gistfile1.js

    var fs = require('fs');
    var stream = fs.createReadStream('./test/fixtures/current');
    var currentData = '';

    stream.on('error', function (err) {
      throw err;
    });
    stream.on('data', function (data) {
      currentData += data;
    }); 

    // I want currentData to contain the contents of my file but
    // console.log(currentData) shows nothing is in there.
    // what am I missing?
    console.log(currentData);

You'll have to put the console.log inside of the event listener:

stream.on('data', function (data) {
    currentData += data;
    console.log(currentData);
});

The current call to console.log executes on a separate path, probably before a data event is generated.

If you are just try to collect all the output of that file/pipe, you could use one of the blocking fs functions, such as fs.readFileSync