如何将此参数传递给侦听器？

Question

use strict;

var fs = require('fs);

var readable = fs.createReadStream(__dirname + '/greet.txt', {encoding: 'utf8', highwaterMark: 32 * 1024 });

readable.on('data', function(chunk) {
  console.log(chunk.length)
});

If I understand correctly, in node, the readable stream inherits from EventEmitter, which allows it to make use of the listener / .on method.

But, how does the callback : , function(chunk){ console.log(chunk.length)} know the value of the chunk parameter?

Answer 1

EventEmitter instance has a method called "emit", so somewhere inside the fs readstream, it is called like this

emitter.emit('data', chunk);

https://nodejs.org/api/events.html#events_passing_arguments_and_this_to_listeners

Answer 2

Answering to your comment, no, the listener doesn't detect or call anything. It's the other way around, fs.createReadStream() is an event emitter. readable.on('data', function() { ... }) means "readable, please call this function when you emit 'data' event". readable emits data events when it has data to "give", calls all registered listeners (only function(chunk) in your example) and provides the parameters like detailed in the doc.