Simple function to flatten an array. To easily verify results, download zip and run `test-cases.js` in Node

flatten.js

/**
 * Flatten any combination of nested ints/arrays
 * in an array recursively into a flat array
 * @param input {Array}
 * @returns {Array}
 */
function flatten(input) {
	let output = [];

	for(let i = 0; i < input.length; i++) {
		const item = input[i];
		
		// recursively add arrays
		if(typeof item == 'object' && Array.isArray(item) && item.length) {
			output.push(...flatten(item));
		}

		// add simple number instances
		if(typeof item == 'number') {
			output.push(item);
		}
	}

	return output;
}

module.exports = flatten;

test-cases.js

const flatten = require('./flatten');

console.log('flatten([]) ->', flatten([]));
console.log('flatten([1,2,3]) ->', flatten([1,2,3]));
console.log('flatten([[1,2,[3]],4]) ->', flatten([[1,2,[3]],4]));
console.log('flatten([1,[],[2]) ->', flatten([1,[],[2]]));
console.log('flatten([1,[[]],[3]) ->', flatten([1,[[]],[3]]));