Install the @defer/client

yarn add @defer/client

Create a defer/ folder

Defer expects to find all the background functions in the defer/ folder.

In an Express/Koa/hapi app, we recommend creating an src/defer/ folder as follows:

 - src/
   - api.ts
+  - defer/
 - .env.example
 - package.json
 - tsconfig.json


  • The defer/ folder can be located anywhere in your project
  • The defer/ folder can contain nested folders

Write a background function

A background function is a function equivalent to a background job.

All background functions must live in the defer/ folder.

Choose how to write your first background function:

Our background function is ready to be used; let’s see how to call it from your application.

Call your background function

Whenever you choose to use the helloWorld() function or create your own, the following example showcasing how to call helloWorld() from a Express, Koa or Hapi API route demonstrates how easily you can trigger a background function execution.

  • Express

  • Koa

  • hapi

import express from "express";

// we import our `helloWorld()` background function
import helloWorld from "../../defer/helloWorld";

const app = express();
const port = 3000;

app.get("/", (req, res) => {
  // calling a background function triggers an execution on Defer
  await helloWorld("Charly");

  res.send("Hello World!");

app.listen(port, () => {
  console.log(`Example app listening on port ${port}`);

How does Defer work?

If you are wondering how calling a background function triggers an execution on Defer, please refer to this guide: How Defer works.

Once your new API route is defined, please commit and push it to your branch.

Now that your API contains a background function and an API route that calls it, let’s ensure that your Defer token is properly configured.

Setup your Node.js / Bun version

Node.js is the default runtime, and version 18.x will be used if you don’t specify your own.

Node.js versions 18.x and 20.x are supported, and Bun versions 1.x are supported.

To use Defer with Bun, you need the @defer/client version to be >=1.11.0

To change your runtime and its version, edit the engines field of your package.json:

  "engines": {
    "node": "18.x"

For more details about the supported runtimes, like how dependencies are installed, check our Deploy section


Create your Defer application

First, sign in to the Defer Console, then, go to the “Setup application page.”

Once your application is correctly configured, click the “Create” button.

You should land on the application’s page with a build running; you can go to the Defer Console to check its status.

Your first successful build on the Defer Console

Your first successful build on the Defer Console

You can now get a Defer Token by navigating to the Settings page:

Copy your Defer token

Copy your Defer token for the next step

Configure your API deployment

Your API deployment should expose your Defer Token as an environment variable in order to get your background function deferred when called.


Now, with your API deployment containing your Defer Token and your project’s builds ready on Defer, open your browser and test your /helloWorld API route.

In the helloWorld() background function scenario, visit the / route.

You should see that your API answered quickly, and by going to your Defer Console, you should see that your background function has 1 execution running - or finished:

Your first function on the Defer Console

By clicking on see executions, you will access the executions list:

Your first successful function un on the Defer Console

Next steps