Function to add a variable when clicking on JavaScript

You have to put the output inside the function and better use textContent .

var a = 0;
function myFunction() {
  a = a + 1;
  document.getElementById("demo").textContent = a;
}

<button onclick="myFunction()">Click me</button>
<p id="demo"></p>

Variables that are created within a function do not work for the rest of the code outside of the function.

<button onclick="myFunction()">Click me</button>
<p id="demo"></p>
<script>
var a = 0;
function myFunction() {
    a = a + 1;
    document.getElementById("demo").innerHTML = a;
}
</script>

This is a perfect example to implement a lock pattern. The code is elegantly encapsulated without global initializations.

 var contador = (function() {
  var c = 0;
  return function() {
     return ++c;
  }
})();
var eventoClick = function() {
   document.getElementById("demo").innerHTML = contador();
}

<button onclick="eventoClick()">Click me</button>
<p id="demo"></p>

You can try something like this:

<button onclick="myFunction()">Click me</button>
<p id="demo">0</p>
<script>
function myFunction() {
  var a = parseInt(document.getElementById("demo"));
  a = a + 1;
  document.getElementById("demo").innerHTML = a;
    }

</script>

The problem is that you have not started the value to 0, and that's why when you try to do the sum, you do not receive any results