Recently, I stumbled upon a code snippet on a blog showcasing the usage of the stream Transform class to modify data streams and display the altered output. However, there are certain aspects of this code that leave me puzzled.
var stream = require('stream');
var util = require('util');
// To support node v0.10+ native Transform, if not polyfill is used
var Transform = stream.Transform ||
require('readable-stream').Transform;
I am unsure why there is a need to check if the 'this' variable points to an instance of the Upper constructor. The Upper constructor is utilized to create the upper object below, so what could possibly be the reason for this validation? Additionally, when I attempted to log the options, it resulted in null or undefined - making me question the purpose of that parameter.
function Upper(options) {
// Allowing use without new keyword
if (!(this instanceof Upper)) {
return new Upper(options);
}
I assume the Transform.call method is used here to explicitly define the 'this' variable. But why does the program do so, given that Transform is never actually invoked anyway?
// Initialize Transform
Transform.call(this, options);
}
Upon researching the util package, it seems that it's employed here to enable Upper to inherit prototypal methods from Transform. Am I understanding this correctly?
util.inherits(Upper, Transform);
The function below is particularly perplexing. Although I comprehend that it defines a method on Upper's prototype to transform input data, I'm unable to locate where this function is called within the code!
Upper.prototype._transform = function (chunk, enc, cb) {
var upperChunk = chunk.toString().toUpperCase();
this.push(upperChunk);
cb();
};
// Testing it out based on the original code
var upper = new Upper();
upper.pipe(process.stdout); // Output to stdout
After inspecting the code with a debugger, I noticed that upper.write triggers the mentioned Upper.prototype._transform method. This leaves me wondering why this is happening - as upper is an instance of the Upper constructor, and write doesn't seem connected to the _transform method defined in Upper's prototype.
upper.write('hello world\n'); // Input line 1
upper.write('another line'); // Input line 2
upper.end(); // Finish