Flatten Array

flattenArray.js

const assert = require('assert');

function flattenArray(array) {
	return array.reduce((acc, current) => {
		if (Array.isArray(current)) {
			return acc.concat(flattenArray(current));
		}

		return acc.concat([current]);
	}, []);
}

function emptyArrayTest() {
	const test = [];
	const result = flattenArray(test);
	const expected = [];
	assert.deepEqual(result, expected);
}

emptyArrayTest();

function noNestingTest() {
	const test = [1, 2, 3];
	const result = flattenArray(test);
	const expected = [1, 2, 3];
	assert.deepEqual(result, expected);
}

noNestingTest();

function deeplyNestedTest() {
	const test = [[1, [2, [[3, 4], [5]]], [6]]];
	const result = flattenArray(test);
	const expected = [1, 2, 3, 4, 5, 6];
	assert.deepEqual(result, expected);
}

deeplyNestedTest();