flatten.js

/**
 * Takes an array or a single number and flattens it into a result array.
 *
 * @param input: Array|any
 * @param resultArray: Array|null - optional. array into which all inputs get flattened into. Uses an empty array if none provided.
 */
const flatten = function (input, resultArray = []) {

    if (!Array.isArray(resultArray))
        throw new Error('ResultArray must be an array')

    if (Array.isArray(input)) {
        for (const item of input) {
            flatten(item, resultArray)
        }
    }
    else if (input) {
        resultArray.push(input)
    }

    return resultArray;
}
exports.flatten = flatten;

flatten.test.js

/**
To run this test, you may put it in the same directory of flatten.js and then use a test runner like mocha. i.e
- npm i -g mocha
- mocha flatten.test.js
**/

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

describe('flatten', function () {

    it('flattens an array', function (done) {
        const input = [[1, 2, [3]], 4];
        const expected = [1, 2, 3, 4];
        const result = flatten(input);
        assert.deepStrictEqual(result, expected);
        done();
    });

    it('flattens very deep arrays', function (done) {
        const input = [[1, 2, [3]], [[[[[[5]], 4]]]], 7];
        const expected = [1, 2, 3, 5, 4, 7];
        const result = flatten(input);
        assert.deepStrictEqual(result, expected);
        done();
    });

    it('flattens already-flattened Array', function (done) {
        const input = [1, 2, 3, 9, 3];
        const expected = [1, 2, 3, 9, 3];
        const result = flatten(input);
        assert.deepStrictEqual(result, expected);
        done();
    });

    it('Returns item wrapped in array if singular item', function (done) {
        const input = 9;
        const expected = [9];
        const result = flatten(input);
        assert.deepStrictEqual(result, expected);
        done();
    });

    it('Returns an empty Array if nothing to flatten', function (done) {
        const input = null;
        const expected = [];
        const result = flatten(input);
        assert.deepStrictEqual(result, expected);
        done();
    });

    it('Appends to ResultArray', function (done) {
        const input = [1, 2, 3];
        const expected = [5, 6, 7, 1, 2, 3];
        const result = flatten(input, [5, 6, 7]);
        assert.deepStrictEqual(result, expected);
        done();
    });

    it('only accepts an array for ResultArray Param', function (done) {
        const input = [1, 2, 3];

        try {
            const result = flatten(input, null);
            assert.fail('An error should throw as ResultArray param was not an array');
        } catch (err) {
            done();
        }
    });
});