Node.js flatten()

index.js

const { expect } = require("chai")

function flatten(array) {
  return array.reduce((acc, val) => acc.concat(Array.isArray(val) ? flatten(val) : val), [])
}

describe("#flatten", () => {
  it("should return flattened array", () => {
    const input = [[1, 2, [3]], 4]
    expect(flatten(input)).to.deep.equal([1, 2, 3, 4])
  })
})

package.json

{
  "name": "flatten",
  "version": "1.0.0",
  "description": "Node.js",
  "scripts": {
    "test": "mocha index.js"
  },
  "keywords": [
    "Javascript",
    "Mocha",
    "Chai"
  ],
  "author": "Adam Faryna",
  "license": "MIT",
  "dependencies": {
    "chai": "^4.2.0",
    "mocha": "^5.2.0"
  }
}