An array flattener

flatten.js

/**
 * @param arr the array to flatten
 * @param ret used internally, a starting point if passed
 * @returns {Array} a flattened array such that `. [[1,2,[3]],4] -> [1,2,3,4]`
 * @example
 *  const flatten = require('./flatten');
 *  console.warn(flatten([1, [2, [3, [4]], 5]]));
 *  // [1, 2, 3, 4, 5]
 */
const flatten = (arr, ret = []) => {
    let index = -1;
    const length = (arr || []).length;
    while (++index < length) {
        const value = arr[index];
        if (Array.isArray(value)) {
            flatten(value, ret);
        } else {
            ret[ret.length] = value;
        }
    }
    return ret;
};
module.exports = flatten;

flatten.test.js

// to run tests, use `Jest`
const flatten = require('./flatten');

describe('flatten', () => {
    it('should flatten empty arrays', () => {
        const arr1 = [];
        expect(flatten(arr1)).toEqual([]);
    });
    it('should return empty arrays on invalid values', () => {
        expect(flatten(null)).toEqual([]);

        expect(flatten(0)).toEqual([]);
    });
    it('should flatten shallow arrays', () => {
        const arr1 = [1, 2, 3];
        expect(flatten(arr1)).toEqual([1, 2, 3]);

        const arr2 = ['a', 'b'];
        expect(flatten(arr2)).toEqual(['a', 'b']);
    });
    it('should flatten deep arrays', () => {
        const arr1 = [1, [2, [3, [4]], 5]];
        expect(flatten(arr1)).toEqual([1, 2, 3, 4, 5]);

        const arr2 = [[1, 2, [3]], 4];
        expect(flatten(arr2)).toEqual([1, 2, 3, 4]);
    })
});