randomFromStream

loop.js

function randomFromStream() {
  var length = 0;
  var result = null;
  var potentialResult;
  
  while(potentialResult = gfs()) {
    length += 1;
    if (Math.floor(Math.random() * length) === 0) {
      result = potentialResult;
    }
  }
  
  return result;
}

var result = randomFromStream();

randomFromStream.js

function randomFromStream(currentResult, length) {
  var potentialResult = gfs();

  // no more numbers in stream
  if (potentialResult === null) {
    return currentResult;
  }
  
  length += 1;
  
  if (Math.floor(Math.random() * length) === 0) {
    currentResult = potentialResult;
  }
  
  return randomFromStream(currentResult, length);
}

var result = randomFromStream(null, 0);