user input with node.js

Question

I have the following Node.js code that behaves strangely:

#!/usr/bin/env node

"use strict";

var readline = require('readline');

var rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

function input(prompt) {
    rl.question(prompt, function (x) {
        rl.close();

        console.log("debug: " + x);
        return x;
    });
}

function main() {
    var n = input("Number: ");
//    console.log("value: " + n);    // problematic line
}

main();

I want to mimic Python's raw_input, i.e. reading a line from the user. After showing the prompt, the program should be blocked until the user presses Enter.

If the "problematic line" is in comments, it works, the program is waiting for input. However, if this line is not in comments, then the program doesn't wait for input and n becomes undefined. Why? How to write a function that returns the user input?

Luis Sieira · Accepted Answer · 2021-02-25 23:20:03Z

That's because you are expecting the execution of input wait until return is called, which is not how this will work. The problematic line is indeed the previous one. First, input does not return anything, the return statement is the return of the question callback function, but then, you seem to misunderstand the execution flow, as we all have at some point (you'll get it pretty quick after some dead-ends like this one)

Your script is loaded
You declare and define rl, input and main
Main executes
You define n as the result of input And here is where things start getting asynchronously funny
since question is asynchronous, its execution start but does not block the process
input returns undefined (while you're still waiting for the input)
you print that undefined
You write something on the input
question() finishes its execution and calls the callback (the function you gave as second parameter)
rl is closed
the callback function returns the line, and it is swallowed by the void (this is not technical terminology, just a metaphor)

You may want to do it like this:

#!/usr/bin/env node

"use strict";

var readline = require('readline');

var rl = readline.createInterface({
    input: process.stdin,
    output: process.stdout
});

function input(prompt, callback) {
    rl.question(prompt, function (x) {
        rl.close();
        callback(x);
    });
}

function main() {
    var n = input("Number: ", console.log);
}

main();

If you're new to javascript and node, you may find very useful to use learnyounode and the node code school path or even, if you have the time, the money and the opportunity, read Node.js, MongoDB, and AngularJS Web Development, by Brad Dayley.

Thanks, but how to do anything with n afterwards? For instance, if I add this line to the end of main(): console.log(Number(n)+1);, then it's problematic again. Or if I want to read two numbers and add n1 and n2 and print the result in main. Yes, I'm new to it and thanks for the useful links.
Anything you ant to do afterwards has to be done in a callback. Instead of passing console.log, pass a custom function with the next operations.
If you pass, for instance, main itself, you'll get a pretty parrot prompt repeating you input and asking again. If you pass a function making a system call with your input, and calling main afterwards you'll get a shell, and so on...
If you add that line to the main, you're trying to make a number from undefined, which is the result you get from input (it doesn't return anything), I thought that would give you NaN, but whatever...

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