How to prevent reloading page fs.writeFileSync()

Question

I can't prevent reloading page when i uncomment string fs.writeFile function. When i haven't this function in my code, browser keeps it's state and doesn't reload the page. How can i put my file in local directory and work with it without reloading?

I tried different ways of sending request to client and server, tried to write file in directory synchronously and nothing helps. I have such code of my form in html:

<form id="metadata-form">
   <input type="file" id="file-input" required/>
   <button id="process-file-button" type="button">Check file</button>
</form>

There is my client code:

document.getElementById("process-file-button").addEventListener("click", async (event) => {
  event.preventDefault();

  const formData = new FormData();
  const fileInput = document.getElementById("file-input");

  if (!fileInput.files.length) {
    alert("Выберите файл для загрузки!");
    return;
  }

  formData.append("file", fileInput.files[0]);

  try {
    const response = await fetch("http://localhost:3000/process-file", {
      method: "POST",
      body: formData,
    });

    if (!response.ok) {
      throw new Error("Error sending to server");
    }

    const result = await response.json();
    console.log(result)
  } catch (error) {
    console.error("Error:", error);
  }
});

And part of server:

app.post("/process-file", async (req, res) => {
  if (!req.files || !req.files.file) {
    return res.status(400).json({
      error: "Файл не загружен"
    });
  }

  const uploadedFile = req.files.file;
  const tempFilePath = path.join(__dirname, "uploads", uploadedFile.name);

  fs.writeFile(tempFilePath, uploadedFile.data, (err) => {
    if (err) {
      console.error("Ошибка при сохранении файла:", err);
      return res.status(500).json({
        error: "Ошибка сохранения файла"
      });
    }
    console.log("Файл успешно сохранен:", tempFilePath);
    res.json("Success")
  });
});

How can i put my file in local directory and work with it without reloading?