了解节点流的操作顺序

Question

I am learning about Node Streams using Stream-Adventure ( https://github.com/substack/stream-adventure ). I'm having a hard time understanding the HTML-Stream Lesson.

This is the challenge :

Your program will get some html written to stdin. Convert all the inner html to upper-case for elements with a class name of "loud".

You can use trumpet and through to solve this adventure.

I found the solution here :

var trumpet = require('trumpet');
var through = require('through');
var to_upper = function (buffer) {
  this.queue(buffer.toString().toUpperCase())
};
var tr = trumpet();

// Here I expect the output to stdout to have printed as is.
process.stdin.pipe(tr).pipe(process.stdout);

// In the below lines, there is no reference to the above stream 
// which should have already started to send to stdout.
// How are the lines below modifying the above stream?
var stream = tr.select('.loud').createStream()
stream.pipe(through(to_upper)).pipe(stream)

I cannot seem to understand the program flow above.

How do the last two lines modify the stream output, even though there is no reference/callbacks to use the above stream?

Answer 1

The key is to think asynchronously. Think of process.stdin.pipe(tr).pipe(process.stdout) as 'take the input put it through tr, which we'll elaborate on later, and print to stdout'.

Then below we fill out the logic and say exactly what tr will be doing to the input. There is only one tr object in this program. And the stream object is defined in terms of it. The code using stream at the bottom is manipulating the same tr being used to filter stdin and stdout . Remember the hint Now stream outputs all the inner html content at '.beep' and the data you write to stream will appear as the new inner html content. That's exactly what the last line is doing.