Recursively flattens nested arrays

flatten.js

/**
* Recursively flattens nested arrays
* ex: [[1,2,[3]],4] -> [1,2,3,4]
*
* @param Array arr
* @return Array
*/
const flatten = (arr) => (
	// reduce will iterate and 
	// add to an array 
	arr.reduce((a, b) => (
		// concat will join the inner 
		// arrays to the final array
		a.concat(Array.isArray(b) ? flatten(b) : b)
	), [])
);