Flatten example written in Node with jest tests

flatten.js

/**
 * Flattens provided array.
 *
 * If an array is provided, this function will flatten it. 
 * EXAMPLE: flatten([[1, 2, [3]], 4]) returns [1, 2, 3, 4].
 *
 * @throws Throws an exception if provided parameter is not an Array.
 * @param {Array}   arr     Array to flatten.
 * @return {array}  Flattend Array. 
 **/
function flatten(arr)
{
    // Sanatize input, throw if not an Array.
    if (!Array.isArray(arr)) {
        throw 'Provided parameter is not an array.';
    }

    var result = [];

    // Run recursive reducer with array and result parameters.
    (function reducer(arr, state) {
        arr.forEach((item) => {
            // Continue reducing if array item is a nested Array.
            if (Array.isArray(item)) {
                reducer(item, state);
            }
            // Add array item to result if not an Array.
            else {
                state.push(item);
            }
        });

        return state;
    })(arr, result);

    return result;
}

module.exports = flatten;

flatten.test.js

/**
 * Meant to be run with jest. If jest is not part of the project 
 * you may install it globally to the system with npm i --global jest
 **/

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

describe("Testing Flatten.js", () => {
    test('flatten() Throws Exception', () => {
        expect(() => { flatten() }).toThrow();
    });
    test('flatten(undefined) Throws Exception', () => {
        expect(() => { flatten(undefined) }).toThrow();
    });
    test('flatten(5) Throws Exception', () => {
        expect(() => { flatten(5) }).toThrow();
    });
    test('flatten(\'z\') Throws Exception', () => {
        expect(() => { flatten('z') }).toThrow();
    });
    test('flatten([]) = []', () => {
        expect(flatten([]))
            .toEqual([]);
    });
    test('flatten([1, 2, [], 4]) = [1, 2, 4]', () => {
        expect(flatten([1, 2, [], 4]))
            .toEqual([1, 2, 4]);
    });
    test('flatten([1, 2, 3, 4]) = [1, 2, 3, 4]', () => {
        expect(flatten([1, 2, 3, 4]))
            .toEqual([1, 2, 3, 4]);
    });
    test('flatten([[1, 2, [3]], 4]) = [1, 2, 3, 4]', () => {
        expect(flatten([[1, 2, [3]], 4]))
            .toEqual([1, 2, 3, 4]);
    });
    test('flatten([1, 2, [1, 2, 3]] = [1, 2, 1, 2, 3]', () => {
        expect(flatten([1, 2, [1, 2, 3]]))
            .toEqual([1, 2, 1, 2, 3]);
    });
    test('flatten([1, 2, [1, 2, 3]] != [1, 1, 2, 2, 3]', () => {
        expect(flatten([1, 2, [1, 2, 3]]))
            .not.toEqual([1, 1, 2, 2, 3]);
    });
    test('flatten([1, 2, [3, 2, 1, [1, 2, 3, 4, 5], 1, 2, 3], 3, 4, 5]) = [1, 2, 3, 2, 1, 1, 2, 3, 4, 5, 1, 2, 3, 3, 4, 5]', () => {
        expect(flatten([1, 2, [3, 2, 1, [1, 2, 3, 4, 5], 1, 2, 3], 3, 4, 5]))
            .toEqual([1, 2, 3, 2, 1, 1, 2, 3, 4, 5, 1, 2, 3, 3, 4, 5]);
    });
    test('flatten([[[1], 2]]) = [1, 2]', () => {
        expect(flatten([[[1], 2]]))
            .toEqual([1, 2]);
    });
});