node.js: Cannot read property 'defaultEncoding' of undefined

Question

I wanted to write a changeable write() function.

var write = function(s) {
    process.stdout.write(s);
}
write("Hello world!");

I thought you could just write it shorter:

var write = process.stdout.write;
write("Hello world!");

But here I will receive this error:

TypeError: Cannot read property 'defaultEncoding' of undefined
    at Writable.write (_stream_writable.js:172:21)
    at Socket.write (net.js:613:40)
    at repl:1:2
    at REPLServer.self.eval (repl.js:110:21)
    at Interface.<anonymous> (repl.js:239:12)
    at Interface.EventEmitter.emit (events.js:95:17)
    at Interface._onLine (readline.js:202:10)
    at Interface._line (readline.js:531:8)
    at Interface._ttyWrite (readline.js:760:14)
    at ReadStream.onkeypress (readline.js:99:10)

Why is that?

Answer 1

It all has to do with how javascript handles this. Inside the function process.stdout.write there is a call to defaultEncoding() using this variable.

In javascript, this is not assigned a value until an object invokes the function where this is defined and it is relative to the calling object.

So in your first example, this points to process.stdout object and it has the method defaultEncoding. In your second example, this is undefined since the function is being called from the global namespace. When process.stdout.write tries to call defaultEncoding, it will throw the error you mentioned.

You can manually define the this value for a function using Function.prototype.call() method. Example:

var write = process.stdout.write;
write.call(process.stdout, "Hello world!");

The first argument of call is the object to be used as this inside the function.

I recommend reading this article, it explains a lot about this in javascript.