User input in Node.js

Question

I am writing a program which will create an array of numbers, and double the content of each array, and storing the result as key/value pair. Earlier, I had hardcoded the array, so everything was fine.

Now, I have changed the logic a bit, I want to take the input from users and then, store the value in an array.

My problem is that I am not able to figure out, how to do this using node.js. I have installed the prompt module using npm install prompt, and also, have gone through the documentation, but nothing is working.

I know that I am making a small mistake here.

Here's my code:

//Javascript program to read the content of array of numbers //Double each element //Storing the value in an object as key/value pair. //var Num=[2,10,30,50,100]; //Array initialization var Num = new Array(); var i; var obj = {}; //Object initialization function my_arr(N) { return N;} //Reads the contents of array function doubling(N_doubled) //Doubles the content of array { doubled_number = my_arr(N_doubled); return doubled_number * 2; } //outside function call var prompt = require('prompt'); prompt.start(); while(i!== "QUIT") { i = require('prompt'); Num.push(i); } console.log(Num); for(var i=0; i< Num.length; i++) { var original_value = my_arr(Num[i]); //storing the original values of array var doubled_value = doubling(Num[i]); //storing the content multiplied by two obj[original_value] = doubled_value; //object mapping } console.log(obj); //printing the final result as key/value pair

Kindly help me in this, Thanks.

function my_arr(N) { return N;} What is the point of this?? — Dan455
– Dan455, Commented Jul 24, 2013 at 14:53
Written a function to call the element, though I will remove it. — vamosrafa
– vamosrafa, Commented Jul 24, 2013 at 15:07

Rick · Accepted Answer · 2018-06-28 15:55:09Z

46

For those that do not want to import yet another module you can use the standard nodejs process.

function prompt(question, callback) { var stdin = process.stdin, stdout = process.stdout; stdin.resume(); stdout.write(question); stdin.once('data', function (data) { callback(data.toString().trim()); }); }

Use case

prompt('Whats your name?', function (input) { console.log(input); process.exit(); });

edited Jun 28, 2018 at 15:55

answered Aug 28, 2015 at 17:10

Rick

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4 Comments

Clint Over a year ago

Excellent, much more lightweight than bringing in prompt and its dependencies

labyrinth Over a year ago

Is stdin.resume(); necessary? I'm new to this, but reading the docs on readable streams and it says setting the 'data' event handler takes it out automatically (c.f. nodejs.org/api/stream.html#stream_readable_streams)

Ahmed Fasih Over a year ago

@JECompton it might not be necessary but I’d do it, because you may stdin.pause() somewhere else, to e.g. allow Node to exit cleanly after a prompt, and you’ll need to resume it for this to work. See my little example: gist.github.com/fasiha/8ee923fd1a27d596af1d287785465231

Can Over a year ago

It's great to have something without a dependency. Much appreciated.

Govind Rai · Accepted Answer · 2020-11-21 16:12:23Z

Modern Node.js Example w/ ES6 Promises & no third-party libraries.

Rick has provided a great starting point, but here is a more complete example of how one prompt question after question and be able to reference those answers later. Since reading/writing is asynchronous, promises/callbacks are the only way to code such a flow in JavaScript.

const { stdin, stdout } = process; function prompt(question) { return new Promise((resolve, reject) => { stdin.resume(); stdout.write(question); stdin.on('data', data => resolve(data.toString().trim())); stdin.on('error', err => reject(err)); }); } async function main() { try { const name = await prompt("What's your name? ") const age = await prompt("What's your age? "); const email = await prompt("What's your email address? "); const user = { name, age, email }; console.log(user); stdin.pause(); } catch(error) { console.log("There's an error!"); console.log(error); } process.exit(); } main();

Then again, if you're building a massive command line application or want to quickly get up and running, definitely look into libraries such as inquirer.js and readlineSync which are powerful, tested options.

Make sure you call stdin.pause() or node will keep that stream alive...

josh3736 · Accepted Answer · 2013-07-24 14:56:26Z

Prompt is asynchronous, so you have to use it asynchronously.

var prompt = require('prompt') , arr = []; function getAnother() { prompt.get('number', function(err, result) { if (err) done(); else { arr.push(parseInt(result.number, 10)); getAnother(); } }) } function done() { console.log(arr); } prompt.start(); getAnother();

This will push numbers to arr until you press Ctrl+C, at which point done will be called.

@vamosrafa the radix. (base we count in, for example 2 would have been binary).
So after this, I will have to do the manipulation of doubling the numbers inside the done function right? Thanks.

Shay · Accepted Answer · 2018-12-31 09:07:06Z

Node.js has implemented a simple readline module that does it asynchronously:

https://nodejs.org/api/readline.html

const readline = require('readline'); const rl = readline.createInterface({ input: process.stdin, output: process.stdout }); rl.question('What do you think of Node.js? ', (answer) => { // TODO: Log the answer in a database console.log(`Thank you for your valuable feedback: ${answer}`); rl.close(); });

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