Input with readline-sync
console.log works fine for printing static (unchanging) messages to the
screen. If we wanted to print a phrase greeting a specific user, then
console.log("Hello, Dave."); would be OK as long as Dave is the actual
What if we want to greet someone else? We could change the string inside the
() to be
'Hello, Sarah' or
'Hello, Elastigirl' or any other name we
need. However, this is inefficient. Also, what if we do not know the name of
the user beforehand? We need to make our code more general and able to respond
to different conditions.
It would be great if we could ask the user to enter a name, store that string
in a variable, and then print a personalized greeting using
Variables to the rescue!
To personalize the greeting, we have to get input from the user. This involves displaying a prompt on the screen (e.g. “Please enter a number:”), and then waiting for the user to respond. Whatever information the user enters gets stored for later use.
As we saw earlier, each programming language has its own way of accomplishing the same task. For example, the Python syntax is
input("Please enter your name: "), while C# uses
readline-sync. Unfortunately, using this module requires more than a single
line of code.
Gathering input from the user requires the following setup:
const input = require('readline-sync'); let info = input.question("Question text... ");
There is a lot going on here behind the scenes, but for now you should follow this bit of wisdom:
I turn the key, and it goes.
Most of us do not need to know all the details about how cars, phones, or
microwave ovens work. We just know enough to interact with them in our day to
day lives. Similarly, we do not need to understand how
at this time. We just need to know enough to collect information from a user.
As you move through the course, you WILL learn about all of the pieces that fit together to make this process work. For now, here is a brief overview.
Load the Module
In line 1,
const input = require('readline-sync') pulls in all the functions
that allow us to get data from the user and assigns them to the variable
const ensures that
input cannot be changed.
How to Prompt the User
To display a prompt and wait for a response, we use the following syntax:
let info = input.question("Question text... ");.
Question texton the screen.
- Wait for the user to respond.
- Store the data in the variable
For our greeting program, we would code
let name = input.question("Enter your name: ");. The user enters a name and
presses the Return or Enter key. When this happens, any text entered is collected by the input function and stored in
const input = require('readline-sync'); let name = input.question("Enter your name: ");
Note that after entering a name, the program does not actually DO anything with the information. If we want to print the data as part of a message, we need to put
name inside a
After line 3, add
console.log("Hello, " + name + "!");, then run the code several times, trying different responses to the input prompt.
By storing the user’s name inside
name, we gain the ability to hold onto the data and use it when and where we see fit.
Try adding another
+ name term inside the
console.log statement and see what happens. Next, add code to prompt the user for a second name. Store the response in
otherName, then print both names using
Update your code to request a user’s first and last name, then print an output that looks like:
First name: Elite Last name: Coder Last, First: Coder, Elite
Critical Input Detail
There is one very important quirk about the input function that we need to remember. Given
console.log(7 + 2);, the output would be
Now explore the following code, which prompts the user for two numbers and then prints their sum:
const input = require('readline-sync'); let num1 = input.question("Enter a number: "); let num2 = input.question("Enter another number: "); console.log(num1 + num2);
Run the program, enter your choice of numbers, and examine the output. Do you see what you expected?
If we enter
2, we expect an output of
9. We do NOT expect
72, but that is the result printed. What gives?!?!?
The quirk with the
input function is that it treats all entries as
strings, so numbers get concatenated rather than added. Just like
“Hello, " + “World” outputs as
Hello, World, “7” + “2” outputs as
If we want our program to perform math on the entered numbers, we must use type conversion to change the string values into numbers.
num2from strings to numbers. Run the program and examine the result.
- Instead of using two steps to assign
num1and then convert it, combine the steps in line 3. Place
input.question("Enter a number: ")inside the
Numberfunction. Run the program and examine the result.
- Repeat step 2 for
- What happens if a user enters
Hiinstead of a number?
Check Your Understanding
What is printed when the following program runs?
const input = require('readline-sync'); let info = input.question("Please enter your age: "); //The user enters 25. console.log(typeof info);