how to take the value of an input and plus it by any number in javascript and html?

Question

this my code and I can't seem to find out why it returns NaN. I tried just given it a certain number in the addnum function but that also didn't work.

 <script>
     var output = document.getElementById("output");
     var input = document.getElementById("input").value;
     var int_input = parseInt(input);

    function addnum(N) {
        int_input = int_input + N;
        output.innerHTML = int_input;
    }

    document.getElementById("addinput").addEventListener("keyup", function (event) {
        if (event.keyCode == 13) {
            addnum(6);
        }
    })
 </script>

Answer 1

When your script loads, input is empty. Later, in the function the script is trying to add 6 to NaN, which will give you NaN.

This should work:

<script>
     var output = document.getElementById("output");
     var input = document.getElementById("input");


    function addnum(N) {
       var int_input = parseInt(input.value); 
       int_input = int_input + N;
       output.innerHTML = int_input;
    }
    // Your eventListener function can be placed here
</script>

Answer 2

This works in console

var input = "12";
var int_input = parseInt(input);

function addnum(N) {
  int_input = int_input + N;
  console.log(int_input);
}
addnum(6);

do a console.log() with the input, to see what value it has.