Build and serve lambda function with webpack compilation

Usage no npm install needed!

<script type="module">
  import netlifyLambda from '';


Netlify Lambda

This is an optional tool that helps with building or locally developing Netlify Functions with a simple webpack/babel build step. For function folders, there is also a small utility to install function folder dependencies.

The goal is to make it easy to write Lambda's with transpiled JS/TypeScript features and imported modules.

Multiple ways to deploy functions on Netlify

There are 3 ways to deploy functions to Netlify:

  1. each function as a single JS or Go file, possibly bundled by a build tool like netlify-lambda or tsc
  2. each function as a zip of a folder of files
  3. as of CLI v2.7, a non-bundled, non-zipped, folder of files.

Netlify-Lambda uses webpack to bundle up your functions and their dependencies for you, suiting the first approach. However, if you have native node modules (or other dependencies that don't expect to be bundled like the Firebase SDK) then you may want to try the other approaches. In particular, try Netlify Dev.

If this sounds confusing, support is available through our regular channels.

When to use Netlify Dev or netlify-lambda or both?

Netlify Dev is incrementally adoptable. Use `netlify-lambda` only if you need a build step for your functions. Expand this to read more on when to use either or both
  • When to use Netlify Dev: Part of Netlify Dev serves unbundled function folders through zip-it-and-ship-it with no build step. This is likely to be attractive to many users who previously just needed netlify-lambda for bundling multi-file functions or functions with node_module dependencies.
  • When to use Netlify Lambda: However, if you need a build step for your functions (e.g. for webpack import/export syntax, running babel transforms or typescript), you can use netlify-lambda, tsc or your own build tool to do this, just point Netlify Dev at your build output with the functions field in netlify.toml.
  • These responsibilities aren't exactly the same. Therefore you can use Netlify Dev and Netlify Lambda together to have BOTH a build step for functions from netlify-lambda and the full proxy environment from Netlify Dev. If you have a npm script in package.json for running netlify-lambda serve ${functionsSourceFolder}, Netlify Dev will detect it and run it for you. This way, existing netlify-lambda users will be able to use Netlify Dev with no change to their workflow

Function Builder detection is a very new feature with only simple detection logic for now, that we aim to improve over time. If it doesn't work well for you, you can simply not use Netlify Dev for now while we work out all your bug reports. 🙏🏼

You can see how to convert a Netlify-Lambda project to Netlify Dev as well as why and how they work together in this 48 min video here


We recommend installing locally rather than globally:

npm install netlify-lambda

This will ensure your build scripts don't assume a global install which is better for your CI/CD (for example with Netlify's buildbot).

If you don't have a netlify.toml file, you'll need one (example). Define the functions field where the functions will be built to and served from, e.g.

# example netlify.toml
  command = "npm run build"
  functions = "lambda" #  netlify-lambda reads this
  publish = "build"


We expose three commands:

netlify-lambda build <folder>
netlify-lambda install [folder]

## legacy command - only preserved for backward compatibility
netlify-lambda serve <folder> 

netlify-lambda install

Sometimes your function folders will have dependencies unique to them, managed by a package.json local to that folder. This is a small utility function for installing those dependencies either on your local machine or as part of your build commands.

By default it just runs on the functions folder specified in netlify.toml. Here's all you need to add to your package.json (see this example):

// package.json
   "scripts": {
       "postinstall": "netlify-lambda install"

This is what you should do if you are just using Netlify Dev without netlify-lambda.

If you're using netlify-lambda serve or build, however, you will want to run this install on the source folder rather than the dist/netlify.toml functions folder, so you should run it with the same exact folder name as with those other commands:

netlify-lambda install <folderName>

We don't anticipate you will use this as often but it can be handy.

netlify-lambda build

At a high level, netlify-lambda takes a source folder (e.g. src/lambda, specified in your command) and outputs it to a built folder, (e.g. built-lambda, specified in your netlify.toml file).

The build function will run a single build of the functions in the folder.

The serve function will start a dev server for the source folder and route requests with a .netlify/functions/ prefix, with a default port of 9000:

folder/hello.js -> http://localhost:9000/.netlify/functions/hello

It also watches your files and restarts the dev server on change. Note: if you add a new file you should kill and restart the process to pick up the new file.


Environment variables in build and branch context

Read Netlify's documentation on environment variables. netlify-lambda should respect the env variables you supply in netlify.toml accordingly (except for deploy previews, which make no sense to locally emulate).

However, this is a relatively new feature, so if you encounter issues, file one.

If you need local-only environment variables that you don't place in netlify.toml for security reasons, you can configure webpack to use a .env file like in this example.

Lambda function examples If you are new to writing Lambda functions, this section may help you. Function signatures should conform to one of either two styles. Traditional callback style:
// legacy callback style - not encouraged anymore, but you'll still see examples doing this
exports.handler = function(event, context, callback) {
// your server-side functionality
callback(null, {
  statusCode: 200,
  body: JSON.stringify({
    message: `Hello world ${Math.floor(Math.random() * 10)}`

or you can use async/await:

// modern JS style - encouraged
export async function handler(event, context) {
  return {
    statusCode: 200,
    body: JSON.stringify({ message: `Hello world ${Math.floor(Math.random() * 10)}` })

:warning: The above example only works with netlify-lambda because it uses ES module syntax! If you get Function invocation failed: SyntaxError: Unexpected token 'export'. errors, this is why.

For more Functions examples, check:

netlify-lambda serve (legacy command)

This command is pretty much superceded by Netlify Dev. We only keep it around for legacy/backward compatibility support reasons.

netlify-lambda serve (legacy command): Using with create-react-app, Gatsby, and other development servers

Why you need to proxy (for beginners)

react-scripts (the underlying library for create-react-app) and other popular development servers often set up catchall serving for you; in other words, if you try to request a route that doesn't exist, the dev server will try to serve you /index.html. This is problematic when you are trying to hit a local API endpoint like netlify-lambda sets up for you - your browser will attempt to parse the index.html file as JSON. This is why you may see this error:

Uncaught (in promise) SyntaxError: Unexpected token < in JSON at position 0

If this desribes your situation, then you need to proxy for local development. Read on. Don't worry it's easier than it looks.

netlify-lambda serve (legacy command): Proxying for local development


When your function is deployed on Netlify, it will be available at /.netlify/functions/function-name for any given deploy context. It is advantageous to proxy the netlify-lambda serve development server to the same path on your primary development server.

Say you are running webpack-serve on port 8080 and netlify-lambda serve on port 9000. Mounting localhost:9000 to /.netlify/functions/ on your webpack-serve server (localhost:8080/.netlify/functions/) will closely replicate what the final production environment will look like during development, and will allow you to assume the same function url path in development and in production.

Example webpack config:

module.exports = {
  mode: "development",
  devServer: {
    proxy: {
      "/.netlify": {
        target: "http://localhost:9000",
        pathRewrite: { "^/.netlify/functions": "" }
Using with Angular CLI

CORS issues when trying to use netlify-lambdas locally with angular? you need to set up a proxy.

Firstly make sure you are using relative paths in your app to ensure that your app will work locally and on Netlify, example below...


Then place a proxy.config.json file in the root of your project, the contents should look something like...

  "/.netlify/functions/*": {
    "target": "http://localhost:9000",
    "secure": false,
    "logLevel": "debug",
    "changeOrigin": true
  • The key should match up with the location of your Transpiled functions as defined in your netlify.toml
  • The target should match the port that the lambdas are being served on (:9000 by default)

When you run up your Angular project you need to pass in the proxy config with the flag --proxy-config like so...

  ng serve --proxy-config proxy.config.json

To make your life easier you can add these to your scripts in package.json

  "scripts": {
    "start": "ng serve --proxy-config proxy.config.json",
    "build": "ng build --prod --aot && yarn nlb",
    "nls": "netlify-lambda serve src_functions",
    "nlb": "netlify-lambda build src_functions"

Obviously you need to run up netlify-lambda & angular at the same time.

Using with Next.js

Next.js doesnt use Webpack Dev Server, so you can't modify any config in next.config.js to get a proxy to run. However, since the CORS proxy issue only happens in dev mode (Functions are on the same domain when deployed on Netlify) you can run Next.js through a Node server for local development:

touch server.js
yarn add -D http-proxy-middleware express
// server.js
/* eslint-disable no-console */
const express = require("express");
const next = require("next");

const devProxy = {
  "/.netlify": {
    target: "http://localhost:9000",
    pathRewrite: { "^/.netlify/functions": "" }

const port = parseInt(process.env.PORT, 10) || 3000;
const env = process.env.NODE_ENV;
const dev = env !== "production";
const app = next({
  dir: ".", // base directory where everything is, could move to src later

const handle = app.getRequestHandler();

let server;
  .then(() => {
    server = express();

    // Set up the proxy.
    if (dev && devProxy) {
      const proxyMiddleware = require("http-proxy-middleware");
      Object.keys(devProxy).forEach(function(context) {
        server.use(proxyMiddleware(context, devProxy[context]));

    // Default catch-all handler to allow Next.js to handle all other routes
    server.all("*", (req, res) => handle(req, res));

    server.listen(port, err => {
      if (err) {
        throw err;
      console.log(`> Ready on port ${port} [${env}]`);
  .catch(err => {
    console.log("An error occurred, unable to start the server");

run your server and netlify-lambda at the same time:

// package.json
  "scripts": {
    "start": "cross-env NODE_ENV=dev npm-run-all --parallel start:app start:server",
    "start:app": "PORT=3000 node server.js",
    "start:server": "netlify-lambda serve functions"

and now you can ping Netlify Functions with locally emulated by netlify-lambda!

For production deployment, you have two options:

Just remember to configure your netlify.toml to point to the Next.js build folder and your netlify-lambda functions folder accordingly.

Webpack Configuration

By default the webpack configuration uses babel-loader to load all js files. netlify-lambda will search for a valid babel config file in the functions directory first and look upwards up to the directory netlify-lambda is run from (similar to how babel-loader looks for a Babel config file). If no babel config file is found, a few basic settings are used.

If you need to use additional webpack modules or loaders, you can specify an additional webpack config with the -c/--config option when running either serve or build.

For example, have a file with:

// webpack.functions.js
module.exports = {
  optimization: { minimize: false }

Then specify netlify-lambda serve --config ./webpack.functions.js. If using VSCode, it is likely that the sourceMapPathOverrides have to be adapted for breakpoints to work. Read here for more info on how to modify the webpack config.

If you're using firebase SDK and other native modules, check this issue and use this plugin:

const nodeExternals = require('webpack-node-externals');

module.exports = {
  externals: [nodeExternals()],

The additional webpack config will be merged into the default config via webpack-merge's method.

Babel configuration

The default webpack configuration uses babel-loader with a few basic settings.

However, if any valid Babel config file is found in the directory netlify-lambda is run from, or folders above it (useful for monorepos), it will be used instead of the default one.

It is possible to disable this behaviour by passing --babelrc false.

If you need to run different babel versions for your lambda and for your app, check this issue to override your webpack babel-loader.

Use with TypeScript

We added .ts and .mjs support recently - check here for the PR and usage tips.

  1. Install @babel/preset-typescript
npm install --save-dev @babel/preset-typescript

You may also want to add typescript @types/node @types/aws-lambda.

  1. Create a Babel config file, e.g. .babelrc:
  "presets": [
        "targets": {
          "node": true
  "plugins": [
  1. (Optional) if you have @types/aws-lambda installed, your lambda functions can use the community typings for Handler, Context, Callback. See the typescript instructions in create-react-app-lambda for an example.

Check for a CRA + Lambda full Typescript experience.

CLI flags/options

There are additional CLI options:

-h --help
-c --config
-p --port
-s --static
-t --timeout
-b --babelrc

--config option

If you need to use additional webpack modules or loaders, you can specify an additional webpack config with the -c/--config option when running either serve or build.

For example, have a file with:

// webpack.functions.js
module.exports = {
  optimization: { minimize: false }

Then specify netlify-lambda serve --config ./webpack.functions.js.

--timeout option

(This is for local dev/serving only) The default function timeout is 10 seconds. If you need to adjust this because you have requested extra timeout, pass a timeout number here. Thanks to @naipath for this feature.

--port option

The serving port can be changed with the -p/--port option.

--static option

If you need an escape hatch and are building your lambda in some way that is incompatible with our build process, you can skip the build with the -s or --static flag. More info here.


Defaults to true

Use a Babel config file found in the directory netlify-lambda is run from. This can be useful when you have conflicting babel-presets, more info here

Netlify Identity

Make sure to read the docs on how Netlify Functions and Netlify Identity work together. Basically you have to make your request with an authorization header and a Bearer token with your Netlify Identity JWT supplied. You can get this JWT from any of our Identity solutions from gotrue-js to netlify-identity-widget.

Since for practical purposes we cannot fully emulate Netlify Identity locally, we provide simple JWT decoding inside the context of your function. This will give you back the user info you need to work with.

Minor note: For the identity field, since we are not fully emulating Netlify Identity, we can't give you details on the Identity instance, so we give you unambiguous strings so you know not to rely on it locally: NETLIFY_LAMBDA_LOCALLY_EMULATED_IDENTITY_URL and NETLIFY_LAMBDA_LOCALLY_EMULATED_IDENTITY_TOKEN. In production, of course, Netlify Functions will give you the correct identity.url and identity.token fields. We find we dont use this info often in our functions so it is not that big of a deal in our judgment.


To debug lambdas, it can be helpful to turn off minification and enable logging. Prepend the serve command with npm's package runner npx, e.g. npx --node-arg=--inspect netlify-lambda serve ....

  1. make sure that sourcemaps are built along the way (e.g. in the webpack configuration and the tsconfig.json if typescript is used)
  2. webpack's minification/uglification is turned off (see below):

For example, to customize the webpack config you can have a file with:

// webpack.functions.js
module.exports = {
  optimization: { minimize: false }

You can see a sample project with this setup here.

So you can run something like npx --node-arg=--inspect netlify-lambda serve --config ./webpack.functions.js. If using VSCode, it is likely that the sourceMapPathOverrides have to be adapted for breakpoints to work. Read here for more info on how to modify the webpack config.

Netlify Functions run in Node v8.10 and you may need to run the same version to mirror the environment locally. Also make sure to check that you aren't committing one of these common Node 8 mistakes in Lambda!

Special warning on node-fetch: node-fetch and webpack currently don't work well together. You will have to use the default export in your code:

const fetch = require("node-fetch").default; // not require('node-fetch')

Don't forget to search our issues in case someone has run into a similar problem you have!

Example functions and Tutorials

You can do a great deal with lambda functions! Here are some examples for inspiration:

These libraries pair very well for extending your functions capability:

Other community approaches

If you wish to serve the full website from lambda, check this issue.

If you wish to run this server for testing, check this issue.

If you wish to emulate more Netlify functionality locally, check this repo: We are considering merging the projects here.

All of the above are community maintained and not officially supported by Netlify.