How to update javascript variable value in html without a page refresh

Question

I am collecting a text area value(number in decimal ) using its id to a javascript variable, adding another variable value (number in decimal), and displaying the result in same html page

text area code:

<div class="input-resp"><span><input  class="textbox" id="num" name="count" type="text" size="5" maxlength="3"  value="" /></span></div>

passing value to var and adding it to another var

var a = document.getElementById('num').value;
var b = 1000;
var c = parseFloat(a) + parseFloat(b); 

passing var c to div id test

document.getElementById("test").innerHTML = c;

html for test id

<div id="test"></div>

But here, whenever I am updating the text area with new number, the final output is not updating instantly.I have refresh the page manually to get new value. Is there anyway I update the value without a page refresh ?

Answer 1

If it's going to update whenever you type into the input, you need an event handler:

document.getElementById('num').onkeyup = function() {
    var a = 1000 + parseFloat(this.value);
    document.getElementById("test").innerHTML = a || 0;
}

FIDDLE