How to pass variable to function and set new value to it?

Question

I find it very hard to explain or search for this problem, so I'm asking Stack Overflow.

I'm asking user for user-input in the terminal several times and want to make a function out of it. A function that takes a question and a variable, and the input should be added to the variable.

This is my code:

var name = 'hello',
    age = '11'

var readline = require('readline');
// var rl = readline.createInterface(process.stdin, process.stdout);
var rl = readline.createInterface({
  input: process.stdin,
  output: process.stdout
});
var getInput = function(inputVariable, question, cb) {
  rl.question(question, function(answer) {
    inputVariable = answer;
    rl.close();
    cb();
  });
}

var askForName = function() {
  console.log(1,age, name);
  getInput(name, "What's your name? ", askForAge);
}
var askForAge = function() {
  console.log(2,age, name);
  getInput(age, "How old are you? ", printIt);
}
var printIt = function() {
  console.log("Hello, " + name);
  console.log(".. and " + age + " old.");
}

askForName();

Thanks!

Answer 1

In node.js applications, this is usually solved using either a callback chain or promises.

Using callbacks, you could write your code like this:

var getInput = function(question, cb) {
  rl.question(question, function(answer) {
    // Don't set the variable, but return value to callback
    rl.close();
    cb(answer);
  });
}

// ask* take a callback which they call with the value
var askForName = function(cb) {
  getInput("What's your name? ", cb);
}
var askForAge = function(cb) {
  getInput("How old are you? ", cb);
}
// printIt takes the values as parameters
var printIt = function(name, age) {
  console.log("Hello, " + name);
  console.log(".. and " + age + " old.");
}

// Now ask for name and age and store them as local variables
// inside callbacks
askForName(function(name) {
  askForAge(function(age) {
    printIt(name, age);
  });
});

I added comments to explain the changes. Basically, the values are only being passed between callbacks, never exposed to another scope.

Answer 2

When you pass a variable to a function, changing the reference to it will not affect the original variable. You can modify the variable's properties. However, that shouldn't be used as the main method of passing around variables.

A common way of passing a response from an asynchronous process is to use a callback function. You've almost got it:

var getInput = function( question, cb ) {
  rl.question(question, function(answer) {
    rl.close();
    cb(answer);
  });
}

var askForName = function( cb ) {
  getInput("What's your name? ", cb);
}
var askForAge = function( cb ) {
  getInput("How old are you? ", cb);
}
var printIt = function( name, age ) {
  console.log("Hello, " + name);
  console.log(".. and " + age + " old.");
}

askForName(function(name) {
  askForAge(function(age) {
    printIt( name, age );
  });
});