Is this okay? The peek will just act as a benign through stream letting us see the first item. Would this add much overhead to the time do you think?

peek.js

var peek = module.exports = function (fn) {
  var peeked = false;
  return through(function write(data) {
    this.emit('data', data);
    if (peeked) {
      return;
    }
    peeked = true;
    var stream = this;
    var cont = function (end) {
      if (end === true) {
        stream.end();
      } else {
        stream.resume();
      }
    };

    this.pause();
    if (fn.length > 1) {
      fn(data, cont);
    } else {
      cont(fn(data));
    }
  });
};

usage.js

var array = new Pump(10);

array.pipe(peek(function (data) {
  console.log('first item is', data); // 1
}));

array.pipe(slice(5)).on('data', collect(result)).on('end', function () {
  t.equal(5, result.length);
  t.end();
}));

Oh, the usage handles both sync and async usage. If the peek handler only receives data, then it's sync. But it can be async with:

peek(array, function (data, done) {
  // do something async with data
  done();  // continue stream, or `done(true)` to end the stream
});