我的代码是这样的，为什么执行出来只输出一个第一次push进去的内容啊

var stream = require('stream');
var util = require('util');

function ReadStream () {
    stream.Readable.call(this);
}
util.inherits(ReadStream, stream.Readable);


ReadStream.prototype._read = function () {
    this.push(' I');
    this.push(' love');
    this.push(' imooc,');
    this.push(null);
};

function WriteStream () {
    stream.Writable.call(this);
    this._cached = new Buffer.from('');
}

util.inherits(WriteStream, stream.Writable);

WriteStream.prototype._write = function (chunk) {
    console.log(chunk.toString());
};

function TransformStream () {
    stream.Transform.call(this);
}

util.inherits(TransformStream, stream.Transform);

TransformStream.prototype._transform = function(chunk){
    this.push(chunk);
};

TransformStream.prototype._flush = function(cb){
    this.push('12345');
    cb && cb();
};

var rs = new ReadStream();
var ws = new WriteStream();
var ts = new TransformStream();

rs.pipe(ts).pipe(ws);