Are node.js variables none blocking - node.js

Question

I have some node code and I'm trying to set this up but it seems like it's processing one item before another is complete and I'm wondering if I'm right and if so if there is some kind of work around
var input = function(text){
process.stdout.write(text);
process.stdin.setEncoding('utf-8')
process.stdin.once("data", function(input_data){
return input_data;
}).resume();
}
var test = function(var1, var2){
var var1 = (var1 == null) ? input("Var One: ") : var1;
var var2 = (var2 == null) ? input("Var Two: ").split(" ") : var1;
console.log(var1, var2);
}
Though, when I execute test(null,null) I expected it to ask me for Var one then define var1 as the data then after that I thought it would prompt me with Var Two and then split that by spaces into a list however that did not work and it just errored saying "Cannot call method of undefined "
I came from python where this was possible and it wouldn't do any other executions till the previously defined one was completed so I'm wondering if node has something similar to this and note I am not using a server or website for this I'm just testing code on my computer.

Answer 1

I'm fairly new to node myself, but here's my understanding of how it will execute:
test(null, null);
// enter test() function
// see that var1 == null, run input("Var One: ")
// write "Var One: " to stdout
// set stdin encoding to utf-8
// set an event listener for 'data' on stdin, and provide a callback for that event
... let's pause there for a moment, because this is the important bit.
When you set a listener and a callback, you've entered the land of asynchronicity. Your code will continue to march on without waiting and do the next things that you've told it to do. Which, in this case, is just sending back an undefined return from input(), and then moving on to handle var2 similarly, where you try to call undefined.split(), and the whole process pukes.
If, on the other hand, you just remove .split(), you'll probably get an output like this:
Var One: Var Two : [waiting for input]
At this point, when you enter the first value, it'll take it and return it from your callback to... nowhere... then it'll wait for your next value and do the same.
This is where you have to start to break your mind away form your python background and procedural, synchronous habits.
I'll risk beating a dead horse, and comment up your code directly:
var input = function(text){
process.stdout.write(text);
process.stdin.setEncoding('utf-8')
// .once() is an asynchronous call
process.stdin.once("data", function(input_data){
// the return value of this anonymous function isn't used anywhere
return input_data;
}).resume();
// there is no return value of the "input" function
}
Functionally, what you're looking for is something like the following (though this is ugly, something like async.waterfall makes this sort of structure much more palatable, and there may be even better ways to do it that I haven't learned yet):
function test(var1, var2) {
if (!var1 || !var2) {
process.stdin.resume();
process.stdin.setEncoding('utf-8');
if (!var1) {
process.stdout.write('Var One: ');
process.stdin.once('data', function(input_data) {
// var1 & var2 get pulled in from the parent context
var1 = input_data;
if (!var2) {
process.stdout.write('Var Two: ');
process.stdin.once('data', function(input_data) {
var2 = input_data;
console.log(var1, var2);
});
}
else {
console.log(var1, var2);
}
});
}
else if (!var2) {
process.stdout.write('Var Two: ');
process.stdin.once('data', function(input_data) {
var2 = input_data;
console.log(var1, var2);
});
}
else {
// there is no else, either var1 or var2 is missing
// per the first conditional
}
}
else {
console.log(var1, var2);
}
}

Answer 2

Here is how you can do it :
function input (text, val, cb) {
if (val) return cb(null, val)
process.stdout.write(text)
process.stdin.setEncoding('utf-8')
process.stdin.once('data', function(data){
process.stdin.pause()
cb(null, data)
}).resume()
}
function test (var1, var2) {
input('Var One: ', var1, function (err, var1) {
input('Var Two: ', var2, function (err, var2) {
console.log(var1)
console.log(var2)
})
})
}
test(null, null)
Basically, since stdin is async, so is input function. You need to use callback-based function style. This works, though you strongly recommend not using stdin this way. Try readline core module or some special userland modules from npm.
You can see that writing callback based code can be a little messy (aka callback hell). Here is a fancy way to address this issue using co module (you need node 0.11.x for this and use --harmony-generators flag):
var co = require('co')
function input (text) {
return function (cb) {
process.stdout.write(text)
process.stdin.setEncoding('utf-8')
process.stdin.once('data', function(data){
process.stdin.pause()
cb(null, data)
}).resume()
}
}
function * test (var1, var2) {
var1 = var1 || (yield input('Var One: '))
var2 = var2 || (yield input('Var Two: '))
console.log(var1)
console.log(var2)
}
co(test)(null, null)

Answer 3

I would use this built-in Node.js module: http://nodejs.org/api/readline.html
var readline = require('readline');
var rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});
rl.question("What do you think of node.js? ", function(answer) {
// TODO: Log the answer in a database
console.log("Thank you for your valuable feedback:", answer);
rl.question("What do you think of JavaScript?", function(answer2) {
console.log("Answer2 is ", answer2);
});
rl.close();
});
There is a relatively new language derived from CoffeeScript that makes async code more readable. It basically looks like sync code, and you don't have to nest indents for callbacks all the time. Its called ToffeeScript https://github.com/jiangmiao/toffee-script Coming from Python you may appreciate it.
readline = require 'readline'
rl = readline.createInterface { input: process.stdin, output: process.stdout }
answer = rl.question! "What do you think of node.js?"
console.log "Thank you for your valuable feedback:", answer
answer2 = rl.question! "What do you think of ToffeeScript?"
console.log "Thanks again."
rl.close()