---
title: Scheduler
description: Use the scheduler.wait() API to delay execution in Workers.
image: https://edgetunnel-b2h.pages.dev/dev-products-preview.png
---

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# Scheduler

## Background

The `scheduler` global provides task scheduling APIs based on the [WICG Scheduling APIs proposal ↗](https://github.com/WICG/scheduling-apis). Workers currently implement the `scheduler.wait()` method.

`scheduler.wait()` returns a Promise that resolves after a given number of milliseconds. It is an `await`\-able alternative to `setTimeout()` that does not require a callback.

Like other [timers in Workers](https://edgetunnel-b2h.pages.dev/workers/runtime-apis/web-standards/#timers), `scheduler.wait()` does not advance during CPU execution when deployed to Cloudflare. This is a [security measure to mitigate against Spectre attacks](https://edgetunnel-b2h.pages.dev/workers/reference/security-model/#step-1-disallow-timers-and-multi-threading). In local development, timers advance regardless of whether I/O occurs.

## Syntax

**JavaScript**

```js
await scheduler.wait(delay);
await scheduler.wait(delay, options);
```

## Parameters

* `delay` number

  * The number of milliseconds to wait before the returned Promise resolves.
* `options` object optional

  * Optional configuration for the wait operation.
  * `signal` AbortSignal optional

    * An [AbortSignal](https://edgetunnel-b2h.pages.dev/workers/runtime-apis/web-standards/#abortcontroller-and-abortsignal) that cancels the wait. When the signal is aborted, the returned Promise rejects with an `AbortError`.

## Return value

A `Promise<void>` that resolves after `delay` milliseconds. If an `AbortSignal` is provided and aborted before the delay elapses, the Promise rejects with an `AbortError`.

## Examples

### Basic delay

Use `scheduler.wait()` to pause execution for a specified duration.

* [  JavaScript ](#tab-panel-12947)
* [  TypeScript ](#tab-panel-12948)

**JavaScript**

```js
export default {
  async fetch(request) {
    // Wait for 1 second
    await scheduler.wait(1000);
    return new Response("Delayed response");
  },
};
```

**TypeScript**

```ts
export default {
  async fetch(request): Promise<Response> {
    // Wait for 1 second
    await scheduler.wait(1000);
    return new Response("Delayed response");
  },
} satisfies ExportedHandler;
```

### Retry with exponential backoff

Use `scheduler.wait()` to implement a delay between retry attempts. This example uses exponential backoff with jitter.

* [  JavaScript ](#tab-panel-12951)
* [  TypeScript ](#tab-panel-12952)

**JavaScript**

```js
async function fetchWithRetry(url, maxAttempts = 3) {
  const baseBackoffMs = 100;
  const maxBackoffMs = 10000;


  for (let attempt = 0; attempt < maxAttempts; attempt++) {
    try {
      return await fetch(url);
    } catch (err) {
      if (attempt + 1 >= maxAttempts) {
        throw err;
      }
      const backoffMs = Math.min(
        maxBackoffMs,
        baseBackoffMs * Math.random() * Math.pow(2, attempt),
      );
      await scheduler.wait(backoffMs);
    }
  }
  throw new Error("unreachable");
}


export default {
  async fetch(request) {
    const response = await fetchWithRetry("https://example.com/api");
    return new Response(response.body, response);
  },
};
```

**TypeScript**

```ts
async function fetchWithRetry(url: string, maxAttempts = 3): Promise<Response> {
  const baseBackoffMs = 100;
  const maxBackoffMs = 10000;


  for (let attempt = 0; attempt < maxAttempts; attempt++) {
    try {
      return await fetch(url);
    } catch (err) {
      if (attempt + 1 >= maxAttempts) {
        throw err;
      }
      const backoffMs = Math.min(
        maxBackoffMs,
        baseBackoffMs * Math.random() * Math.pow(2, attempt),
      );
      await scheduler.wait(backoffMs);
    }
  }
  throw new Error("unreachable");
}


export default {
  async fetch(request): Promise<Response> {
    const response = await fetchWithRetry("https://example.com/api");
    return new Response(response.body, response);
  },
} satisfies ExportedHandler;
```

### Cancel with AbortSignal

Use an [AbortController](https://edgetunnel-b2h.pages.dev/workers/runtime-apis/web-standards/#abortcontroller-and-abortsignal) to cancel a pending wait.

* [  JavaScript ](#tab-panel-12949)
* [  TypeScript ](#tab-panel-12950)

**JavaScript**

```js
export default {
  async fetch(request) {
    const controller = new AbortController();


    // Cancel the wait after 500ms
    setTimeout(() => controller.abort(), 500);


    try {
      await scheduler.wait(5000, { signal: controller.signal });
      return new Response("Wait completed");
    } catch (err) {
      if (err instanceof DOMException && err.name === "AbortError") {
        return new Response("Wait was cancelled", { status: 408 });
      }
      throw err;
    }
  },
};
```

**TypeScript**

```ts
export default {
  async fetch(request): Promise<Response> {
    const controller = new AbortController();


    // Cancel the wait after 500ms
    setTimeout(() => controller.abort(), 500);


    try {
      await scheduler.wait(5000, { signal: controller.signal });
      return new Response("Wait completed");
    } catch (err) {
      if (err instanceof DOMException && err.name === "AbortError") {
        return new Response("Wait was cancelled", { status: 408 });
      }
      throw err;
    }
  },
} satisfies ExportedHandler;
```

## Related resources

* [Timers](https://edgetunnel-b2h.pages.dev/workers/runtime-apis/web-standards/#timers) — `setTimeout()` and `setInterval()` APIs
* [Performance and timers](https://edgetunnel-b2h.pages.dev/workers/runtime-apis/performance/) — `performance.now()` and timer security behavior
* [AbortController and AbortSignal](https://edgetunnel-b2h.pages.dev/workers/runtime-apis/web-standards/#abortcontroller-and-abortsignal) — cancel asynchronous operations
* [WICG Scheduling APIs proposal ↗](https://github.com/WICG/scheduling-apis) — the specification this API is based on

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