Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>In this example, myFunction is a function constructor:</p>

<p id="demo"></p>

<script>
function myFunction(arg1, arg2) {
this.firstName = arg1;
this.lastName = arg2;
}

var x = new myFunction("John","Doe")
document.getElementById("demo").innerHTML = x.firstName;
</script>

</body>
</html>

<!DOCTYPE html>
<html>
<body>

<p>In this example, myFunction is a function constructor:</p>

<p id="demo"></p>

<script>
function myFunction(arg1, arg2) {
    this.firstName = arg1;
    this.lastName  = arg2;
}

var x = new myFunction("John","Doe")
document.getElementById("demo").innerHTML = x.firstName; 
</script>

</body>
</html>