Plugins

The plugin API is new and still experimental. It may change in the future before version 1.0.0 of esbuild as new use cases are uncovered. Consider this particular plugin API as being specific to the 0.8.x version of esbuild. You can follow the tracking issue for updates about this feature.

The plugin API allows you to inject code into various parts of the build process. Unlike the rest of the API, it's not available from the command line. You must write either JavaScript or Go code to use the plugin API. Plugins can also only be used with the build API, not with the transform API.

Finding plugins

If you're looking for an existing esbuild plugin, you should check out the list of existing esbuild plugins. Plugins on this list have been deliberately added by the author and are intended to be used by others in the esbuild community.

If you want to share your esbuild plugin, you should:

  1. Publish it to npm so others can install it.
  2. Add it to the list of existing esbuild plugins so others can find it.

Using plugins

An esbuild plugin is an object with a name and a setup function. They are passed in an array to the build API call. The setup function is run once for each build API call.

Here's a simple plugin example that allows you to import the current environment variables at build time:

JS Go
let envPlugin = {
  name: 'env',
  setup(build) {
    // Intercept import paths called "env" so esbuild doesn't attempt
    // to map them to a file system location. Tag them with the "env-ns"
    // namespace to reserve them for this plugin.
    build.onResolve({ filter: /^env$/ }, args => ({
      path: args.path,
      namespace: 'env-ns',
    }))

    // Load paths tagged with the "env-ns" namespace and behave as if
    // they point to a JSON file containing the environment variables.
    build.onLoad({ filter: /.*/, namespace: 'env-ns' }, () => ({
      contents: JSON.stringify(process.env),
      loader: 'json',
    }))
  },
}

require('esbuild').build({
  entryPoints: ['app.js'],
  bundle: true,
  outfile: 'out.js',
  plugins: [envPlugin],
}).catch(() => process.exit(1))
package main

import "encoding/json"
import "os"
import "strings"
import "github.com/evanw/esbuild/pkg/api"

var envPlugin = api.Plugin{
  Name: "env",
  Setup: func(build api.PluginBuild) {
    // Intercept import paths called "env" so esbuild doesn't attempt
    // to map them to a file system location. Tag them with the "env-ns"
    // namespace to reserve them for this plugin.
    build.OnResolve(api.OnResolveOptions{Filter: `^env$`},
      func(args api.OnResolveArgs) (api.OnResolveResult, error) {
        return api.OnResolveResult{
          Path:      args.Path,
          Namespace: "env-ns",
        }, nil
      })

    // Load paths tagged with the "env-ns" namespace and behave as if
    // they point to a JSON file containing the environment variables.
    build.OnLoad(api.OnLoadOptions{Filter: `.*`, Namespace: "env-ns"},
      func(args api.OnLoadArgs) (api.OnLoadResult, error) {
        mappings := make(map[string]string)
        for _, item := range os.Environ() {
          if equals := strings.IndexByte(item, '='); equals != -1 {
            mappings[item[:equals]] = item[equals+1:]
          }
        }
        bytes, err := json.Marshal(mappings)
        if err != nil {
          return api.OnLoadResult{}, err
        }
        contents := string(bytes)
        return api.OnLoadResult{
          Contents: &contents,
          Loader:   api.LoaderJSON,
        }, nil
      })
  },
}

func main() {
  result := api.Build(api.BuildOptions{
    EntryPoints: []string{"app.js"},
    Bundle:      true,
    Outfile:     "out.js",
    Plugins:     []api.Plugin{envPlugin},
    Write:       true,
  })

  if len(result.Errors) > 0 {
    os.Exit(1)
  }
}

You would use it like this:

import { PATH } from 'env'
console.log(`PATH is ${PATH}`)

Concepts

Writing a plugin for esbuild works a little differently than writing a plugin for other bundlers. The concepts below are important to understand before developing your plugin:

Namespaces

Every module has an associated namespace. By default esbuild operates in the file namespace, which corresponds to files on the file system. But esbuild can also handle "virtual" modules that don't have a corresponding location on the file system. One case when this happens is when a module is provided using stdin.

Plugins can be used to create virtual modules. Virtual modules usually use a namespace other than file to distinguish them from file system modules. Usually the namespace is specific to the plugin that created them. For example, the sample HTTP plugin below uses the http-url namespace for downloaded files.

Filters

Every callback must provide a regular expression as a filter. This is used by esbuild to skip calling the callback when the path doesn't match its filter, which is done for performance. Calling from esbuild's highly-parallel internals into single-threaded JavaScript code is expensive and should be avoided whenever possible for maximum speed.

You should try to use the filter regular expression instead of using JavaScript code for filtering whenever you can. This is faster because the regular expression is evaluated inside of esbuild without calling out to JavaScript at all. For example, the sample HTTP plugin below uses a filter of ^https?:// to ensure that the performance overhead of running the plugin is only incurred for paths that start with http:// or https://.

The allowed regular expression syntax is the syntax supported by Go's regular expression engine. This is slightly different than JavaScript. Specifically, look-ahead, look-behind, and backreferences are not supported. Go's regular expression engine is designed to avoid the catastrophic exponential-time worst case performance issues that can affect JavaScript regular expressions.

Note that namespaces can also be used for filtering. Callbacks must provide a filter regular expression but can optionally also provide a namespace to further restrict what paths are matched. This can be useful for "remembering" where a virtual module came from. Keep in mind that namespaces are matched using an exact string equality test instead of a regular expression, so unlike module paths they are not intended for storing arbitrary data.

Resolve callbacks

A callback added using onResolve will be run on each import path in each module that esbuild builds. The callback can customize how esbuild does path resolution. For example, it can intercept import paths and redirect them somewhere else. It can also mark paths as external. Here is an example:

JS Go
let exampleOnResolvePlugin = {
  name: 'example',
  setup(build) {
    let path = require('path')

    // Redirect all paths starting with "images/" to "./public/images/"
    build.onResolve({ filter: /^images\// }, args => {
      return { path: path.join(args.resolveDir, 'public', args.path) }
    })

    // Mark all paths starting with "http://" or "https://" as external
    build.onResolve({ filter: /^https?:\/\// }, args => {
      return { path: args.path, external: true }
    })
  },
}

require('esbuild').build({
  entryPoints: ['app.js'],
  bundle: true,
  outfile: 'out.js',
  plugins: [exampleOnResolvePlugin],
  loader: { '.png': 'binary' },
}).catch(() => process.exit(1))
package main

import "os"
import "path/filepath"
import "github.com/evanw/esbuild/pkg/api"

var exampleOnResolvePlugin = api.Plugin{
  Name: "example",
  Setup: func(build api.PluginBuild) {
    // Redirect all paths starting with "images/" to "./public/images/"
    build.OnResolve(api.OnResolveOptions{Filter: `^images/`},
      func(args api.OnResolveArgs) (api.OnResolveResult, error) {
        return api.OnResolveResult{
          Path: filepath.Join(args.ResolveDir, "public", args.Path),
        }, nil
      })

    // Mark all paths starting with "http://" or "https://" as external
    build.OnResolve(api.OnResolveOptions{Filter: `^https?://`},
      func(args api.OnResolveArgs) (api.OnResolveResult, error) {
        return api.OnResolveResult{
          Path:     args.Path,
          External: true,
        }, nil
      })
  },
}

func main() {
  result := api.Build(api.BuildOptions{
    EntryPoints: []string{"app.js"},
    Bundle:      true,
    Outfile:     "out.js",
    Plugins:     []api.Plugin{exampleOnResolvePlugin},
    Write:       true,
    Loader: map[string]api.Loader{
      ".png": api.LoaderBinary,
    },
  })

  if len(result.Errors) > 0 {
    os.Exit(1)
  }
}

The callback can return without providing a path to pass on responsibility for path resolution to the next callback. For a given import path, all onResolve callbacks from all plugins will be run in the order they were registered until one takes responsibility for path resolution. If no callback returns a path, esbuild will run its default path resolution logic.

Keep in mind that many callbacks may be running concurrently. In JavaScript, if your callback does expensive work that can run on another thread such as fs.existsSync(), you should make the callback async and use await (in this case with fs.promises.exists()) to allow other code to run in the meantime. In Go, each callback may be run on a separate goroutine. Make sure you have appropriate synchronization in place if your plugin uses any shared data structures.

Resolve options

The onResolve API is meant to be called within the setup function and registers a callback to be triggered in certain situations. It takes a few options:

JS Go
interface OnResolveOptions {
  filter: RegExp;
  namespace?: string;
}
type OnResolveOptions struct {
  Filter    string
  Namespace string
}

Resolve arguments

When esbuild calls the callback registered by onResolve, it will provide these arguments with information about the imported path:

JS Go
interface OnResolveArgs {
  path: string;
  importer: string;
  namespace: string;
  resolveDir: string;
  kind: ResolveKind;
  pluginData: any;
}

type ResolveKind =
  | 'entry-point'
  | 'import-statement'
  | 'require-call'
  | 'dynamic-import'
  | 'require-resolve'
  | 'import-rule'
  | 'url-token'
type OnResolveArgs struct {
  Path       string
  Importer   string
  Namespace  string
  ResolveDir string
  Kind       ResolveKind
  PluginData interface{}
}

const (
  ResolveEntryPoint        ResolveKind
  ResolveJSImportStatement ResolveKind
  ResolveJSRequireCall     ResolveKind
  ResolveJSDynamicImport   ResolveKind
  ResolveJSRequireResolve  ResolveKind
  ResolveCSSImportRule     ResolveKind
  ResolveCSSURLToken       ResolveKind
)

Resolve results

This is the object that can be returned by a callback added using onResolve to provide a custom path resolution. If you would like to return from the callback without providing a path, just return the default value (so undefined in JavaScript and OnResolveResult{} in Go). Here are the optional properties that can be returned:

JS Go
interface OnResolveResult {
  errors?: Message[];
  external?: boolean;
  namespace?: string;
  path?: string;
  pluginData?: any;
  pluginName?: string;
  warnings?: Message[];
  watchDirs?: string[];
  watchFiles?: string[];
}

interface Message {
  text: string;
  location: Location | null;
  detail: any; // The original error from a JavaScript plugin, if applicable
}

interface Location {
  file: string;
  namespace: string;
  line: number; // 1-based
  column: number; // 0-based, in bytes
  length: number; // in bytes
  lineText: string;
}
type OnResolveResult struct {
  Errors     []Message
  External   bool
  Namespace  string
  Path       string
  PluginData interface{}
  PluginName string
  Warnings   []Message
  WatchDirs  []string
  WatchFiles []string
}

type Message struct {
  Text     string
  Location *Location
  Detail   interface{} // The original error from a Go plugin, if applicable
}

type Location struct {
  File      string
  Namespace string
  Line      int // 1-based
  Column    int // 0-based, in bytes
  Length    int // in bytes
  LineText  string
}

Load callbacks

A callback added using onLoad will be run for each unique path/namespace pair that has not been marked as external. Its job is to return the contents of the module and to tell esbuild how to interpret it. Here's an example plugin that converts .txt files into an array of words:

JS Go
let exampleOnLoadPlugin = {
  name: 'example',
  setup(build) {
    let fs = require('fs')

    // Load ".txt" files and return an array of words
    build.onLoad({ filter: /\.txt$/ }, async (args) => {
      let text = await fs.promises.readFile(args.path, 'utf8')
      return {
        contents: JSON.stringify(text.split(/\s+/)),
        loader: 'json',
      }
    })
  },
}

require('esbuild').build({
  entryPoints: ['app.js'],
  bundle: true,
  outfile: 'out.js',
  plugins: [exampleOnLoadPlugin],
}).catch(() => process.exit(1))
package main

import "encoding/json"
import "io/ioutil"
import "os"
import "strings"
import "github.com/evanw/esbuild/pkg/api"

var exampleOnLoadPlugin = api.Plugin{
  Name: "example",
  Setup: func(build api.PluginBuild) {
    // Load ".txt" files and return an array of words
    build.OnLoad(api.OnLoadOptions{Filter: `\.txt$`},
      func(args api.OnLoadArgs) (api.OnLoadResult, error) {
        text, err := ioutil.ReadFile(args.Path)
        if err != nil {
          return api.OnLoadResult{}, err
        }
        bytes, err := json.Marshal(strings.Fields(string(text)))
        if err != nil {
          return api.OnLoadResult{}, err
        }
        contents := string(bytes)
        return api.OnLoadResult{
          Contents: &contents,
          Loader:   api.LoaderJSON,
        }, nil
      })
  },
}

func main() {
  result := api.Build(api.BuildOptions{
    EntryPoints: []string{"app.js"},
    Bundle:      true,
    Outfile:     "out.js",
    Plugins:     []api.Plugin{exampleOnLoadPlugin},
    Write:       true,
  })

  if len(result.Errors) > 0 {
    os.Exit(1)
  }
}

The callback can return without providing the contents of the module. In that case the responsibility for loading the module is passed to the next registered callback. For a given module, all onLoad callbacks from all plugins will be run in the order they were registered until one takes responsibility for loading the module. If no callback returns contents for the module, esbuild will run its default module loading logic.

Keep in mind that many callbacks may be running concurrently. In JavaScript, if your callback does expensive work that can run on another thread such as fs.readFileSync(), you should make the callback async and use await (in this case with fs.promises.readFile()) to allow other code to run in the meantime. In Go, each callback may be run on a separate goroutine. Make sure you have appropriate synchronization in place if your plugin uses any shared data structures.

Load options

The onLoad API is meant to be called within the setup function and registers a callback to be triggered in certain situations. It takes a few options:

JS Go
interface OnLoadOptions {
  filter: RegExp;
  namespace?: string;
}
type OnLoadOptions struct {
  Filter    string
  Namespace string
}

Load arguments

When esbuild calls the callback registered by onLoad, it will provide these arguments with information about the module to load:

JS Go
interface OnLoadArgs {
  path: string;
  namespace: string;
  pluginData: any;
}
type OnLoadArgs struct {
  Path       string
  Namespace  string
  PluginData interface{}
}

Load results

This is the object that can be returned by a callback added using onLoad to provide the contents of a module. If you would like to return from the callback without providing any contents, just return the default value (so undefined in JavaScript and OnLoadResult{} in Go). Here are the optional properties that can be returned:

JS Go
interface OnLoadResult {
  contents?: string | Uint8Array;
  errors?: Message[];
  loader?: Loader;
  pluginData?: any;
  pluginName?: string;
  resolveDir?: string;
  warnings?: Message[];
  watchDirs?: string[];
  watchFiles?: string[];
}

interface Message {
  text: string;
  location: Location | null;
  detail: any; // The original error from a JavaScript plugin, if applicable
}

interface Location {
  file: string;
  namespace: string;
  line: number; // 1-based
  column: number; // 0-based, in bytes
  length: number; // in bytes
  lineText: string;
}
type OnLoadResult struct {
  Contents   *string
  Errors     []Message
  Loader     Loader
  PluginData interface{}
  PluginName string
  ResolveDir string
  Warnings   []Message
  WatchDirs  []string
  WatchFiles []string
}

type Message struct {
  Text     string
  Location *Location
  Detail   interface{} // The original error from a Go plugin, if applicable
}

type Location struct {
  File      string
  Namespace string
  Line      int // 1-based
  Column    int // 0-based, in bytes
  Length    int // in bytes
  LineText  string
}

Caching your plugin

Since esbuild is so fast, it's often the case that plugin evaluation is the main bottleneck when building with esbuild. Caching of plugin evaluation is left up to each plugin instead of being a part of esbuild itself because cache invalidation is plugin-specific. If you are writing a slow plugin that needs a cache to be fast, you will have to write the cache logic yourself.

A cache is essentially a map that memoizes the transform function that represents your plugin. The keys of the map usually contain the inputs to your transform function and the values of the map usually contain the outputs of your transform function. In addition, the map usually has some form of least-recently-used cache eviction policy to avoid continually growing larger in size over time.

The cache can either be stored in memory (beneficial for use with esbuild's incremental build API), on disk (beneficial for caching across separate build script invocations), or even on a server (beneficial for really slow transforms that can be shared between different developer machines). Where to store the cache is case-specific and depends on your plugin.

Here is a simple caching example. Say we want to cache the function slowTransform() that takes as input the contents of a file in the *.example format and transforms it to JavaScript. An in-memory cache that avoids redundant calls to this function when used with esbuild's incremental build API) might look something like this:

let examplePlugin = {
  name: 'example',
  setup(build) {
    let fs = require('fs')
    let cache = new Map

    build.onLoad({ filter: /\.example$/ }, async (args) => {
      let input = await fs.promises.readFile(args.path, 'utf8')
      let key = args.path
      let value = cache.get(key)

      if (!value || value.input !== input) {
        let contents = slowTransform(input)
        value = { input, output: { contents } }
        cache.set(key, value)
      }

      return value.output
    })
  }
}

Some important caveats about the caching code above:

Accessing build options

Plugins can access the initial build options from within the setup method. This lets you inspect how the build is configured as well as modify the build options before the build starts. Here is an example:

JS Go
let examplePlugin = {
  name: 'auto-node-env',
  setup(build) {
    const options = build.initialOptions
    options.define = options.define || {}
    options.define['process.env.NODE_ENV'] =
      options.minify ? '"production"' : '"development"'
  },
}
package main

import "github.com/evanw/esbuild/pkg/api"

var examplePlugin = api.Plugin{
  Name: "auto-node-env",
  Setup: func(build api.PluginBuild) {
    options := build.InitialOptions
    if options.Define == nil {
      options.Define = map[string]string{}
    }
    if options.MinifyWhitespace && options.MinifyIdentifiers && options.MinifySyntax {
      options.Define[`process.env.NODE_ENV`] = `"production"`
    } else {
      options.Define[`process.env.NODE_ENV`] = `"development"`
    }
  },
}

func main() {
}

Note that modifications to the build options after the build starts do not affect the build. In particular, incremental rebuilds, watch mode, and serve mode do not update their build options if plugins mutate the build options object after the first build has started.

Example plugins

The example plugins below are meant to give you an idea of the different types of things you can do with the plugin API.

HTTP plugin

This example demonstrates: using a path format other than file system paths, namespace-specific path resolution, using resolve and load callbacks together.

This plugin allows you to import HTTP URLs into JavaScript code. The code will automatically be downloaded at build time. It enables the following workflow:

import { zip } from 'https://unpkg.com/lodash-es@4.17.15/lodash.js'
console.log(zip([1, 2], ['a', 'b']))

This can be accomplished with the following plugin. Note that for real usage the downloads should be cached, but caching has been omitted from this example for brevity:

JS Go
let httpPlugin = {
  name: 'http',
  setup(build) {
    let https = require('https')
    let http = require('http')

    // Intercept import paths starting with "http:" and "https:" so
    // esbuild doesn't attempt to map them to a file system location.
    // Tag them with the "http-url" namespace to associate them with
    // this plugin.
    build.onResolve({ filter: /^https?:\/\// }, args => ({
      path: args.path,
      namespace: 'http-url',
    }))

    // We also want to intercept all import paths inside downloaded
    // files and resolve them against the original URL. All of these
    // files will be in the "http-url" namespace. Make sure to keep
    // the newly resolved URL in the "http-url" namespace so imports
    // inside it will also be resolved as URLs recursively.
    build.onResolve({ filter: /.*/, namespace: 'http-url' }, args => ({
      path: new URL(args.path, args.importer).toString(),
      namespace: 'http-url',
    }))

    // When a URL is loaded, we want to actually download the content
    // from the internet. This has just enough logic to be able to
    // handle the example import from unpkg.com but in reality this
    // would probably need to be more complex.
    build.onLoad({ filter: /.*/, namespace: 'http-url' }, async (args) => {
      let contents = await new Promise((resolve, reject) => {
        function fetch(url) {
          console.log(`Downloading: ${url}`)
          let lib = url.startsWith('https') ? https : http
          let req = lib.get(url, res => {
            if ([301, 302, 307].includes(res.statusCode)) {
              fetch(new URL(res.headers.location, url).toString())
              req.abort()
            } else if (res.statusCode === 200) {
              let chunks = []
              res.on('data', chunk => chunks.push(chunk))
              res.on('end', () => resolve(Buffer.concat(chunks)))
            } else {
              reject(new Error(`GET ${url} failed: status ${res.statusCode}`))
            }
          }).on('error', reject)
        }
        fetch(args.path)
      })
      return { contents }
    })
  },
}

require('esbuild').build({
  entryPoints: ['app.js'],
  bundle: true,
  outfile: 'out.js',
  plugins: [httpPlugin],
}).catch(() => process.exit(1))
package main

import "io/ioutil"
import "net/http"
import "net/url"
import "os"
import "github.com/evanw/esbuild/pkg/api"

var httpPlugin = api.Plugin{
  Name: "http",
  Setup: func(build api.PluginBuild) {
    // Intercept import paths starting with "http:" and "https:" so
    // esbuild doesn't attempt to map them to a file system location.
    // Tag them with the "http-url" namespace to associate them with
    // this plugin.
    build.OnResolve(api.OnResolveOptions{Filter: `^https?://`},
      func(args api.OnResolveArgs) (api.OnResolveResult, error) {
        return api.OnResolveResult{
          Path:      args.Path,
          Namespace: "http-url",
        }, nil
      })

    // We also want to intercept all import paths inside downloaded
    // files and resolve them against the original URL. All of these
    // files will be in the "http-url" namespace. Make sure to keep
    // the newly resolved URL in the "http-url" namespace so imports
    // inside it will also be resolved as URLs recursively.
    build.OnResolve(api.OnResolveOptions{Filter: ".*", Namespace: "http-url"},
      func(args api.OnResolveArgs) (api.OnResolveResult, error) {
        base, err := url.Parse(args.Importer)
        if err != nil {
          return api.OnResolveResult{}, err
        }
        relative, err := url.Parse(args.Path)
        if err != nil {
          return api.OnResolveResult{}, err
        }
        return api.OnResolveResult{
          Path:      base.ResolveReference(relative).String(),
          Namespace: "http-url",
        }, nil
      })

    // When a URL is loaded, we want to actually download the content
    // from the internet. This has just enough logic to be able to
    // handle the example import from unpkg.com but in reality this
    // would probably need to be more complex.
    build.OnLoad(api.OnLoadOptions{Filter: ".*", Namespace: "http-url"},
      func(args api.OnLoadArgs) (api.OnLoadResult, error) {
        res, err := http.Get(args.Path)
        if err != nil {
          return api.OnLoadResult{}, err
        }
        defer res.Body.Close()
        bytes, err := ioutil.ReadAll(res.Body)
        if err != nil {
          return api.OnLoadResult{}, err
        }
        contents := string(bytes)
        return api.OnLoadResult{Contents: &contents}, nil
      })
  },
}

func main() {
  result := api.Build(api.BuildOptions{
    EntryPoints: []string{"app.js"},
    Bundle:      true,
    Outfile:     "out.js",
    Plugins:     []api.Plugin{httpPlugin},
    Write:       true,
  })

  if len(result.Errors) > 0 {
    os.Exit(1)
  }
}

The plugin first uses a resolver to move http:// and https:// URLs to the http-url namespace. Setting the namespace tells esbuild to not treat these paths as file system paths. Then, a loader for the http-url namespace downloads the module and returns the contents to esbuild. From there, another resolver for import paths inside modules in the http-url namespace picks up relative paths and translates them into full URLs by resolving them against the importing module's URL. That then feeds back into the loader allowing downloaded modules to download additional modules recursively.

WebAssembly plugin

This example demonstrates: working with binary data, creating virtual; modules using import statements, re-using the same path with different namespaces.

This plugin allows you to import .wasm files into JavaScript code. It does not generate the WebAssembly files themselves; that can either be done by another tool or by modifying this example plugin to suit your needs. It enables the following workflow:

import load from './example.wasm'
load(imports).then(exports => { ... })

When you import a .wasm file, this plugin generates a virtual JavaScript module in the wasm-stub namespace with a single function that loads the WebAssembly module exported as the default export. That stub module looks something like this:

import wasm from '/path/to/example.wasm'
export default (imports) =>
  WebAssembly.instantiate(wasm, imports).then(
    result => result.instance.exports)

Then that stub module imports the WebAssembly file itself as another module in the wasm-binary namespace using esbuild's built-in binary loader. This means importing a .wasm file actually generates two virtual modules. Here's the code for the plugin:

JS Go
let wasmPlugin = {
  name: 'wasm',
  setup(build) {
    let path = require('path')
    let fs = require('fs')

    // Resolve ".wasm" files to a path with a namespace
    build.onResolve({ filter: /\.wasm$/ }, args => {
      // If this is the import inside the stub module, import the
      // binary itself. Put the path in the "wasm-binary" namespace
      // to tell our binary load callback to load the binary file.
      if (args.namespace === 'wasm-stub') {
        return {
          path: args.path,
          namespace: 'wasm-binary',
        }
      }

      // Otherwise, generate the JavaScript stub module for this
      // ".wasm" file. Put it in the "wasm-stub" namespace to tell
      // our stub load callback to fill it with JavaScript.
      //
      // Resolve relative paths to absolute paths here since this
      // resolve callback is given "resolveDir", the directory to
      // resolve imports against.
      if (args.resolveDir === '') {
        return // Ignore unresolvable paths
      }
      return {
        path: path.isAbsolute(args.path) ? args.path : path.join(args.resolveDir, args.path),
        namespace: 'wasm-stub',
      }
    })

    // Virtual modules in the "wasm-stub" namespace are filled with
    // the JavaScript code for compiling the WebAssembly binary. The
    // binary itself is imported from a second virtual module.
    build.onLoad({ filter: /.*/, namespace: 'wasm-stub' }, async (args) => ({
      contents: `import wasm from ${JSON.stringify(args.path)}
        export default (imports) =>
          WebAssembly.instantiate(wasm, imports).then(
            result => result.instance.exports)`,
    }))

    // Virtual modules in the "wasm-binary" namespace contain the
    // actual bytes of the WebAssembly file. This uses esbuild's
    // built-in "binary" loader instead of manually embedding the
    // binary data inside JavaScript code ourselves.
    build.onLoad({ filter: /.*/, namespace: 'wasm-binary' }, async (args) => ({
      contents: await fs.promises.readFile(args.path),
      loader: 'binary',
    }))
  },
}

require('esbuild').build({
  entryPoints: ['app.js'],
  bundle: true,
  outfile: 'out.js',
  plugins: [wasmPlugin],
}).catch(() => process.exit(1))
package main

import "encoding/json"
import "io/ioutil"
import "os"
import "path/filepath"
import "github.com/evanw/esbuild/pkg/api"

var wasmPlugin = api.Plugin{
  Name: "wasm",
  Setup: func(build api.PluginBuild) {
    // Resolve ".wasm" files to a path with a namespace
    build.OnResolve(api.OnResolveOptions{Filter: `\.wasm$`},
      func(args api.OnResolveArgs) (api.OnResolveResult, error) {
        // If this is the import inside the stub module, import the
        // binary itself. Put the path in the "wasm-binary" namespace
        // to tell our binary load callback to load the binary file.
        if args.Namespace == "wasm-stub" {
          return api.OnResolveResult{
            Path:      args.Path,
            Namespace: "wasm-binary",
          }, nil
        }

        // Otherwise, generate the JavaScript stub module for this
        // ".wasm" file. Put it in the "wasm-stub" namespace to tell
        // our stub load callback to fill it with JavaScript.
        //
        // Resolve relative paths to absolute paths here since this
        // resolve callback is given "resolveDir", the directory to
        // resolve imports against.
        if args.ResolveDir == "" {
          return api.OnResolveResult{}, nil // Ignore unresolvable paths
        }
        if !filepath.IsAbs(args.Path) {
          args.Path = filepath.Join(args.ResolveDir, args.Path)
        }
        return api.OnResolveResult{
          Path:      args.Path,
          Namespace: "wasm-stub",
        }, nil
      })

    // Virtual modules in the "wasm-stub" namespace are filled with
    // the JavaScript code for compiling the WebAssembly binary. The
    // binary itself is imported from a second virtual module.
    build.OnLoad(api.OnLoadOptions{Filter: `.*`, Namespace: "wasm-stub"},
      func(args api.OnLoadArgs) (api.OnLoadResult, error) {
        bytes, err := json.Marshal(args.Path)
        if err != nil {
          return api.OnLoadResult{}, err
        }
        contents := `import wasm from ` + string(bytes) + `
          export default (imports) =>
            WebAssembly.instantiate(wasm, imports).then(
              result => result.instance.exports)`
        return api.OnLoadResult{Contents: &contents}, nil
      })

    // Virtual modules in the "wasm-binary" namespace contain the
    // actual bytes of the WebAssembly file. This uses esbuild's
    // built-in "binary" loader instead of manually embedding the
    // binary data inside JavaScript code ourselves.
    build.OnLoad(api.OnLoadOptions{Filter: `.*`, Namespace: "wasm-binary"},
      func(args api.OnLoadArgs) (api.OnLoadResult, error) {
        bytes, err := ioutil.ReadFile(args.Path)
        if err != nil {
          return api.OnLoadResult{}, err
        }
        contents := string(bytes)
        return api.OnLoadResult{
          Contents: &contents,
          Loader:   api.LoaderBinary,
        }, nil
      })
  },
}

func main() {
  result := api.Build(api.BuildOptions{
    EntryPoints: []string{"app.js"},
    Bundle:      true,
    Outfile:     "out.js",
    Plugins:     []api.Plugin{wasmPlugin},
    Write:       true,
  })

  if len(result.Errors) > 0 {
    os.Exit(1)
  }
}

The plugin works in multiple steps. First, a resolve callback captures .wasm paths in normal modules and moves them to the wasm-stub namespace. Then load callback for the wasm-stub namespace generates a JavaScript stub module that exports the loader function and imports the .wasm path. This invokes the resolve callback again which this time moves the path to the wasm-binary namespace. Then the second load callback for the wasm-binary namespace causes the WebAssembly file to be loaded using the binary loader, which tells esbuild to embed the file itself in the bundle.

Svelte plugin

This example demonstrates: supporting a compile-to-JavaScript language, reporting warnings and errors, integrating source maps.

This plugin allows you to bundle .svelte files, which are from the Svelte framework. You write code in an HTML-like syntax that is then converted to JavaScript by the Svelte compiler. Svelte code looks something like this:

<script>
  let a = 1;
  let b = 2;
</script>
<input type="number" bind:value={a}>
<input type="number" bind:value={b}>
<p>{a} + {b} = {a + b}</p>

Compiling this code with the Svelte compiler generates a JavaScript module that depends on the svelte/internal package and that exports the component as a a single class using the default export. This means .svelte files can be compiled independently, which makes Svelte a good fit for an esbuild plugin. This plugin is triggered by importing a .svelte file like this:

import Button from './button.svelte'

Here's the code for the plugin (there is no Go version of this plugin because the Svelte compiler is written in JavaScript):

let sveltePlugin = {
  name: 'svelte',
  setup(build) {
    let svelte = require('svelte/compiler')
    let path = require('path')
    let fs = require('fs')

    build.onLoad({ filter: /\.svelte$/ }, async (args) => {
      // This converts a message in Svelte's format to esbuild's format
      let convertMessage = ({ message, start, end }) => {
        let location
        if (start && end) {
          let lineText = source.split(/\r\n|\r|\n/g)[start.line - 1]
          let lineEnd = start.line === end.line ? end.column : lineText.length
          location = {
            file: filename,
            line: start.line,
            column: start.column,
            length: lineEnd - start.column,
            lineText,
          }
        }
        return { text: message, location }
      }

      // Load the file from the file system
      let source = await fs.promises.readFile(args.path, 'utf8')
      let filename = path.relative(process.cwd(), args.path)

      // Convert Svelte syntax to JavaScript
      try {
        let { js, warnings } = svelte.compile(source, { filename })
        let contents = js.code + `//# sourceMappingURL=` + js.map.toUrl()
        return { contents, warnings: warnings.map(convertMessage) }
      } catch (e) {
        return { errors: [convertMessage(e)] }
      }
    })
  }
}

require('esbuild').build({
  entryPoints: ['app.js'],
  bundle: true,
  outfile: 'out.js',
  plugins: [sveltePlugin],
}).catch(() => process.exit(1))

This plugin only needs a load callback, not a resolve callback, because it's simple enough that it just needs to transform the loaded code into JavaScript without worrying about where the code comes from.

It appends a //# sourceMappingURL= comment to the generated JavaScript to tell esbuild how to map the generated JavaScript back to the original source code. If source maps are enabled during the build, esbuild will use this to ensure that the generated positions in the final source map are mapped all the way back to the original Svelte file instead of to the intermediate JavaScript code.

Plugin API limitations

This API does not intend to cover all use cases. It's not possible to hook into every part of the bundling process. For example, it's not currently possible to modify the AST directly. This restriction exists to preserve the excellent performance characteristics of esbuild as well as to avoid exposing too much API surface which would be a maintenance burden and would prevent improvements that involve changing the AST.

One way to think about esbuild is as a "linker" for the web. Just like a linker for native code, esbuild's job is to take a set of files, resolve and bind references between them, and generate a single file containing all of the code linked together. A plugin's job is to generate the individual files that end up being linked.

Plugins in esbuild work best when they are relatively scoped and only customize a small aspect of the build. For example, a plugin for a special configuration file in a custom format (e.g. YAML) is very appropriate. The more plugins you use, the slower your build will get, especially if your plugin is written in JavaScript. If a plugin applies to every file in your build, then your build will likely be very slow. If caching is applicable, it must be done by the plugin itself.