Lambda compatibility for Functions

Select your function language:

Prepare project

Before creating a TypeScript function, prepare your project by doing the following:

Add the @netlify/functions module to your project to import typings for TypeScript.

npm install @netlify/functions

Set up tsconfig.json. We recommend that you enable the properties esModuleInterop and verbatimModuleSyntax in your TypeScript configuration file for better compatibility. During the build process, Netlify automatically loads any tsconfig.json configuration files found in your functions directory, the repository root directory, or the base directory, if set.

Create function file

To add a serverless function to your project, create a TypeScript file in your functions directory following the instructions below for naming and coding your function. Netlify will access the functions directory during every build, preparing and deploying each supported code file as a function.

Directory

The default functions directory is YOUR_BASE_DIRECTORY/netlify/functions. You can optionally configure a custom functions directory.

Name your function

You can store your function file directly under the functions directory or in a subdirectory dedicated to the function. The function’s endpoint name is case-sensitive and determined by its filename or the name of its dedicated parent directory.

For example, to create a function with an endpoint name of hello, save the function file in one of the following ways:

netlify/functions/hello.ts
netlify/functions/hello/hello.ts
netlify/functions/hello/index.ts

These formats would deploy a synchronous function that can be called on the following endpoint: /.netlify/functions/hello. The endpoint is relative to the base URL of your site. Here are some example full endpoint URLs: mysitename.netlify.app/.netlify/functions/hello for a site using the default Netlify subdomain or www.yourcustomdomain.com/.netlify/functions/hello for a site using a custom domain.

To create a background function, append the function name with -background. For example, to create a background function with an endpoint name of hello-background, save the function file in one of these ways:

netlify/functions/hello-background.ts
netlify/functions/hello-background/hello-background.ts
netlify/functions/hello-background/index.ts

These formats would deploy a background function that can be called on the following endpoint: /.netlify/functions/hello-background. The endpoint is relative to the base URL of your site. Here are some example full endpoint URLs: mysitename.netlify.app/.netlify/functions/hello-background for a site using the default Netlify subdomain or www.yourcustomdomain.com/.netlify/functions/hello-background for a site using a custom domain.

Code your function

This section will help you learn how to write functions. It covers the syntax for synchronous functions, both buffered and streaming, as well as background functions.

Synchronous function format

To create a synchronous function, use the following general syntax in your TypeScript function file to export a handler method:

import type { Handler, HandlerEvent, HandlerContext } from "@netlify/functions";

const handler: Handler = async (event: HandlerEvent, context: HandlerContext) => {
  // your server-side functionality
};

export { handler };

Import types before declaring the function.

If you import the Handler type as demonstrated in the previous code sample, the event and context arguments and the response are typed accordingly.
Alternatively, you can import the types HandlerEvent, HandlerContext, and HandlerResponse separately and use them to construct a handler function.

Netlify provides the event and context parameters when the function is invoked.

The event object received by the handler is similar to the following:

{
  "path": "Path parameter (original URL encoding)",
  "httpMethod": "Incoming request’s method name",
  "headers": {Incoming request headers},
  "queryStringParameters": {Query string parameters},
  "body": "A JSON string of the request payload",
  "isBase64Encoded": "A boolean flag to indicate if the applicable request payload is Base64-encoded"
}

If you need to parse a multipart/form-data-encoded event.body, we recommend using busboy. To learn more, visit our blog article on processing multipart form data with Netlify Functions.

The context object received by the handler includes information about the context in which the function was called, like certain Identity user information, for example.

Here’s a basic example function, hello.ts:

import type { Handler, HandlerEvent, HandlerContext } from "@netlify/functions";

const handler: Handler = async (event: HandlerEvent, context: HandlerContext) => {
  return {
    statusCode: 200,
    body: JSON.stringify({ message: "Hello World" }),
  };
};

export { handler };

This function deploys to an endpoint at /.netlify/functions/hello relative to the base URL of your site.
A successful invocation returns the 200 status code and the string, “Hello, World”.

Synchronous functions can return a response object that includes the following information:

{
  "isBase64Encoded": true|false,
  "statusCode": httpStatusCode,
  "headers": { "headerName": "headerValue", ... },
  "multiValueHeaders": { "headerName": ["headerValue", "headerValue2", ...], ... },
  "body": "..."
}

Because async function code returns a promise, we recommend returning a response with at least an HTTP status code instead of allowing the function to time out. Using async in your function code doesn’t automatically designate a function as a background function.

Response streaming

This feature is in Beta and is available on all pricing plans.

Netlify Functions can stream data to clients as it becomes available, rather than returning a buffered payload at the end of the computation. This lets developers and frameworks create faster experiences by using streaming and partial hydration to get content and interactions in front of people as quickly as possible.

To stream a function’s response:

wrap your handler export with the stream decorator
return a ReadableStream as the body

Here’s a simplified example that you can use to test that your function response streams to the client:

import { stream } from "@netlify/functions";

export const handler = stream(async () => {
  const encoder = new TextEncoder();
  const formatter = new Intl.DateTimeFormat("en", { timeStyle: "medium" });
  const body = new ReadableStream({
    start(controller) {
      controller.enqueue(encoder.encode("<html><body><ol>"));
      let i = 0;
      const timer = setInterval(() => {
        controller.enqueue(
          encoder.encode(
            `<li>Hello at ${formatter.format(new Date())}</li>\n\n`
          )
        );
        if (i++ >= 5) {
          controller.enqueue(encoder.encode("</ol></body></html>"));
          controller.close();
          clearInterval(timer);
        }
      }, 1000);
    }
  });
  return {
    headers: {
      "content-type": "text/html"
    },
    statusCode: 200,
    body
  };
});

For an extended example that demonstrates a more likely response streaming use case, consider this function:

import { stream } from "@netlify/functions";

export const handler = stream(async event => {
  // Get the request from the request query string, or use a default
  const pie =
    event.queryStringParameters?.pie ??
    "something inspired by a springtime garden";

  // The response body returned from "fetch" is a "ReadableStream",
  // so you can return it directly in your streaming response
  const res = await fetch("https://api.openai.com/v1/chat/completions", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      // Set this environment variable to your own key
      Authorization: `Bearer ${process.env.OPENAI_API_KEY}`
    },
    body: JSON.stringify({
      model: "gpt-3.5-turbo",
      messages: [
        {
          role: "system",
          content:
            "You are a baker. The user will ask you for a pie recipe. You will respond with the recipe. Use markdown to format your response"
        },
        // Use "slice" to limit the length of the input to 500 characters
        { role: "user", content: pie.slice(0, 500) }
      ],
      // Use server-sent events to stream the response
      stream: true
    })
  });

  return {
    headers: {
      // This is the mimetype for server-sent events
      "content-type": "text/event-stream"
    },
    statusCode: 200,
    // Pipe the event stream from OpenAI to the client
    body: res.body
  };
});

When invoked this function sends a prompt to OpenAI and streams the response back to the client like a chatbot.

Keep the following limitations in mind when working with streaming functions:

10 second execution limit. If the limit is reached, the response stops streaming.
20 MB response size limit. Responses larger than 20 MB cannot be streamed.
Background Functions can’t stream responses.
On-demand Builders can’t stream responses.
Scheduled Functions can’t stream responses.

Background function format

This feature is available on Credit-based plans, including Free, Personal, and Pro and on Enterprise plans.

If you are on the legacy Pro plan, this feature is available only until December 15, 2025.
To keep using the feature, consider switching to an updated plan. Learn more.

To create a background function, append the function name with -background. For example, netlify/functions/hello-background.ts or netlify/functions/hello-background/index.ts.

Background function syntax is similar to synchronous function syntax with a couple of key differences:

You will generally pass the function result to a destination other than the originating client.
Background functions don’t support response streaming because they don’t return responses.

Like the synchronous function format, each TypeScript file to be deployed as a background function must import types and export a handler method. As with synchronous function invocation, Netlify provides the event and context parameters when a background function is invoked. Although background functions rely on asynchronous invocation, they don’t require the use of async TypeScript syntax.

Here’s a simplified example background function that you can use to test that your function runs longer than 30 seconds, count-background.ts:

import type { BackgroundHandler, HandlerEvent, HandlerContext } from "@netlify/functions";

export const handler: BackgroundHandler = (event: HandlerEvent, context: HandlerContext) => {
  const sleep = (ms: number) => {
    return new Promise((resolve) => setTimeout(resolve, ms));
  };

  (async () => {
    for (let i = 0; i <= 60; i++) {
      const date = new Date();
      await sleep(1000);
      console.log(date.toLocaleString(), i);
    }
    console.log("Done");
  })();
};

This function deploys to an endpoint at /.netlify/functions/count-background relative to the base URL of your site.
When invoked, the function returns an initial 202 success response.
When successfully executed, the function logs numbers 1 through 60.

For an extended example that demonstrates a more likely background function use case, consider this send-pdf-background.ts function:

import { jsPDF } from "jspdf";
import mailgun from "mailgun-js";
import type { BackgroundHandler, HandlerEvent } from "@netlify/functions";

type Context = {
  content: string;
  destination: string;
};

const mg = mailgun({
  apiKey: process.env.MAILGUN_API_KEY,
  domain: process.env.MAILGUN_DOMAIN,
});

const handler: BackgroundHandler = async (event: HandlerEvent) => {
  if (!event.body) {
    return;
  }

  const { content, destination } = JSON.parse(event.body) as Context;

  console.log(`Sending PDF report to ${destination}`);

  const report = Buffer.from(
    new jsPDF().text(content, 10, 10).output("arraybuffer")
  );

  const info = await mg.messages().send({
    from: process.env.MAILGUN_SENDER,
    to: destination,
    subject: "Your report is ready!",
    text: "Details in attached report PDF",
    attachments: [
      {
        filename: `report-${new Date().toDateString()}.pdf`,
        content: report,
        contentType: "application/pdf",
      },
    ],
  });

  console.log(`PDF report sent: %s`, info.id);
};

export { handler };

This background function deploys to an endpoint at /.netlify/functions/send-pdf-background relative to the base URL of your site.
When invoked, the function returns an initial 202 success response.
When successfully executed, a PDF report is generated and emailed as an attachment.

Environment variables

Netlify Functions have access to environment variables in the runtime environment. If you have the option to set specific scopes for your environment variables, the scope must include Functions to be available to functions during runtime.

You can also leverage build environment variables to configure how Netlify builds your functions. For example, you can use an environment variable to set the Node.js version.

Learn more about how to set and use environment variables with functions.

Manage dependencies

To optimize build performance, specify function dependencies in the top-level package.json file in the site’s base directory. Here’s an example of a recommended layout:

my-base-directory/
├─ package.json
├─ node_modules
└─ netlify/
   └─ functions/
        ├─ hello.ts
        └─ send-pdf-background.ts

With continuous deployment, Netlify automatically zips dependencies with your functions for deployment. To do this, Netlify’s build system installs your dependencies and parses each function file to note its dependencies. For each function, Netlify then pulls the required dependencies from the associated node_modules folder and zips them with the function file for deployment. For more details about how this bundling functionality works, visit the repository for the underlying module: @netlify/zip-it-and-ship-it.

If you’re using a Netlify configuration file to configure custom [functions] settings, note that the node_bundler value for TypeScript functions always overrides to esbuild.

Test locally

To streamline writing and testing your functions on Netlify, run a local development environment with Netlify Dev. This feature of Netlify CLI includes tools for local function development through a simulated Netlify production environment. The netlify dev command starts a framework server if a framework is detected and handles redirects, proxy rules, environment variables, and Netlify Functions.

To simulate Netlify Functions in a standalone server without the full overhead of Netlify Dev, serve functions locally with the netlify functions:serve CLI command. Then to debug functions, inspect the functions server process.

Type check

Because Netlify’s build system doesn’t perform type checking, we recommend that you type check locally with one of these options:

Use your code editor to type check during local development.
Use the official TypeScript compiler, running tsc --noEmit locally before committing the files.

Next steps

Push your function source files to your Git provider for continuous deployment where Netlify’s build system automatically detects, builds, and deploys your functions. For more control over the process, learn about other workflows for deploying your functions including custom builds with continuous deployment and manual deploys with the Netlify CLI or API.

Monitor function logs and metrics in the Netlify UI to observe and help troubleshoot your deployed functions.

Netlify function logs are found in the Netlify UI. You can also stream Netlify function logs to the console with the Netlify CLI.

Learn how to use Netlify Blobs with lambda-compatible functions.

Create function file

To add a serverless function to your project, create a JavaScript file in your functions directory following the instructions below for naming and coding your function. Netlify will access the functions directory during every build, preparing and deploying each supported code file as a function.

Directory

The default functions directory is YOUR_BASE_DIRECTORY/netlify/functions. You can optionally configure a custom functions directory.

Name your function

For example, to create a function with an endpoint name of hello, save the function file in one of the following ways:

netlify/functions/hello.js
netlify/functions/hello/hello.js
netlify/functions/hello/index.js

netlify/functions/hello-background.js
netlify/functions/hello-background/hello-background.js
netlify/functions/hello-background/index.js

Code your function

This section will help you learn how to write functions. It covers the syntax for synchronous functions, both buffered and streaming, as well as background functions.

Synchronous function format

To create a synchronous function, use the following general syntax in your JavaScript function file to export a handler method:

exports.handler = async function(event, context) {
  // your server-side functionality
};

Netlify provides the event and context parameters when the function is invoked.

The event object received by the handler is similar to the following:

{
  "path": "Path parameter (original URL encoding)",
  "httpMethod": "Incoming request’s method name",
  "headers": {Incoming request headers},
  "queryStringParameters": {Query string parameters},
  "body": "A JSON string of the request payload",
  "isBase64Encoded": "A boolean flag to indicate if the applicable request payload is Base64-encoded"
}

If you need to parse a multipart/form-data-encoded event.body, we recommend using busboy. To learn more, visit our blog article on processing multipart form data with Netlify Functions.

The context object received by the handler includes information about the context in which the function was called, like certain Identity user information, for example.

Here’s a basic example function, hello.js:

exports.handler = async function(event, context) {
  return {
    statusCode: 200,
    body: JSON.stringify({ message: "Hello World" })
  };
};

This function deploys to an endpoint at /.netlify/functions/hello relative to the base URL of your site.
A successful invocation returns the 200 status code and the string, “Hello, World”.

Synchronous functions can return a response object that includes the following information:

{
  "isBase64Encoded": true|false,
  "statusCode": httpStatusCode,
  "headers": { "headerName": "headerValue", ... },
  "multiValueHeaders": { "headerName": ["headerValue", "headerValue2", ...], ... },
  "body": "..."
}

Because async function code returns a promise, we recommend returning a response with at least an HTTP status code instead of allowing the function to time out. Using async in your function code doesn’t automatically designate a function as a background function.

Response streaming

This feature is in Beta and is available on all pricing plans.

To stream a function’s response:

wrap your handler export with the stream decorator
return a ReadableStream as the body

Here’s a simplified example that you can use to test that your function response streams to the client:

const { stream } = require("@netlify/functions");

exports.handler = stream(async () => {
  const encoder = new TextEncoder();
  const formatter = new Intl.DateTimeFormat("en", { timeStyle: "medium" });
  const body = new ReadableStream({
    start(controller) {
      controller.enqueue(encoder.encode("<html><body><ol>"));
      let i = 0;
      const timer = setInterval(() => {
        controller.enqueue(
          encoder.encode(
            `<li>Hello at ${formatter.format(new Date())}</li>\n\n`
          )
        );
        if (i++ >= 5) {
          controller.enqueue(encoder.encode("</ol></body></html>"));
          controller.close();
          clearInterval(timer);
        }
      }, 1000);
    }
  });
  return {
    headers: {
      "content-type": "text/html"
    },
    statusCode: 200,
    body
  };
});

For an extended example that demonstrates a more likely response streaming use case, consider this function:

const { stream } = require("@netlify/functions");

exports.handler = stream(async event => {
  // Get the request from the request query string, or use a default
  const pie =
    event.queryStringParameters?.pie ??
    "something inspired by a springtime garden";

  // The response body returned from "fetch" is a "ReadableStream",
  // so you can return it directly in your streaming response
  const res = await fetch("https://api.openai.com/v1/chat/completions", {
    method: "POST",
    headers: {
      "Content-Type": "application/json",
      // Set this environment variable to your own key
      Authorization: `Bearer ${process.env.OPENAI_API_KEY}`
    },
    body: JSON.stringify({
      model: "gpt-3.5-turbo",
      messages: [
        {
          role: "system",
          content:
            "You are a baker. The user will ask you for a pie recipe. You will respond with the recipe. Use markdown to format your response"
        },
        // Use "slice" to limit the length of the input to 500 characters
        { role: "user", content: pie.slice(0, 500) }
      ],
      // Use server-sent events to stream the response
      stream: true
    })
  });

  return {
    headers: {
      // This is the mimetype for server-sent events
      "content-type": "text/event-stream"
    },
    statusCode: 200,
    // Pipe the event stream from OpenAI to the client
    body: res.body
  };
});

When invoked this function sends a prompt to OpenAI and streams the response back to the client like a chatbot.

Keep the following limitations in mind when working with streaming functions:

10 second execution limit. If the limit is reached, the response stops streaming.
20 MB response size limit. Responses larger than 20 MB cannot be streamed.
Background Functions can’t stream responses.
On-demand Builders can’t stream responses.
Scheduled Functions can’t stream responses.

Background function format

To create a background function, append the function name with -background. For example, netlify/functions/hello-background.js or netlify/functions/hello-background/index.js.

Background function syntax is similar to synchronous function syntax with a couple of key differences:

You will generally pass the function result to a destination other than the originating client.
Background functions don’t support response streaming because they don’t return responses.

Like the synchronous function format, each JavaScript file to be deployed as a background function must export a handler method. As with synchronous function invocation, Netlify provides the event and context parameters when a background function is invoked. Although background functions rely on asynchronous invocation, they don’t require the use of async JavaScript syntax.

Here’s a simplified example background function that you can use to test that your function runs longer than 30 seconds, count-background.js:

exports.handler = function(event, context) {
  function sleep(ms) {
    return new Promise(resolve => setTimeout(resolve, ms));
  }

  async function demo() {
    for (let i = 0; i < 60; i++) {
      let date = new Date();
      await sleep(1000);
      console.log(date.toLocaleString(), i);
    }
    console.log("Done");
  }

  demo();
};

This function deploys to an endpoint at /.netlify/functions/count-background relative to the base URL of your site.
When invoked, the function returns an initial 202 success response.
When successfully executed, the function logs numbers 1 through 60.

For an extended example that demonstrates a more likely background function use case, consider this send-pdf-background.js function:

const { jsPDF } = require("jspdf");
const nodemailer = require("nodemailer");
const mg = require("nodemailer-mailgun-transport");

const transporter = nodemailer.createTransporter(
  mg({
    auth: {
      api_key: process.env.MAILGUN_API_KEY,
      domain: process.env.MAILGUN_DOMAIN
    }
  })
);

exports.handler = async function(event) {
  const { content, destination } = JSON.parse(event.body);
  console.log(`Sending PDF report to ${destination}`);

  const report = Buffer.from(
    new jsPDF().text(content, 10, 10).output("arraybuffer")
  );
  const info = await transporter.sendMail({
    from: process.env.MAILGUN_SENDER,
    to: destination,
    subject: "Your report is ready!",
    text: "Details in attached report PDF",
    attachments: [
      {
        filename: `report-${new Date().toDateString()}.pdf`,
        content: report,
        contentType: "application/pdf"
      }
    ]
  });

  console.log(`PDF report sent: ${info.messageId}`);
};

This background function deploys to an endpoint at /.netlify/functions/send-pdf-background relative to the base URL of your site.
When invoked, the function returns an initial 202 success response.
When successfully executed, a PDF report is generated and emailed as an attachment.

Environment variables

You can also leverage build environment variables to configure how Netlify builds your functions. For example, you can use an environment variable to set the Node.js version.

Learn more about how to set and use environment variables with functions.

Manage dependencies

To optimize build performance, specify function dependencies in the top-level package.json file in the site’s base directory. Here’s an example of a recommended layout:

my-base-directory/
├─ package.json
├─ node_modules
└─ netlify/
   └─ functions/
        ├─ hello.js
        └─ send-pdf-background.js

Test locally

Next steps

Monitor function logs and metrics in the Netlify UI to observe and help troubleshoot your deployed functions.

Netlify function logs are found in the Netlify UI. You can also stream Netlify function logs to the console with the Netlify CLI.

Learn how to use Netlify Blobs with lambda-compatible functions.

Create function file

To add a serverless function to your project, create a Go file in a dedicated function subdirectory in your functions directory following the instructions below for naming and coding your function. Netlify will access the functions directory during every build, preparing and deploying each supported code file as a function.

Environment variables

You can also leverage build environment variables to configure how Netlify builds your functions. For example, you can use an environment variable to set the Node.js version.

Learn more about how to set and use environment variables with functions.

Test locally

To simulate Netlify Functions in a standalone server without the full overhead of Netlify Dev, serve functions locally with the netlify functions:serve CLI command.

Next steps

Monitor function logs and metrics in the Netlify UI to observe and help troubleshoot your deployed functions.

Netlify function logs are found in the Netlify UI. You can also stream Netlify function logs to the console with the Netlify CLI.

Did you find this doc useful?

Your feedback helps us improve our docs.

Lambda compatibility for Functions

Prepare project

Create function file

Directory

Name your function

Code your function

Synchronous function format

Response streaming

Background function format

Environment variables

Manage dependencies

Test locally

Type check

Next steps

Create function file

Directory

Name your function

Code your function

Synchronous function format

Response streaming

Background function format

Environment variables

Manage dependencies

Test locally

Next steps

Create function file

Directory

Name your function

Code your function

Synchronous function format

Access the ClientContext

Background function format

Environment variables

Test locally

Next steps

Did you find this doc useful?

Access the `ClientContext`