blocking field input from a non-=integer value - JS

Question

i am quite new to JavaScript. I am trying to make it so that if anything other than a number is entered it either blocks it or it takes their entry and tells them to enter a number (because their entry is not acceptable) - I don't know which is easier.

The code I have so far:

const calculateEntered = document.querySelector(".js- 
    calculateButton ")

    const calculateTotal = () => {

      let enteredValue = document.querySelector(".js-enterValue").value


      if (enteredValue < 40) {
        console.log(`<40>`)
        enteredValue = parseFloat(enteredValue) + 10
        document.querySelector(".js-totalText").innerHTML = `Your 
    total is: £${enteredValue}`
      } else if (enteredValue => 40 && enteredValue != NaN) {


        document.querySelector(".js-totalText").innerHTML = `Your total 
    is: £${enteredValue}`
      }
    }

<button class="js-calculateButton">Calculate total</button>
<div>
  <div>
    <p class="js - enterTotalHere">Enter order total here: £</p>
  </div>
  <div><input placeholder="cost of order" class="js- 
    enterValue"></input>
  </div>
</div>
<p class="js-totalText"> Your total is: </p>

Any characters added lead to the "else if" statement. I tried looking this up but the solutions seemed very difficult - I didn't want to use any 3rd party method.

If you use an input element, you can set it to type="number" and the browser will display a message if the value that's entered is not a number. — devlin carnate
– devlin carnate, Commented Jun 16, 2023 at 16:50
Also, why are there spaces here? class="js - enterTotalHere" — devlin carnate
– devlin carnate, Commented Jun 16, 2023 at 16:53
Specically, to your case type="number" can be used with min="0" to set a minimum value, zero here as it is a cost, it won't be negative — Kaddath
– Kaddath, Commented Jun 16, 2023 at 16:54

chrwahl · Accepted Answer · 2023-06-25 18:37:41Z

You can listen for the input event on the input element and then calculate the result.

document.forms.form01.cost.addEventListener('input', e => {
  let form = e.target.form;
  let costNum = Number(e.target.value);
  form.result.value = (costNum < 40) ? costNum + 10 : costNum;
});

<form name="form01">
  <button class="js-calculateButton">Calculate total</button>
  <p class="js - enterTotalHere">Enter order total here: £</p>
  <div>
    <input name="cost" type="number" placeholder="cost of order" class="js-enterValue">
  </div>
  <p class="js-totalText"> Your total is: £<output name="result"></output></p>
</form>

If you don't like the arrows on the input element (type="number") an alternative could be to use type="text" inputmode="numeric" pattern="\d*":

document.forms.form01.cost.addEventListener('input', e => {
  let form = e.target.form;
  let costNum = Number(e.target.value);
  form.result.value = (costNum < 40) ? costNum + 10 : costNum;
});

<form name="form01">
  <button class="js-calculateButton">Calculate total</button>
  <p class="js - enterTotalHere">Enter order total here: £</p>
  <div>
    <input name="cost" type="text" inputmode="numeric" pattern="\d*" placeholder="cost of order" class="js-enterValue">
  </div>
  <p class="js-totalText"> Your total is: £<output name="result"></output></p>
</form>

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