How can I use document.querySelector in TypeScript/ Angular?

I’m trying to write a login/register form where the user can toggle between the two forms (only one is shown at a time). I use a simple button to perceive the changes.
I’ve written it in a simple html file an it works. Now I’ve copied to my Angular project and nothing happens.

First I tried this one (it’s in the html file):

<script>
  function showlogin() {
    document.querySelector('#login').classList.remove("d-none");
    document.querySelector('#register').classList.add("d-none");
  }


  function showregister() {
    document.querySelector('#login').classList.add("d-none");
    document.querySelector('#register').classList.remove("d-none");
  }


</script>

But then I got an error that says that "Property ‘showregister’ does not exist".
So I remove the code from the html file and paste it in the typescript file.
Now I get an new error "Object is possibly ‘null’."

Why is the Object null? What can I do to have access to the button and the function?

I’m very thankful for any help

>Solution :

Use the Angular way!

Code Behind:

loginVisible: boolean = true;
...
function showLogin() {
    loginVisible = true;
}

function showRegister() {
    loginVisible = false;
}

And the html part:

<div *ngIf="loginVisible">Login Data</div>

<div *ngIf="!loginVisible">Register Data</div>

Here is a simple Stackblitz example.

The ngIf will only render the component if the condition is true.