How do I handle 429 errors from eleventy-fetch?

I'm making a request with eleventy-fetch like this:

let response = await EleventyFetch(url, {
    duration: "1h",
    type: "json"
  });

However, sometimes it fails with an error:

Bad response for [url] (429): Too Many Requests (via Error)

I know that I'm requesting too often, so I want to build exponential backoff. However, my initial attempt didn't work:

let response;
let attempts = 0;
const maxAttempts = 5;

while (attempts < maxAttempts) {
  try {
    response = await EleventyFetch(url, {
      duration: "1h",
      type: "json"
    });
    break; // If request is successful, exit the loop
  } catch (err) {
    if (err.response && err.response.status === 429) {
      attempts++;
      const waitTime = Math.pow(2, attempts) * 1000; // Exponential backoff
      console.warn(`Rate limit exceeded. Retrying in ${waitTime / 1000} seconds...`);
      await new Promise(resolve => setTimeout(resolve, waitTime));
    } else {
      throw err; // If it's not a 429 error, rethrow the error
    }
  }
}

The error is still being thrown and the exponential backoff never happens. What have I done wrong?

You want to be looking for err.cause, not err.response.

The source code of eleventy-fetch's RemoteAssetCache.js shows where the error is generated:

if (!response.ok) {
    throw new Error(
        `Bad response for ${this.displayUrl} (${response.status}): ${response.statusText}`,
        { cause: response },
    );
}

Here, the response is attached to the error as a property cause.

To make your exponential backoff work, you just need to switch err.response to err.cause:

let response;
let attempts = 0;
const maxAttempts = 5;

while (attempts < maxAttempts) {
  try {
    response = await EleventyFetch(url, {
      duration: "1h",
      type: "json"
    });
    break; // If request is successful, exit the loop
  } catch (err) {
    if (err.cause && err.cause.status === 429) {
      attempts++;
      const waitTime = Math.pow(2, attempts) * 1000; // Exponential backoff
      console.warn(`Rate limit exceeded. Retrying in ${waitTime / 1000} seconds...`);
      await new Promise(resolve => setTimeout(resolve, waitTime));
    } else {
      throw err; // If it's not a 429 error, rethrow the error
    }
  }
}