selectMany.js

function selectMany(func) {
  return this.reduce(
    (acc, val) => [
      ...acc,
      ...func(val),
    ],
    [],
  ); 
}

['josh', 'steve', 'steve', 'craig']::selectMany(x => [x, `hello ${x}']);

/* [
  'josh',
  'hello josh',
  'steve',
  'hello steve',
  ...etc
]
*/