Local Lambda development environment

Usage no npm install needed!

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  import funRuntimesTest from '';



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Local serverless function λ development runtime.

  • Programmatic. A TypeScript API is exposed to trigger invocations.
  • Provider agnostic. AWS Lambda + other cloud providers planned.
  • Runtime agnostic. Node, go, python and custom runtime APIs.
  • Platform agnostic. Functions can be executed natively (e.g. macOS) or via Docker.
  • Zero setup needed. ƒun acquires the necessary runtime files (e.g. node).


Given a Lambda function like this one:

// index.js
exports.handler = function(event, context, callback) {
    callback(null, { hello: 'world' });

You can invoke this function locally using the code below:

const { createFunction } = require('@zeit/fun');

async function main() {
    // Starts up the necessary server to be able to invoke the function
    const fn = await createFunction({
        Code: {
            // `ZipFile` works, or an already unzipped directory may be specified
            Directory: __dirname + '/example'
        Handler: 'index.handler',
        Runtime: 'nodejs8.10',
        Environment: {
            Variables: {
                HELLO: 'world'
        MemorySize: 512

    // Invoke the function with a custom payload. A new instance of the function
    // will be initialized if there is not an available one ready to process.
    const res = await fn({ hello: 'world' });

    // Prints: { hello: 'world' }

    // Once we are done with the function, destroy it so that the processes are
    // cleaned up, and the API server is shut down (useful for hot-reloading).
    await fn.destroy();



ƒun has a concept of pluggable "providers", which are responsible for creating, freezing, unfreezing and shutting down the processes that execute the Lambda function.


The native provider executes Lambda functions directly on the machine executing ƒun. This provides an execution environment that closely resembles the real Lambda environment, with some key differences that are documented here:

  • Lambdas processes are ran as your own user, not the sbx_user1051 user.
  • Processes are not sandboxed nor chrooted, so do not rely on hard-coded locations like /var/task, /var/runtime, /opt, etc. Instead, your function code should use the environment variables that represent these locations (namely LAMBDA_TASK_ROOT and LAMBDA_RUNTIME_DIR).
  • Processes are frozen by sending the SIGSTOP signal to the lambda process, and unfrozen by sending the SIGCONT signal, not using the cgroup freezer.
  • Lambdas that compile to native executables (i.e. Go) will need to be compiled for your operating system. So if you are on macOS, then the binary needs to be executable on macOS.


A docker provider is planned, but not yet implemented. This will allow for an execution environment that more closely matches the AWS Lambda environment, including the ability to execute Linux x64 binaries / shared libraries.


ƒun aims to support all runtimes that AWS Lambda provides. Currently implemented are:

  • nodejs for Node.js Lambda functions using the system node binary
  • nodejs6.10 for Node.js Lambda functions using a downloaded Node v6.10.0 binary
  • nodejs8.10 for Node.js Lambda functions using a downloaded Node v8.10.0 binary
  • python for Python Lambda functions using the system python binary
  • python2.7 for Python Lambda functions using a downloaded Python v2.7.12 binary
  • python3.6 for Python Lambda functions using a downloaded Python v3.6.8 binary
  • python3.7 for Python Lambda functions using a downloaded Python v3.7.2 binary
  • go1.x for Lambda functions written in Go - binary must be compiled for your platform
  • provided for custom runtimes