NodeJS: function vs. anonymous function

Working on the stream-adventure NodeJS tutorial, but am struggling with the duplexer redux exercise.

This is what I've tried to use, but it doesn't work:

var duplexer = require('duplexer2');
var through = require('through2').obj;

module.exports = function (counter) {
  var counts = {};
  var input = through(write, end);
  return duplexer(input, counter);

  var write = function (row, _, next) {
    counts[row.country] = (counts[row.country] || 0) + 1;
    next();
  }

  var end = function (done) {
    counter.setCounts(counts);
    done();
  }
};

This is the proposed solution, which works:

var duplexer = require('duplexer2');
var through = require('through2').obj;

module.exports = function (counter) {
  var counts = {};
  var input = through(write, end);
  return duplexer(input, counter);

  function write (row, _, next) {
    counts[row.country] = (counts[row.country] || 0) + 1;
    next();
  }

  function end (done) {
    counter.setCounts(counts);
    done();
  }
};

Can someone help me understand the difference between using the anonymous function saved into a variable versus just naming a function?