flattenArray code sample - https://codesandbox.io/s/jp5y2k71m3

flattenArray.js

/**
 * recursively flatten a nested array
 * @param {Array} arr to be flattened
 * @return {Array} flattened array
 */
const flattenArray = arr =>
  arr.reduce(
    (acc, val) => acc.concat(Array.isArray(val) ? flattenArray(val) : val),
    []
  );

module.exports = flattenArray;

flattenArray.spec.js

const flattenArray = require("./index");

describe("flattenArray", () => {
  it("should flatten an array", () => {
    const nestedArray = [1, [2], 3];
    expect(flattenArray(nestedArray)).toEqual([1, 2, 3]);
  });
  it("should flatten a deeply nested array", () => {
    const nestedArray = [[1, 2, [3]], 4, [[[5]], 6]];
    expect(flattenArray(nestedArray)).toEqual([1, 2, 3, 4, 5, 6]);
  });
});

package.json

{
  "name": "citrus-byte-flatten-array",
  "version": "0.0.1",
  "description": "flattenArray code sample",
  "main": "flattenArray.js",
  "scripts": {
    "test": "jest"
  },
  "dependencies": {},
  "devDependencies": {
    "jest": "23.6.0"
  }
}