How to flash message in Laravel after api post request with JavaScript?

Goal

Flash a message after a post request is saved successfully to database.

Problem

The message doesn't appear when a post request comes from a react frontend. If the react frontend receives a success message it reload the page, as you can see in the handleSubmit function. When a request is send from a form generated in the "standard laravel way", everythin works, like expected.

So here is the code:

ReportsController:

use Illuminate\Support\Facades\Session;

...

public function store(Request $request)
  {
    $report = new Report([

      'name' => $request->get('name'),

      'body' => $request->get('body'),

      'visible' => $request->get('visible'),

      'show_in_slider' => $request->get('show_in_slider')

    ]);

    if ($report->save()) {
      Session::flash('success', 'Feedback gespeichert!');
      return response()->json(['success' => true, 'url' => '/admin/reports'], 200);
    }
  }

The line in the api.php

Route::resource("reports", "ReportsController");

The blade for the messages:

@if(count($errors) > 0)
@foreach($errors->all() as $error)
<div class="warning-modal">
  {{$error}}
</div>
@endforeach
@endif

@if(session("success"))
<div class="alert-modal">
  {{session("success")}}
</div>
@endif

@if(session("error"))
<div class="warning-modal">
  {{session("error")}}
</div>
@endif

The corresponding functions snippets in the React frontend.

handleSubmit:

async function handleSubmit(event) {
        event.preventDefault();
        const response = await saveData("/api/reports", report);
        if (response.success) {
            window.location = response.url;
        } else {
            alert("Ein Fehler ist aufgetreten.");
        }
    }

Sending the post request in the saveData function:

export default async function saveData(api, data) {
    const token = document
        .querySelector('meta[name="csrf-token"]')
        .getAttribute("content");
    const url = window.location.origin + api;
    const response = await fetch(url, {
        method: "POST",
        headers: {
            "Content-Type": "application/json",
            "X-CSRF-TOKEN": token
        },
        body: JSON.stringify(data)
    });

    const result = await response.json();
    return result;
}

So where is the fault?

Thanks for your time in advance :)

Edit

I solved it without reloading the page and let JavaScript handle the alert. Thanks to elyas :)

export default async function PostData() {
    const report = await CollectData();

    const response = await SaveData("/api/reports", report);

    const alert = document.createElement("div");
    alert.innerHTML = response.message;
    if (response.success) {
        alert.className = "alert-modal";
        ToggleNewReport();
    } else {
        alert.className = "warning-modal";
    }

    document.getElementById("content").appendChild(alert);
}

Depending on the response i add the corresponding alert.