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JavaScript JavaScript Numbers The Math Object Random Number Challenge

boi
boi
14,242 Points
Posted by boi
boi
14,242 Points

JavaScript is weird.

Alright. Since I'm coming from a Python background to JavaScript, I guess it's natural to feel a bit bamboozled.

So, basically, the working of the code below is as follows: It takes a user input (a number) and returns a random number from 1 to the user number. I've included an if clause to hunt for errors. Now here is the part where it got really weird for me.

When I ran the code, and passed a 0 to the prompt, the if clause did not execute and it printed out The random number from 1 to 0, is 1. Ok, so I said to myself "This is because the input is taken as a string which is '0'", and I totally understand that and changed the statement userInput === 0 to userInput == 0 BUT before that, I passed an empty value and if clause did not catch the error. The else clause ran the printed out the message The random number from 1 to NaN, is NaN.

I thought the purpose of the isNaN() method was to give me True in this case but it seems to act strange when passing an empty or null value. Upon research, what I found was that the isNaN() method tries to convert the value passed to it into a number, and when an empty or null value is passed to it, it changes it to 0, and hence the statement isNaN("") = False. This is just absolutely not well designed (IMO, please don't attack me).

let userInput = prompt("Enter a number please: ");

if ( isNaN(userInput) || userInput === 0 ) {

  alert("This input is not valid please try again");

}

else {

  userInput = parseFloat(userInput);

  const randomNumber = Math.floor(Math.random() * userInput) +1;

  document.write(`The random number from 1 to ${userInput}, is ${randomNumber}.`);
  console.log(isNaN("1"))
}

I mean, is JavaScript filled with these types of mechanics? It just scares me sometimes to think these mechanics are everywhere in JavaScript. Anyways, I love learning languages and love learning JavaScript, and will continue to learn JavaScript. I just had a small concern (or misunderstanding). I would highly appreciate any criticism, education, or tips from you. Thanks

Damien Lavizzo
Damien Lavizzo
4,265 Points
Damien Lavizzo
Damien Lavizzo
4,265 Points

I had the same idea as you to check for validity, and this was my code. I've tested everything I can think of and it seems to function properly: 

// Collect input from the user
let userNumber = prompt("Choose a number between 1 and 100:")

// Convert the input to a number
let userNumberInt = Math.trunc(parseInt(userNumber));

// 1. Check for validity, if valid then -> 2
if (isNaN(userNumberInt)) {
    alert("Sorry, but you didn't input a number. Please try again.") 
} else if (userNumberInt < 1 || userNumberInt > 100) {
    alert("Sorry, but you didn't choose a number between 1 and 100. Please try again.")
} else {
    // 2. Create a message displaying the random number 
    const randomNumber = Math.floor(Math.random() * userNumberInt + 1)
    console.log(`${randomNumber} is a number between 1 and your chosen number ${userNumberInt}`)
}

1 Answer

fredyzegdi
seal-mask
.a{fill-rule:evenodd;}techdegree
fredyzegdi
Full Stack JavaScript Techdegree Student 12,899 Points
fredyzegdi
.a{fill-rule:evenodd;}techdegree
fredyzegdi
Full Stack JavaScript Techdegree Student 12,899 Points

Hello: JS treats the exceptions in a very different way than Python does. If something doesn't work in JS, the console of the browser shows you Warnings but the rest of the code still running. In Python if something doesn't work and you if didn't catch the exception, the script crashes. Here, I wrote an alternative solution for your problem:

let userInput = parseInt(prompt("Please type a number [greater than 1]:   "));

if(userInput > 1) {
    const randomNumber = Math.floor(Math.random() * userInput) +1;
    document.querySelector('body').innerHTML = `<h1>The random number from 1 to ${userInput} is ${randomNumber}</h1>`;
} else {
    document.querySelector('body').innerHTML = "<h1>That is not a valid number!</h1>";
}

Have a nice day!