How to use Bun on AWS Lambda — A Step-By-Step Guide

Publish Bun Lambda layer

This is an optional step for those who want to publish Bun Lambda layer by themselves using our public Bun Lambda layer.

Make sure you have AWS CLI setup.

To setup AWS CLI, follow this 5 minute setup guide.

git clone [email protected]:oven-sh/bun.git
cd bun/packages/bun-lambda

bun install
bun run build-layer

bun run publish-layer

Create a Lambda Function with Amazon Linux 2

1. Go to AWS Lambda console and click on Create function or simply go to this link to create a new function.
2. Select Author from scratch.
   1. Put your function name in Function name input. Eg. my-bun-func
3. Select Provide your own bootstrap on Amazon Linux 2 from Custom runtime
4. Select arm64 as Architecture


Now click on Create function button. You’ll get to see the green banner with Successfully created the function.


Write code for Bun Lambda function

Go to the Code source and create a new file with index.ts

Paste the code below where we are just returning Hello from LearnAWS with status 200 and logging the function arn header:

export default {
  async fetch(request: Request): Promise<Response> {
    console.log(request.headers.get("x-amzn-function-arn"));
    // ...
    return new Response("Hello from LearnAWS!", {
      status: 200,
      headers: {
        "Content-Type": "text/plain",
      },
    });
  },
};

This is how your Bun Lambda code should look like:


Click on Deploy deploy the changes.

Update function handler

Scroll down to Runtime settings and click on Edit.


• Update the Handler option with index.fetch
  • index is the name of our file.
  • fetch is the name of function we want Bun to call.
• Click on Save.

You can name your handler whatever you want, just make sure you have the same function name in the file eg. index.main


Attach Bun Lambda layer to the Function

Scroll down to Layers and click on Add a layer.


Enable Lambda Function URL

To test things out we can enable function URL for our Lambda.

• Go to Configuration tab, navigate to Function URL and click on Create function URL.


• Select NONE in Auth type which will make our AWS Lambda function publicly accessible by anyone via AWS Lambda function URL.
• Hit Save to deploy the changes.


Now you’ll see your AWS Lambda Function URL in Lambda Function overview page:


Test your Bun Lambda function using function URL

Click on the blue Lambda Function URL link.

You’ll get to see your Lambda say Hello.


Go to CloudWatch Log groups and navigate to your function name to get the logs from the Function. As you can see here we are getting the Lambda ARN in the INFO of the log.


Parsing Query Params: An Example

I’ve used the URL constructor to get pathname and get the first item after /.

You can learn more on MDN about the Request variable and it’s properties.

As of now search param didn’t work with Bun as it was not appearing in the request.url.

const path = new URL(request.url).pathname;
const name = path.split("/")[1]; // get the first param
return new Response(`You're awesome ${name}!`, {
  //...
});

Don’t forget to Deploy the function.

Test your Lambda Function with SAM Bun

It might be too much work for you to build APIs directly on AWS Lambda.

But with Bun, you can directly run and test your Lambda functions locally with bun run.

import fn from "./index.ts"; // import your lambda function

const server = Bun.serve({
  port: 3000,
  fetch: fn.fetch,
});

console.log(`Listening on http://localhost:${server.port}`);

bun run server.ts

Listening on http://localhost:3000

So is Bun better than NodeJS on AWS Lambda?

Hold your spear before you make any decision.

Bun is not officially supported by AWS Lambda yet (and it won’t be anytime soon), since we had to use the Lambda layer with Amazon Linux.