Recursive implementation to flatten a nested array structure.

JS Flatten

var flatten = function(nested) {
    var head = nested.slice(0, 1),
        tail = nested.slice(1),
        type = Object.prototype.toString.call(head[0]);

    if(nested.length === 0) return [];

    if(type === '[object Array]') return flatten(head[0]).concat(tail);
    
    return head.concat(flatten(tail));
};

var test = ['a', 'b', ['c', 'd'], 'e', 'f'];

console.log(flatten(test));