A Variable in JS is not deleted but here is no error?

Question

I have a variable. var variable; It is variable="Hello World";. Now, without using var and deleting it:

document.write(myVar);
delete myVar;
console.log("This should be an error because myVar has been deleted: ");
console.log(myVar);

<!--This is my module-->
<script>
  myVar = "Hello";
</script>

But with 'use strict';, there is no error with delete, and no error even though it cannot delete a var?

console.log("My Code Is Running");
delete myVar; 
if (!myVar){
document.write("Var Has Been Deleted");
}
console.log("My Code Is Done");

// Why does it not write "Var Has Been Deleted"?
// There is no 'use strict' to stop the delete.

<script>
'use strict';
var myVar = "Hello World";
</script>

What is happening?

Thanks in advance