Flattening nested array

.babelrc

{
  "presets": ["es2015"]
}

flatten-array.js

// API-documentation generated by yuidoc: http://yui.github.io/yuidoc/

/**
 *
 * Flattens a nested array
 *
 * <i>Copyright (c) 2016 Marco Asbreuk</i>
 * New BSD License - http://choosealicense.com/licenses/bsd-3-clause/
 *
 * @module array
 * @class Array
 *
 */

"use strict";

/**
 * Flattens an Array (nested Array-items become items of the array)
 *
 * @method itsa_flatten
 * @param source {Array} the source-array that needs to be flattened
 * @return Array the flattened array
 */
const flattenArray = source => {
    let flattened = Array.prototype.concat.apply([], source),
        prevLen = flattened.length;
    // concat will only flatten 1 level deep, so in case of nested arrays,
    // we might need more flattens. We check this by checking the lenght of the flattened array.
    // If it doesn't increase anymore, then we are finished
    do {
        flattened = Array.prototype.concat.apply([], flattened);
    } while (prevLen !== (prevLen = flattened.length));
    return flattened;
};

export default flattenArray;

package.json

{
  "name": "flatten-array",
  "version": "0.0.1",
  "description": "Flattens a nested array",
  "main": "flatten-array.js",
  "scripts": {
    "test": "mocha test.js --compilers js:babel-core/register"
  },
  "author": "Marco Asbreuk",
  "license": "BSD-3-Clause",
  "dependencies": {},
  "devDependencies": {
    "babel-core": "^6.7.0",
    "babel-preset-es2015": "^6.6.0",
    "chai": "^3.5.0",
    "mocha": "^2.4.5"
  }
}

test.js

/*global describe, it */

"use strict";

import {expect} from "chai";
import flatten from "./flatten-array";

describe("Testing flattenArray", () => {

    const flattened = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

    it("Should leave 1-dim array the same", () => {
        var source = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];
        expect(flatten(source)).to.be.eql(flattened);
    });

    it("Should flatten 2-dim array", () => {
        var source = [1, [2, 3], 4, 5, [6, 7], 8, 9, 10];
        expect(flatten(source)).to.be.eql(flattened);
    });

    it("Should flatten 3-dim array", () => {
        var source = [1, [2, [3, 4]], [5, [6, 7], 8], 9, 10];
        expect(flatten(source)).to.be.eql(flattened);
    });

    it("Should flatten 4-dim array", () => {
        var source = [1, [2, [3, [4, 5], 6, 7], 8], 9, 10];
        expect(flatten(source)).to.be.eql(flattened);
    });

});