Javascript: Average of 2 user input values

I apologize for the basic question but I have been trying to make this work for a long time and I just can't seem to get this code to return a value.

I am embarrassed to admit how long I have been attempting to make it work, and how many StackOverflow questions that were related that I have looked at, however, none were as simple as my code, and when I attempted to make something closer to how mine looked, it just wouldn't alert anything.

The idea is the following:

User inputs 2 numbers,
clicks the button,
and is alerted the average of the numbers they input.

My alert has been NaN no matter how many iterations I have made. I could really use some advice. Thanks in advance!

<html>
	<head>
		<title> Javascript </title>
	<body>
		<p> Enter two number for an average below </p>
		<p> Number 1<input type="text" id="userInput1"></input></p>
    	<p> Number 2<input type="text" id="userInput2"></input></p>
    	<button id="averageButton"> Calculate</button>
   
    <script type="text/javascript">
    	function average(a, b) {
    		return ((a + b) /2);
    	}

    	document.getElementById("averageButton").onclick = function (){
   	 		var a = document.getElementById("userInput1").value;
			var b = document.getElementById("userInput2").value;    		
    		alert(average());
		}
	</script>
    </body>

</html>

Solution

You failed to pass the numbers a,b to your function - a simple mistake.

But since the inputs are seen as strings you also need to convert them to numbers a*1

See commented code

<html>
	<head>
		<title> Javascript </title>
	<body>
		<p> Enter two number for an average below </p>
		<p> Number 1<input type="text" id="userInput1"></input></p>
    	<p> Number 2<input type="text" id="userInput2"></input></p>
    	<button id="averageButton"> Calculate</button>
   
    <script type="text/javascript">
    	function average(a, b) {
    		// force the input as numbers *1
        return ((a*1 + b*1) /2);
    	}

    	document.getElementById("averageButton").onclick = function (){
   	 		var a = document.getElementById("userInput1").value;
			var b = document.getElementById("userInput2").value;    		
    		// pass the numbers to the average function!
        alert(average(a,b));
		}
	</script>
    </body>

</html>