A builder is an npm module that exposes a build function and optionally an analyze function and prepareCache function. Official Builders are published to npmjs.com as a package and referenced in the use property of the now.json configuration file. However, the use property will work with any npm install argument such as a git repo url which is useful for testing your Builder.

See the Builders Overview to view example usage.

Builder Exports


A required exported constant that decides which version of the Builder API to use.

The latest and suggested version is 2.


An optional exported function that returns a unique fingerprint used for the purpose of build de-duplication. If the analyze function is not supplied, a random fingerprint is assigned to each build.

export analyze({
  files: Files,
  entrypoint: String,
  workPath: String,
  config: Object
}) : String fingerprint

If you are using TypeScript, you should use the following types:

import { AnalyzeOptions } from '@now/build-utils'

export analyze(options: AnalyzeOptions) {
  return 'fingerprint goes here'


A required exported function that returns a Files data structure that contains the Build outputs, which can be a Static File or a Lambda.

What's a Lambda? Read about Lambda concepts first, and then continue reading here.

  files: Files,
  entrypoint: String,
  workPath: String,
  config: Object,
  meta?: {
    isDev?: Boolean,
    requestPath?: String,
    filesChanged?: Array<String>,
    filesRemoved?: Array<String>
}) : {
  watch: Array<String>,
  output: Files output,
  routes: Object

If you are using TypeScript, you should use the following types:

import { BuildOptions } from '@now/build-utils'

export build(options: BuildOptions) {
  // Build the code here

  return {
    output: {
      'path-to-file': File,
      'path-to-lambda': Lambda
    watch: [],
    routes: {}


An optional exported function that is equivalent to build, but it executes the instructions necessary to prepare a cache for the next run.

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

If you are using TypeScript, you can import the types for each of these functions by using the following:

import { PrepareCacheOptions } from '@now/build-utils'

export prepareCache(options: PrepareCacheOptions) {
  return { 'path-to-file': File }


An optional exported function that is only used by now dev in Now CLI and indicates whether a Builder wants to be responsible for building a certain request path.

  entrypoint: String,
  files: Files,
  config: Object,
  requestPath: String,
  workPath: String
}) : Boolean

If you are using TypeScript, you can import the types for each of these functions by using the following:

import { ShouldServeOptions } from '@now/build-utils'

export shouldServe(options: ShouldServeOptions) {
  return Boolean

If this method is not defined, Now CLI will default to this function.

Builder Options

The exported functions analyze, build, and prepareCache receive one argument with the following properties.


  • files: All source files of the project as a Files data structure.
  • 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.

Example: html-minifier

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.

Setting up the module

Defining the analyze step

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.

Defining the build step

Your module will need some utilities to manipulate the data structures 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.version = 2

exports.analyze = ({ files, entrypoint }) => files[entrypoint].digest

exports.build = 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 {
    output: {
      [entrypoint]: result
    watch: [],
    routes: {}

Defining a prepareCache step

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.

Technical Details

Execution Context

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 getWriteableDirectory ) is retained.

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

Directory and Cache Lifecycle

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.

Accessing Environment and Secrets

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.

Utilities as peerDependencies

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.



import { File } from '@now/build-utils'
type Files = { [filePath: string]: File }

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.

When used as an input, the Files object will only contain FileRefs. When Files is an output, it may consist of Lambda types as well as FileRefs.

An example of a valid output Files object is:

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


This is an abstract type that can be imported if you are using TypeScript.

import { File } from '@now/build-utils'

Valid File types include:


import { FileRef } from '@now/build-utils'

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


import { FileFsRef } from '@now/build-utils'

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 the absolute 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


import { FileBlob } from '@now/build-utils'

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


import { Lambda } from '@now/build-utils'

This is a JavaScript class that can be created by supplying files, handler, 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:

  • nodejs10.x
  • nodejs8.10
  • go1.x
  • java-1.8.0-openjdk
  • python3.6
  • python2.7
  • dotnetcore2.1
  • dotnetcore2.0
  • dotnetcore1.0

JavaScript API

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


Signature: createLambda(Object spec) : Lambda

import { createLambda } from '@now/build-utils'

Constructor for the Lambda type.

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


Signature: download() : Files

import { download } from '@now/build-utils'

This utility allows you to download the contents of a Files data structure, 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.

If the optional meta property is passed (the argument for build), only the files that have changed are downloaded. This is decided using filesRemoved and filesChanged inside that object.

await download(files, workPath, meta)


Signature: glob() : Files

import { glob } from '@now/build-utils'

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, download } = require('@now/build-utils')

exports.build = ({ files, workPath }) => {
  await download(files, workPath)
  return glob('**', workPath)


Signature: getWriteableDirectory() : String

import { getWriteableDirectory } from '@now/build-utils'

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


Signature: rename(Files) : Files

import { rename } from '@now/build-utils'

Renames the keys of the Files object, which represent the paths. For example, to remove the *.go suffix you can use:

const rename = require('@now/build-utils')
const originalFiles = { 'one.go': fileFsRef1, 'two.go': fileFsRef2 }
const renamedFiles = rename(originalFiles, path => path.replace(/.go$/, '')