11_user_input.md

Getting User Input with NodeJS

Terms

• modules
• require

Lesson

NodeJS handles most user input in an asynchronous manner. To illustrate, let's examine the question function that is part of the readline module. A Module is just some code that exists in another file. In order to access a specific module, we use the require keyword:

var readline = require('readline')

The readline module is built-in to node - if you can run node you should be able to access it. We can give the variable to the left of require any name, not just readline. However, The string to the right of require ('readline') must remain exactly the same. This string would be used to find a file whose name is readline. There is one more step to be made - understanding it is not necessary, so you may simply copy and paste this into your program:

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

Now we can use the variable rl to get input from the user. The method we need is question: This method takes as input a string and a (callback) function. It will log the string to the console, get input from the user, and call the provided function with the user's input as an argument. Let's see it in action:

function log(string){
  console.log('your input was: ' + string)
}

rl.question('enter something', log)

Or using an anonymous function:

rl.question('enter something', function(string) {
  console.log('your input was: ' + string)
})

Note that the input will always be of the string type, even if the user types a number. Of course, we may convert the string to a number if needed. In that case, however, we should make sure that we did not get NaN. First, we try converting the input to a number using parseInt. If parseInt fails it will return NaN to be assigned to number. So we will check if number is NaN using isNaN.

rl.question('enter a number', function(input) {
  // attempting to convert the input to a number
  var number = parseInt(input)
  // checking if input was not a number
  if (isNaN(number)) {
    console.log('that was not a number')
  ) else {
    console.log('your input was: ' + number)
  }
})

Repeatedly asking for input

The above code works to get the user's input a single time. What about if we wanted to repeatedly ask for input? For example: every time we receive input from the user with rl.question, we process that input, and if it is not a number we ask the user for input again. First, let us put the above code into a function:

function getUserInput(){
  rl.question('enter a number: ', function(input) {
    // attempting to convert the input to a number
    var number = parseInt(input)
    // checking if input was not a number
    if (isNaN(number)) {
      console.log('that was not a number')
    ) else {
      console.log('your input was: ' + number)
    }
  })
}

getUserInput()

So far the only thing we changed was that we need to call getUserInput. Before we go further, let us rewrite the callback as a non-anonymous function:

function handleInput(input) {
  // attempting to convert the input to a number
  var number = parseInt(input)
  // checking if input was not a number
  if (isNaN(number)) {
    console.log('that was not a number')
  } else {
    console.log('your input was: ' + number)
  }
}

function getUserInput() {
  rl.question('enter a number: ', handleInput)
}

getUserInput()

Now all we have to do is call getUserInput from handleInput if we get NaN:

function handleInput(input) {
  // attempting to convert the input to a number
  var number = parseInt(input)
  // checking if input was not a number
  if (isNaN(number)) {
    console.log('that was not a number')
    getUserInput()
  } else {
    console.log('your input was: ' + number)
  }
}

function getUserInput() {
  rl.question('enter a number: ', handleInput)
}

getUserInput()

Questions

1. Write a program that asks the user for input. If the input is 'dog', log 'cat'.
2. Write a program that asks the user for input. The input must be of length 10 or greater. Ask the user for input until they provide the correct length.
3. Write a program that asks the user for number input repeatedly. Every time an input is receieved, it will be added to a sum, and the sum will be displayed to the user. If the user inputs a nu,begit