How to flatten an array in JavaScript?

  1. Array.prototype.flat()
  2. reduce + concat + isArray + recursivity
  3. ES3

Array.prototype.flat()

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const arr1 = [0, 1, 2, [3, 4]];

console.log(arr1.flat());
// expected output: [0, 1, 2, 3, 4]

const arr2 = [0, 1, 2, [[[3, 4]]]];

console.log(arr2.flat(2));
// expected output: [0, 1, 2, [3, 4]]

reduce + concat + isArray + recursivity

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const arr = [1, 2, [3, 4, [5, 6]]];

// to enable deep level flatten use recursion with reduce and concat
function flatDeep(arr, d = 1) {
   return d > 0 ? arr.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatDeep(val, d - 1) : val), [])
                : arr.slice();
};

flatDeep(arr, Infinity);
// [1, 2, 3, 4, 5, 6]

ES3

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function flat(arr, depth) {
  depth = typeof depth === 'number' ? depth : 1;
  var flattened = [];
  var isArray = function(arg) {
    return Object.prototype.toString.call(arg) === '[object Array]';
  };
  var doFlat = function(arr, depth) {
    for (var i = 0; i < arr.length; i++) {
      var el = arr[i];
      if (isArray(el) && depth > 0) {
        doFlat(el, depth - 1);
      } else {
        flattened.push(el);
      }
    }
  };
  doFlat(arr, depth);
  return flattened;
}