The difference between "alert (a)" and "alert" (a): var a = 1; '' in javascript?

Question

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

The console log shows: "Unused ReferenceError: a not defined";

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

in the middle of viewing, the log shows: "undefined"

How does this code work in js and what makes this difference

Answer 1

in this code

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

var a ; rises up and becomes

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

therefore, at the time of a it was warned that it was undefined

In this code

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

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