Array flatten for es6

flatten.js

/* This function use array.reduce
* https://developer.mozilla.org/es/docs/Web/JavaScript/Referencia/Objetos_globales/Array/reduce
* to recursively flatten a multidemensional array. 
* The array can be arbitrarly deep
*/
const flatten = list => list.reduce(
    (a, b) => a.concat(Array.isArray(b) ? flatten(b) : b), []
);

// Testing with chai and mocha
describe('Array flatten', function() {
  it('Should return a flat version of a bi-dimensional array', function() {
    const array = [[1,2],[3,4]]
    const flat = flatten(array) 
    const truth = flat.flat(Infinity)
    expect(flat).to.have.length(truth.length)
    expect(flat).to.have.deep.members(truth)

  })
  it('Should get a flat version of complext array', function() {
    /** Boilerplate to generate random custom nested array */
    const genObject = () => ({
      id: Math.ceil(Math.random()*100),
        value: Math.random()
    })

    const genTotal = () => {
      return Math.ceil(Math.random()*100)
    }

    const genNestedBoard = ({ width, height}) => 
       Array.from({ length: height}, () => Array.from({ length: width }, () => genObject() )
    );
    const nested = genNestedBoard({ width: genTotal(), height: genTotal() })
    const flat = flatten(nested)
    const truth = flat.flat(Infinity)
    expect(flat).to.have.length(truth.length)
    expect(flat).to.have.deep.members(truth)

  })
})