How to recursively flatten a nested array.

recursive-alternative.js

// alternative using 'instanceof' check

(function() {


	'use strict';
  
  var array = [
    'l1a',
    'l1b',
    [
      'l2a',
      [
        'l3a'
     	],
      'l2c'
    ],
    'l1d'
  ];
  
  var flat = [];
  
  function flatten(nested) {
      
    for (var i = 0; i < nested.length; i++) {
    
      if ( isArray(nested[i]) ) {
        console.log('nested ', nested[i]);
      	flatten(nested[i]);
      } else {
      	flat.push(nested[i]);
      }
    
    }
    
    return flat;
    
  }
  
  function isArray(arr) {
  	return arr instanceof Array;
  }
  
  
  console.log(flatten(array));
 
}());

recursive-concat-nested-array.js

// using native Array.prototype.concat method
(function() {
	'use strict';

	var myArray = [
		'first!',
		[1,2,3],
		['a','b',['nested', 'x1'],'c'],
		4,
		5,
		[6,7,'d','e'],
		['deep', ['deeper', 'deeperstill', ['deepest', 'deepest', 'yet']]],
		'last!'
	],
		weirdArray = [[[[[0]], [1]], [[[2], [3]]], [[4], [5]]]];
		
	function doConcat(array) {

		var flat = [];

		//console.log('array being worked on: ', array);

		for (var i = 0; i < array.length; i++) {
			if (array[i].constructor === Array) {
					//console.log('nested array found: ');
					flat = flat.concat(doConcat(array[i]));
			} else {
				flat.push(array[i]);
				//console.log('flat is now: ', flat);
			}
		}
		return flat;
	}

	var flattened = doConcat(myArray);
	console.log('totally flat: ', flattened);
}());

recursive-flatten-nested-arrays.js

// avoiding native Array.prototype.concat method... IMHO, this is cleaner, easier to read/understand
(function() {
	'use strict';

	var myArray = [
		'first!',
		[1,2,3],
		['a','b',['nested', 'x1'],'c'],
		4,
		5,
		[6,7,'d','e'],
		['deep', ['deeper', 'deeperstill', ['deepest', 'deepest', 'yet']]],
		'last!'
	],
		weirdArray = [[[[[0]], [1]], [[[2], [3]]], [[4], [5]]]],
		flat = [];


	function doConcat(array) {

		//console.log('array being worked on: ', array);

		for (var i = 0; i < array.length; i++) {
			if (array[i].constructor === Array) {
					//console.log('nested array found: ');
					doConcat(array[i]);
			} else {
				flat.push(array[i]);
				//console.log('flat is now: ', flat);
			}
		} 
	}

	doConcat(myArray);
	console.log('totally flat: ', flat);
}());

Goal was to do this avoiding native Array.prototype.concat method... just 'cause.