Dangerous Promises

dangerous-promises.js

//
// utils
//

const thrush = x => f => f(x);

const indexBy = k => xs => xs.reduce((index, x) => {
  if(index[x[k]] != null) index[x[k]].push(x);
  else index[x[k]] = [x];
  return index;
}, {});

const mapObject = f => o => Object.keys(o).reduce((mapped, k) => {
  mapped[k] = f(o[k]);
  return mapped;
}, {});


//
// program
//

const fetchUsers = _ => Promise.resolve([
  {id: 1, name: 'bob'},
  {id: 2, name: 'frank'},
  {id: 3, name: 'bob'},
]);

const isLongerThan = xs => n => xs.length > n;

fetchUsers()
.then(indexBy('name'))
.then(mapObject(isLongerThan))
.then(mapObject(thrush(1)))
.then(console.log, console.error);

This somewhat-contrived-but-not-unthinkable program loads a list of users and writes to the terminal, for every user name, whether there exist more than one users with this name.

Now what happens when one of the users names themselves then?