HTTP POST request with fetch() method not allowed

Post with fetch() does not work on my localhost on both Apple and Windows.
This line of code `const response = await fetch("/api", options);` throws a 405 error saying that the method is not allowed. So, no data is sent to the server. Any ideas?
Here is the larger snippet `let lat, lon;
const button = document.getElementById("submit");
button.addEventListener("click", async (event) => {
const data = { lat, lon };
const options = {
method: "POST",
headers: {
"Content-Type": "application/json",
},
body: JSON.stringify(data),
};
const response = await fetch("/api", options);
const json = await response.json();
console.log(json);
});
`

Start at the fetch('/api', options) call and trace the /api request handler or route. Reproduce the 405 locally, then verify that the endpoint accepts the shown POST request and no longer rejects it with a 405 response.

索引モデルが issue の本文から書いたものです。