Difference between "alert(a)'' and ''alert(a);var a =1;'' in javascript?

Question

<script type="text/javascript">
    alert(a);
</script>

Console log shows : "Uncaught ReferenceError: a is not defined";

<script type="text/javascript">
    alert(a);
    var a = 1;
</script>

at the middle of the browse, Log shows: "undefined"

How does this code run in js and what causes this difference

Answer 1

in this code

<script type="text/javascript">
    alert(a);
    var a = 1;
</script>

var a ; is hoisted to the top and it becomes

<script type="text/javascript">
    var a;
    alert(a);
    a = 1;
</script>

so by the time a was alerted, it was undefined

In this code

<script type="text/javascript">
    alert(a);
</script>

a was not defined at all, so it gave an error "Uncaught ReferenceError: a is not defined"