How to Check if the Inputted Value is a Number or Letter with JavaScript?

To check if the inputted value is a number or letter with JavaScript, we can get the inputted value of the input element with the value property.

Then we can use the isNaN function to check if the inputted value is a number or not.

For instance, if we have the following form:

<form name="myForm">
  Age: <input type="text" name="age">
  <input type="submit" value="Submit">
</form>

Then we can get the form and check that the age field has a number entered in it by writing:

const {
  myForm
} = document.forms

myForm.addEventListener('submit', (e) => {
  e.preventDefault()
  const x = myForm.age.value;
  if (isNaN(x)) {
    alert("Must input numbers");
  }
})

We get the form by its name attribute value from document.forms .

Then we call addEventListener with 'submit' to add a submit event listener to the form.

The 2nd argument is the submit event handler callback that calls e.preventDefault to prevent default server-side submission.

Then it gets the value of from the myForm.age field, which is the input with name attribute set to age .

Next, we check if the x string is not a number with isNaN .

If it returns true , then we know it’s not a number string, and we display an alert to the user with alert.

Otherwise, it’s a number.

Conclusion

To check if the inputted value is a number or letter with JavaScript, we can get the inputted value of the input element with the value property.

Then we can use the isNaN function to check if the inputted value is a number or not.