Why is this returning 'Undefined?'

Question

Why is this returning 'Undefined?'

I'm learning some of nodes core modules and i've wrote a little command line tool to test out the readline module but on my console.log() outputs, i'm also recieving undefined under it :/

Here's my code..

var rl = require('readline');

var prompts = rl.createInterface(process.stdin, process.stdout);

prompts.question("What is your favourite Star Wars movie? ", function (movie) {

    var message = '';

    if (movie = 1) {
        message = console.log("Really!!?!?? Episode" + movie + " ??!?!!?!?!, Jar Jar Binks was a total dick!");
    } else if (movie > 3) {
        message = console.log("They were great movies!");
    } else {
        message = console.log("Get out...");
    }

  console.log(message);

  prompts.close();
});

And here's what im seeing in my console..

What is your favourite Star Wars movie? 1
Really!!?!?? Episode1 ??!?!!?!?!, Jar Jar Binks was a total dick!
undefined

Why am I getting back undefined?

javascript
osx
node.js
readline

Answer 1

Why am I getting back undefined?

Because console.log doesn't have a return value, so you're assigning undefined to message.

Since you're outputting message later, just remove the console.log calls from the lines where you're setting the message. E.g., change

message = console.log("Really!!?!?? Episode" + movie + " ??!?!!?!?!, Jar Jar Binks was a total dick!");

to

message = "Really!!?!?? Episode" + movie + " ??!?!!?!?!, Jar Jar Binks was a total dick!";

Side note: Your line

if (movie = 1) {

assigns the number 1 to movie and then tests the result (1) to see if it's truthy. So no matter what you type in, it will always take that branch. You probably meant:

if (movie == 1) {

...although I would recommend not relying on implicit type coercion of user-supplied input, and so I'd put this near the top of that callback:

movie = parseInt(movie, 10);

Answer 2

console.log doesn't return a value, so the result is undefined.

Note: comparison is done with ==, eg: movie == 1.