Array#flatten in JS

.babelrc

{ "presets": ["es2015"] }

README.md

Run tests

npm test

Interactive mode

npm run console
var flatten = require("./flatten.js").default
flatten([1,2,[3])

flatten.js

const unpack = (el) => {
    if(Array.isArray(el)) {
        return flatten(el);
    } else {
        return el;
    }
};

export default function flatten(array) {
    if(!Array.isArray(array)) {
        throw "Only arrays can be flatten";
    }
    return array.reduce((a, b) => {
        return unpack(a).concat(unpack(b));
    }, []);
};

flatten_test.js

var test = require('tape');
import flatten from './flatten';

test('flatten of empty array is empty array', (t) => {
    t.plan(3);

    var flatten_empty = flatten([]);
    t.same(flatten_empty, [], 'array with no elements is already flatten');

    var flatten_empty_nested = flatten([[]]);
    t.same(flatten_empty_nested, [], 'array with nested empty array should be flatten to empty array');

    var flatten_empty_two_levels = flatten([[[]]]);
    t.same(flatten_empty_two_levels, [], 'array with multiple levels of empty array should be flatten to empty array');
});

test('flatten of multiple levels', (t) => {
    t.plan(2);

    var flatten_two_levels = flatten([1,2, [3,4]]);
    t.same(flatten_two_levels, [1,2,3,4], 'elements from inside array should be on the same level after flattening')

    var flatten_five_levels = flatten([1,2, [3,[[4]]]]);
    t.same(flatten_two_levels, [1,2,3,4], 'elements from inside array should be on the same level after flattening')
});

test('flatten of example array', (t) => {
    t.plan(1);

    t.same(flatten([1,2,[3],4]), [1,2,3,4]);
});

package.json

{
  "name": "citrusbyte_flatten",
  "version": "1.0.0",
  "description": "",
  "main": "flatten.js",
  "scripts": {
    "pretest": "npm install",
    "test": "babel-node flatten_test.js"
  },
  "author": "",
  "license": "ISC",
  "devDependencies": {
    "babel-cli": "^6.9.0",
    "babel-core": "^6.9.0",
    "babel-preset-es2015": "^6.9.0",
    "tape": "^4.5.1"
  }
}