Stream empty for concatenated passthrough streams about concat-stream HOT 2 CLOSED

var Stream = require("stream")
var CombinedStream = require('combined-stream')
var util = require('util')

var Readable = Stream.Readable
util.inherits(StringStream, Readable)

function StringStream(s) {
    Readable.call(this)
    this.s = s
}

StringStream.prototype._read = function() {
    if(this.s !== undefined) {
        this.push(this.s)
        this.s = undefined
    } else {
        this.push(null)
    }
}




var concatStreams = function(a,b) {
    var combinedStream = CombinedStream.create()
    combinedStream.append(a)
    combinedStream.append(b)

    return combinedStream
}

concatStreams(new StringStream("moo"), new StringStream("boo")).pipe(process.stdout)