flat array

flat-array.js

// flat single level array
arr1.reduce((acc, val) => acc.concat(val), []);

// deep level flatten
function flatDeep(arr1) {
   return arr1.reduce((acc, val) => Array.isArray(val) ? acc.concat(flatDeep(val)) : acc.concat(val), []);
};

//recursive flatten deep
function flatten(array) {
  var flattend = [];
  (function flat(array) {
    array.forEach(function(el) { Array.isArray(el) ? flat(el) : flattend.push(el) });
  })(array);
  return flattend;
}

// deep level flatten recursive (fastest)
function flatDeep(a, res) {
	var i = 0, v;
	for (; i < a.length; i++) {
		v = a[i];
		Array.isArray(v) ? flatDeep(v, res) : res.push(v);
	}
	return res;
}

// resource : https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/flat