This simple test proves, that standard JavaScript arrays are algebraic functors according to the fantasy land spec (https://github.com/fantasyland/fantasy-land)

array_functor_test.js

const eq = (test, a1, a2) => {
  if(a1.length === a2.length && a1.every((v,i) => v === a2[i])) {
    return;
  }

  throw new Error(`Test failed: ${ test }`);
};

{ // Functor
  const u = Array.of([1, 2, 3]);
  const f = (x) => x + 1;
  const g = (x) => x + 2;

  eq('Functor: identity', u.map(x => x), u);
  eq('Functor: composition', u.map(x => f(g(x))), u.map(g).map(f));
}