fs.readstream 如何读取？

想读取一个密码字典，内容大概如下

//dic.txt
a
passw
passwo
password3
密码长度不同

//API上的例子
var fs = require('fs');
var path = "dic.txt";
var dic = fs.createReadStream(path);
var chunk
;

dic.on('readable', function() {
  var chunk;
  while (null !== (chunk = dic.read())) {
    console.log('got %s ', chunk);
  }
});

这样会把全部内容读出来，但是我想一行一行读的话

var fs = require('fs');
var path = "dic.txt";
var dic = fs.createReadStream(path);
var chunk
console.log(chunk=dic.read());

这要返回的却是null,这是为什么？