Builders Developer Guide

A builder is an npm module that exposes a build function and optionally an analyze function and prepareCache function.

In short:

  • The analyze function returns a unique fingerprint used for the purpose of build de-duplication. If the analyze function is not supplied, it's equivalent to it returning a random fingerprint for each build.
  • The build function returns a Files datastructure that contains the build outputs, which can be:

  • The prepareCache function is equivalent to build, but it executes the instructions necessary to prepare a cache for the next run.

The arguments these functions receive are:

  • files: All source files of the project as a Files datastructure
  • entrypoint: Name of entrypoint file for this particular build job. Value files[entrypoint] is guaranteed to exist and be a valid File reference. entrypoint is always a discrete file and never a glob, since globs are expanded into separate builds at deployment time.
  • workPath, a writable temporary directory where you are encouraged to perform your build process. This directory will be populated with the restored cache from the previous run (if any) for analyze and build.
  • cachePath, a writable temporary directory where you can build a cache for the next run. This is only passed to prepareCache
  • config, an arbitrary object passed from by the user in the build definition in now.json.

The signatures of the functions are as follows:

  files: Files,
  entrypoint: String,
  workPath: String,
  config: Object
}) : String fingerprint
  files: Files,
  entrypoint: String,
  workPath: String,
  config: Object
}) : Files output
  files: Files,
  entrypoint: String,
  workPath: String,
  cachePath: String,
  config: Object
}): Files cacheOutput

Let's walk through what it takes to create a simple builder that takes in a HTML source file and yields a minified HTML static file as its build output.

While this is a very simple builder, the approach demonstrated here can be used to return anything: one or more static files and/or one or more lambdas.

To see the source code for this example, check it out on GitHub.

The analyze hook is optional. Its goal is to give the developer a tool to avoid wasting time re-computing a build that has already occurred.

The return value of analyze is a fingerprint: a simple string that uniquely identifies the build process.

If analyze is not specified, its behavior is to use as the fingerprint the combined checksums of all the files in the same directory level as the entrypoint. This is a default that errs on making sure that we re-execute builds when files other than the entrypoint (like dependencies, manifest files, etc) have changed.

For our html-minify example, we know that HTML files don't have dependencies. Therefore, our analyze step can just return the digest of the entrypoint.

Our index.js file looks as follows:

exports.analyze = function ({ files, entrypoint }) {
  return files[entrypoint].digest;

This means that we will only re-minify and re-create the build output only if the file contents (and therefore its digest) change.

Your module will need some utilities to manipulate the datastructures we pass you, create new ones and alter the filesystem.

To that end, we expose our API as part of a @now/build-utils package. This package is always loaded on your behalf, so make sure it's only included as peerDependencies in your package.json.

Builders can include dependencies of their liking. In this case, we'll use the html-minifier npm package:

const htmlMinifier = require('html-minifier');

exports.analyze = ({ files, entrypoint }) => files[entrypoint].digest; = async ({ files, entrypoint, config }) => {
  const stream = files[entrypoint].toStream();
  const options = Object.assign({}, config || {});
  const { data } = await FileBlob.fromStream({ stream });
  const content = data.toString();

  const minified = htmlMinifier(content, options);
  const result = new FileBlob({ data: minified });

  return { [entrypoint]: result };

If our builder had performed work that could be re-used in the next build invocation, we could define a prepareCache step.

In this case, there are not intermediate artifacts that we can cache, and our analyze step already takes care of caching the full output based on the fingerprint of the input.

A lambda is created where the builder logic is executed. The lambda is run using the Node.js 8 runtime. A brand new sandbox is created for each deployment, for security reasons. The sandbox is cleaned up between executions to ensure no lingering temporary files are shared from build to build.

All the APIs you export (analyze, build and prepareCache) are not guaranteed to be run in the same process, but the filesystem we expose (e.g.: workPath and the results of calling getWritableDirectory ) is retained.

If you need to share state between those steps, use the filesystem.

When a new build is created, we pre-populate the workPath supplied to analyze with the results of the prepareCache step of the previous build.

The analyze step can modify that directory, and it will not be re-created when it's supplied to build and prepareCache.

To learn how the cache key is computed and invalidated, refer to the overview.

The env and secrets specified by the user as build.env are passed to the builder process. This means you can access user env via process.env in Node.js.

When you publish your builder to npm, make sure to not specify @now/build-utils (as seen below in the API definitions) as a dependency, but rather as part of peerDependencies.

This is an abstract type that is implemented as a plain JavaScript Object. It's helpful to think of it as a virtual filesystem representation.

An example of a valid Files object is:

  "index.html": FileRef,
  "api/index.js": Lambda

This is an abstract type. Valid File types include:

Exported as: @now/build-utils/file-ref

This is a JavaScript class that represents an abstract file instance stored in our platform, based on the file identifier string (its checksum). When a Files object is passed as an input to analyze or build, all its values will be instances of FileRef.


  • mode : Number file mode
  • digest : String a checksum that represents the file


  • toStream() :Stream creates a Stream of the file body

Exported as: @now/build-utils/file-fs-ref

This is a JavaScript class that represents an abstract instance of a file present in the filesystem that the build process is executing in.


  • mode : Number file mode
  • fsPath : String a path of the file in file system


  • static async fromStream({ mode : Number, stream :Stream, fsPath : String }) :FileFsRef creates an instance of a FileFsRef from Stream, placing file at fsPath with mode
  • toStream() :Stream creates a Stream of the file body

Exported as: @now/build-utils/file-blob

This is a JavaScript class that represents an abstract instance of a file present in memory.


  • mode : Number file mode
  • data : String | Buffer the body of the file


  • static async fromStream({ mode : Number, stream :Stream }) :FileBlob creates an instance of a FileBlob from Stream with mode
  • toStream() :Stream creates a Stream of the file body

Exported from: @now/build-utils/lambda

This is a JavaScript class that can be created by supplying files, handler and runtime and environment as an object to the createLambda helper. The instances of this class should not be created directly. Instead use a call to createLambda.


  • files : Files the internal filesystem of the lambda
  • handler : String path to handler file and (optionally) a function name it exports
  • runtime : LambdaRuntime the name of the lambda runtime
  • environment : Object key-value map of handler-related (aside of those passed by user) environment variables

This is an abstract enumeration type that is implemented by one of the following possible String values:

  • nodejs8.10
  • nodejs6.10
  • go1.x
  • java-1.8.0-openjdk
  • python3.6
  • python2.7
  • dotnetcore2.1
  • dotnetcore2.0
  • dotnetcore1.0

The following is exposed by @now/build-utils to simplify the process of writing Builders, manipulating the file system, using the above types, etc.

Exported from: @now/build-utils/lambda

Constructor for the Lambda type.

const { createLambda } = require('@now/build-utils/lambda');
const FileBlob = require('@now/build-utils/file-blob');
await createLambda({
  runtime: 'nodejs8.10',
  handler: 'index.main',
  files: {
    'index.js': new FileBlob({ data: "exports.main = () => {}" })

Exported as: @now/build-utils/fs/download

This utility allows you to download the contents of a Files datastructure, therefore creating the filesystem represented in it.

Since Files is an abstract way of representing files, you can think of download as a way of making that virtual filesystem real.

await download(files, workPath);

Exported as: @now/build-utils/fs/glob

This utility allows you to scan the filesystem and return a Files representation of the matched glob search string. It can be thought of as the reverse of download.

The following trivial example downloads everything to the filesystem, only to return it back (therefore just re-creating the passed-in files):

const glob = require('@now/build-utils/fs/glob')
const download = require('@now/build-utils/fs/download') = ({ files, workPath }) => {
  await download(files, workPath)
  return glob('**', workPath);

Exported as: @now/build-utils/fs/get-writable-directory

In some occasions, you might want to write to a temporary directory

Exported as: @now/build-utils/fs/rename

Renames the keys of the Files object, which represent the paths. For example, to rename test.go to test you can write:

const rename = require('@now/build-utils/fs/rename');
  'test.go': fileFsRef
}, (path) => path.replace(/.go$/, '');
// returns new Files object