(nextElement);
    }
  }
  return results;
}
// console.log(flattenQueue(oneDepth));
// console.log(flattenQueue(multiDepth));

// flatten recursive
function flattenRec(array) {
  return array.reduce(function (acc, element) {
    return acc.concat(Array.isArray(element) ? flattenRec(element) : element );
  }, []);
}
// console.log(flattenRec(oneDepth));
// console.log(flattenRec(multiDepth));

// flatten recursive with depth
function flattenDepth(array, depth) {
  if (depth === 0) return array;
  return array.reduce(function (acc, element) {
    return acc.concat(Array.isArray(element) ? flattenDepth(element, depth - 1) : element );
  }, []);
}

console.log(flattenDepth(oneDepth, 1));
console.log(flattenDepth(multiDepth, 5));