index.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <h1>SCUFFED CALCUATOR</h1>
    <input id="nrsel" type="text" placeholder="Was yo number">
    <h1 id="output"></h1>


    <script>
        document.getElementById("nrsel").addEventListener("keydown", calc, false)
        let nrs = []
        function calc (e){
        let keyCode = e.keyCode;
            if(keyCode==13) {
                let inp = document.getElementById("nrsel");
                nrs.push(inp.value)
                inp.value = '';
                inp.placeholder = 'Was yo second number'
            } else {
                return
            }
            if(nrs.length == 2){
                let one = parseInt(nrs[0]);
                let two = parseInt(nrs[1]);
                let outpt = one + two; 
                document.getElementById('output').innerText = outpt;
            } else if(nrs.length >= 2){
                alert('It appears that adding 2 numbers is my current limit!');
            }
        }
    </script>
</body>
</html>