Flatten array recursively in JavaScript

flat.js

function flatten(array) {
   return Array.isArray(array) ? [].concat.apply([], array.map(flatten)) : array;
}

console.log(flatten('a')); // ["a"]
console.log(flatten([[['b']]])); // ["b"]
console.log(flatten(['a',['b'],['c']])); // ["a","b","c"]
console.log(flatten([['a'],'b',['c',['d']]])); // ["a","b","c","d"]