How to Fix Generator Error in Deno

When your Deno app throws a Generator Error, it can be frustrating. Let's look at why this happens and how to resolve it.

Root Cause

Generator errors in Deno happen when using function* or async function* incorrectly — missing the asterisk, forgetting yield, or trying to consume generator objects where arrays are expected.

Step-by-Step Fix

The key is to use async function* with for await...of for paginated async iteration:

async function* fetchPages(baseUrl: string) {
  let page = 1;
  while (true) {
    const res = await fetch(`${baseUrl}?page=${page}`);
    const data = await res.json();
    if (data.items.length === 0) return;
    yield data.items;
    page++;
  }
}

for await (const items of fetchPages("/api/data")) {
  console.log(`Got ${items.length} items`);
}

Common Pitfall

A systematic approach works best here: isolate the failing component, verify its inputs, check the Deno docs for breaking changes, and test the fix in an environment that mirrors production. As a follow-up, set up automated tests that would catch this regression. Even a simple smoke test can prevent this from reappearing after a dependency update.

Validate the Solution

Verify by triggering the same action that caused the original error. In Deno, you can also enable verbose logging temporarily to confirm the fix is applied correctly. Once verified, remove or reduce the logging level to keep your logs clean in production.

Stay Ahead of Errors

To prevent this from recurring unnoticed, set up [Bugsly](https://bugsly.dev) for your Deno project — it monitors errors and gives you actionable alerts.