Question 169611 in Javascript Question posted in
A very good W3school tutorial can be found here.

Can Javascript require a user to input a number, not a blank input?

I am making a javascript program in which the program asks the user whether they want to calculate distance, speed or time. The program works but I want to be able to make it where the user cannot enter a blank input as well as not allowing the program to continue if their input is not a number(where numbers are needed)

//This program asks the user to input whether they would like to calculate distance, time or speed.
//Depending their answer they get asked to input the values of the other two options
//The program then calculates either the speed, distance or time

function calculate(){
try{
    let question = prompt("Would you like to calculate Distance(km), Time(h) or Speed(kph)");
    let answer = question.toLowerCase();
        if(answer === "distance" && answer != ""){
            let time = Number(prompt("Please enter your time in hours:"));
            let speed = Number(prompt("Please enter your speed:"));
            let calculation = speed * time;
            console.log(`The Distance is: ${calculation} km`);
        }else if(answer === "time" && answer != ""){
            let distance = Number(prompt("Please enter your distance:"));
            speed = Number(prompt("Please enter your speed:"));
            let calculation2 = distance / speed;
            console.log(`Your Time is: ${calculation2} hours`);
        }else if(answer === "speed" && answer != ""){
            distance = Number(prompt("Please enter your distance:"));
            time = Number(prompt("Please enter your time in hours:"));
            calculation3 = distance / time;
            console.log(`Your speed is: ${calculation3} kph`)
        }else{
          calculate();
        }
}catch(e){
calculate();
}
}

let output = calculate();
Tags:

3

Answers


  1. 0

    You can try to separate the functionality and write a generic function which recursively calls itself until a valid input is given.

    One of the possible implementation:

    function getTypePrompt() {
    const question = prompt("Would you like to calculate Distance(km), Time(h) or Speed(kph)");
    if ((question == null) || (question === "")) {
        return getTypePrompt();
    }

    switch (question) {
        case "distance":
        case "time":
        case "speed":
            return question;
    }

    return getTypePrompt();
}

function getNumberPrompt(message) {
    const response = prompt(message);
    if ((response == null) || (response === "")) {
        return getNumberPrompt(message);
    }

    const data = parseInt(response);

    if (!isNaN(data)) {
        return data;
    }

    return getNumberPrompt(message);
}



try {
    let question = getTypePrompt();
    let answer = question.toLowerCase();
    if (answer === "distance") {
        let time = getNumberPrompt("Please enter your time in hours:");
        let speed = getNumberPrompt("Please enter your speed:");
        let calculation = speed * time;
        document.write(`The Distance is: ${calculation} km`);
    } else if (answer === "time") {
        let distance = getNumberPrompt("Please enter your distance:");
        speed = getNumberPrompt("Please enter your speed:");
        let calculation2 = distance / speed;
        document.write(`Your Time is: ${calculation2} hours`);
    } else if (answer === "speed") {
        distance = getNumberPrompt("Please enter your distance:");
        time = getNumberPrompt("Please enter your time in hours:");
        calculation3 = distance / time;
        document.write(`Your speed is: ${calculation3} kph`)
    } else {
        calculate();
    }
} catch (e) {
    document.write("Something went wrong!");
}
    Login or Signup to reply.
  2. 0 
    let 
  time,
  speed,
  calculation;

time = Number(prompt("Please enter your time in hours:"));

if(!time) {
  time = Number(prompt("Please enter your time in hours:"));
}

speed = Number(prompt("Please enter your speed:"));

if(!speed) {
  speed = Number(prompt("Please enter your speed:"));
}

    You can replace the Number by adding + just before prompting.
    time = +(prompt("Please enter your time in hours:"));

    If you want a faster, thought less clearer sometimes, you can use ternary operator

    which is basically like a short if-else

    time = !time ? +(prompt("Please enter your time in hours:")); : time;
    Login or Signup to reply.
  3. 0

    To prevent the user from entering a blank input, you can add a condition to check if the input is an empty string. To ensure that the user enters a number, you can use the isNaN() function to check if the input is not a number. Here’s an updated version of your code with these changes:

    function calculate() {
  let question = prompt("Would you like to calculate Distance(km), Time(h) or Speed(kph)");
  let answer = question.toLowerCase().trim();

  if (answer === "") {
    alert("Please enter a valid input.");
    calculate();
    return;
  }

  if (answer === "distance") {
    let time = Number(prompt("Please enter your time in hours:"));
    let speed = Number(prompt("Please enter your speed:"));

    if (isNaN(time) || isNaN(speed)) {
      alert("Please enter valid numbers.");
      calculate();
      return;
    }

    let calculation = speed * time;
    console.log(`The Distance is: ${calculation} km`);
  } else if (answer === "time") {
    let distance = Number(prompt("Please enter your distance:"));
    let speed = Number(prompt("Please enter your speed:"));

    if (isNaN(distance) || isNaN(speed)) {
      alert("Please enter valid numbers.");
      calculate();
      return;
    }

    let calculation2 = distance / speed;
    console.log(`Your Time is: ${calculation2} hours`);
  } else if (answer === "speed") {
    let distance = Number(prompt("Please enter your distance:"));
    let time = Number(prompt("Please enter your time in hours:"));

    if (isNaN(distance) || isNaN(time)) {
      alert("Please enter valid numbers.");
      calculate();
      return;
    }

    let calculation3 = distance / time;
    console.log(`Your speed is: ${calculation3} kph`);
  } else {
    alert("Please enter a valid input.");
    calculate();
    return;
  }
}

calculate();
    Login or Signup to reply.
Please signup or login to give your own answer.

Back To Top
Search