DCC-19.js

// Prompt:

// - Write a function named flatten that accepts a single array that may contain nested arrays and returns a new "flattened" array.
// - A flattened array is an array that contains no nested arrays.
// - Arrays maybe nested at any level.
// - If any of the arrays have duplicate values those duplicate values should be present in the returned array.
// - The values in the new array should maintain their ordering as shown in the examples below.

// Hint:

// - This assignment provides an excellent opportunity to use recursion (a function that calls itself).  It can also be solved by using an inner function.

// Examples:

// flatten( [1, [2, 3]] );
// //=> [1, 2, 3]  (a new array) 

// flatten( [1, [2, [3, [4]]], 1, 'a', ['b', 'c']] );
// //=> [1, 2, 3, 4, 1, 'a', 'b', 'c']

const flatten = (arr) => {
	const result = [];
	const stack = [...arr];

	while (stack.length) {
		const next = stack.pop();
		if (Array.isArray(next)) {
			stack.push(...next);
		} else {
			result.push(next);
		}
	}

	return result.reverse();
};