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API Reference ​

The public WebContainer API allows you to build custom applications on top of an in-browser Node.js runtime. Its main entry point is the WebContainer class.

WebContainer ​

The main export of this library. An instance of WebContainer represents a runtime ready to be used.

WebContainer Properties ​


fs: FileSystemAPI ​

Gives access to the underlying file system.


path: string ​

The default value of the PATH environment variable for processes started through spawn.


workdir: string ​

The full path to the working directory (see FileSystemAPI).

WebContainer Methods ​

▸ boot ​

Boots a WebContainer. Only a single instance of WebContainer can be booted concurrently (see teardown). Booting WebContainer is an expensive operation.

Signature

static boot(options: BootOptions = {}): Promise<WebContainer>

Return

Returns a WebContainer instance.

BootOptions

ts
interface BootOptions {
  coep?: 'require-corp' | 'credentialless' | 'none';
  workdirName?: string;
  forwardPreviewErrors?: boolean | 'exceptions-only';
}
interface BootOptions {
  coep?: 'require-corp' | 'credentialless' | 'none';
  workdirName?: string;
  forwardPreviewErrors?: boolean | 'exceptions-only';
}

coep?: 'require-corp' | 'credentialless' | 'none' ​

The value of the COEP header used to load your application.

Choosing 'none' will result in no cross-origin isolation headers being used. This will only work on Chromium-based browsers as long as an Origin Trial is supported.

This value is fixed the first time a WebContainer is booted, and cannot be changed in successive reboots.

workdirName?: string ​

Sets the folder name for the working directory of your WebContainer instance. If not provided, it will be auto-generated.

This is mostly a "cosmetic" option.

forwardPreviewErrors?: boolean | 'exceptions-only' ​

Configure whether errors occurring in embedded preview iframes should be forwarded to the parent page. Captured errors originate from:

  • Calls to console.error
  • Any unhandledrejection events on window
  • Any uncaught error events on window

If set to exceptions-only, console.errors are not forwarded.

Default value is false, so no errors are emitted.

To receive the events, you register an event handler like this:

js
webcontainerInstance.on('preview-message', (message) => {
  // process the message received from a preview
});
webcontainerInstance.on('preview-message', (message) => {
  // process the message received from a preview
});

▸ mount ​

Mounts a tree of files into the filesystem. This can be specified as a FileSystemTree object or as a binary snapshot generated by @webcontainer/snapshot.

Signature

mount(tree: FileSystemTree | Uint8Array | ArrayBuffer, options?: Options): Promise<void>

Options

ts
interface Options {
  mountPoint?: string;
}
interface Options {
  mountPoint?: string;
}

▸ on ​

Listens for an event. The listener is called every time the event gets emitted.

Signature


on(event: 'port' | 'error' | 'server-ready' | 'preview-message', listener: PortListener | ErrorListener | ServerReadyListener | PreviewMessageListener): () => void


Returns

Returns a function to unsubscribe from the events. Once unsubscribed, the listener will no longer be called.


Overloads


▸ on(event: 'port', listener: PortListener): () => void

Listens for port events, which are emitted when a port is opened or closed by some process.

PortListener (Function)

ts
(port: number, type: "open" | "close", url: string): void
(port: number, type: "open" | "close", url: string): void

▸ on(event: 'error', listener: ErrorListener): () => void

Listens for error events, emitted when an internal error is triggered.

ErrorListener (Function)

ts
(error: { message: string }): void
(error: { message: string }): void

▸ on(event: 'preview-message', listener: PreviewMessageListener): () => void

Listens for preview-message events, emitted when an internal error is triggered.

PreviewMessageListener (Function)

ts
(message: PreviewMessage): void
(message: PreviewMessage): void

PreviewMessage

ts
type PreviewMessage = (UncaughtExceptionMessage | UnhandledRejectionMessage | ConsoleErrorMessage) & BasePreviewMessage;

interface BasePreviewMessage {
    previewId: string;
    port: number;
    pathname: string;
    search: string;
    hash: string;
}

interface UncaughtExceptionMessage {
    type: PreviewMessageType.UncaughtException;
    message: string;
    stack: string | undefined;
}

interface UnhandledRejectionMessage {
    type: PreviewMessageType.UnhandledRejection;
    message: string;
    stack: string | undefined;
}

interface ConsoleErrorMessage {
    type: PreviewMessageType.ConsoleError;
    args: any[];
}
type PreviewMessage = (UncaughtExceptionMessage | UnhandledRejectionMessage | ConsoleErrorMessage) & BasePreviewMessage;

interface BasePreviewMessage {
    previewId: string;
    port: number;
    pathname: string;
    search: string;
    hash: string;
}

interface UncaughtExceptionMessage {
    type: PreviewMessageType.UncaughtException;
    message: string;
    stack: string | undefined;
}

interface UnhandledRejectionMessage {
    type: PreviewMessageType.UnhandledRejection;
    message: string;
    stack: string | undefined;
}

interface ConsoleErrorMessage {
    type: PreviewMessageType.ConsoleError;
    args: any[];
}

▸ on(event: 'server-ready', listener: ServerReadyListener): () => void

Listens for server-ready events, emitted when a server was started and ready to receive traffic.

ServerReadyListener (Function)

ts
(port: number, url: string): void
(port: number, url: string): void

▸ spawn ​

Spawns a process. When no args are provided, spawns a process without command-line arguments.

Signature

spawn(command: string, args: string[], options?: SpawnOptions): Promise<WebContainerProcess>

Example

With args:

js
const install = await webcontainerInstance.spawn('npm', ['i']);
const install = await webcontainerInstance.spawn('npm', ['i']);

Without args:

js
const install = await webcontainerInstance.spawn('yarn');
const install = await webcontainerInstance.spawn('yarn');

Returns

Returns a WebContainerProcess.


Overloads


▸ spawn(command: string, args: string[], options?: SpawnOptions): () => void

Spawns a process with additional arguments.

▸ spawn(command: string, options?: SpawnOptions): () => void

Spawns a process without additional arguments.

▸ teardown ​

Destroys the WebContainer instance, turning it unusable, and releases its resources. After this, a new WebContainer instance can be obtained by calling boot.

All entities derived from this instance (e.g. processes, the file system, etc.) also become unusable after calling this method.

Signature

teardown(): void

reloadPreview ​

Added in version 1.2.2.

Reload the provided iframe by sending a message to the iframe and falling back to resetting the src if the iframe didn't respond in time.

Signature


reloadPreview(preview: HTMLIFrameElement, hardRefreshTimeout?: number = 200): Promise<void>


Returns

Returns a Promise that resolves when the reload has completed.

auth ​

The authentication API is exported under the auth namespace. It allows you to authenticate users visiting your website via StackBlitz. In order for users to be authenticated via this method, they must:

  • Be logged in on StackBlitz.
  • Belong to the organisation you used to generate your clientId for use with the WebContainer API.
  • Authorize your website.

Once logged in, you'll be able to install private packages that those users have access to within WebContainer.

auth Functions ​

▸ init ​

Intialize the authentication for use in WebContainer. This method should be called as soon as possible as part of the loading phase of your page. For example at the top of a module that gets loaded as soon as the page loads. This is important for multiple reasons:

  • If you do client side routing, and the OAuth flow is happening, then query parameters might be populated with values related to the OAuth flow. The init function removes them after they've been consumed.

  • If you do the authentication in popup mode, you likely want the popup to be closed as soon as the authentication completed.

Signature

init(options: AuthInitOptions): { status: 'need-auth' | 'authorized' } | AuthFailedError

AuthInitOptions

ts
interface AuthInitOptions {
  /**
   * StackBlitz' origin.
   *
   * @default https://stackblitz.com
   */
  editorOrigin?: string;

  /**
   * The client id for this OAuth application.
   */
  clientId: string;

  /**
   * OAuth scope. The value can be found under your `Teams Settings` > `API`.
   *
   * @see https://www.rfc-editor.org/rfc/rfc6749#section-3.3
   */
  scope: string;
}
interface AuthInitOptions {
  /**
   * StackBlitz' origin.
   *
   * @default https://stackblitz.com
   */
  editorOrigin?: string;

  /**
   * The client id for this OAuth application.
   */
  clientId: string;

  /**
   * OAuth scope. The value can be found under your `Teams Settings` > `API`.
   *
   * @see https://www.rfc-editor.org/rfc/rfc6749#section-3.3
   */
  scope: string;
}

AuthFailedError

ts
interface AuthFailedError {
  status: 'auth-failed';

  /**
   * A short description of the error.
   */
  error: string;

  /**
   * A detailed description of the error.
   */
  description: string;
}
interface AuthFailedError {
  status: 'auth-failed';

  /**
   * A short description of the error.
   */
  error: string;

  /**
   * A detailed description of the error.
   */
  description: string;
}

▸ startAuthFlow ​

This starts the OAuth flow, redirecting the current page to the StackBlitz editor to authenticate the user unless popup is set to true in which case it's done in a popup.

startAuthFlow(options?: { popup?: boolean }): void

▸ loggedIn ​

Returns a promise that resolves when the user authorized your application. This promise is guaranteed to never be rejected.

If the user never authorizes or declines your application, this promise never resolves.

Signature

loggedIn(): Promise<void>

Example

ts
const instance = await WebContainer.boot();

// wait until the user is logged in
await auth.loggedIn();

// we can now fetch private packages from our organisation
await instance.spawn('npm', ['install']);
const instance = await WebContainer.boot();

// wait until the user is logged in
await auth.loggedIn();

// we can now fetch private packages from our organisation
await instance.spawn('npm', ['install']);

▸ logout ​

Logout the user and clear any credentials that were saved locally.

If ignoreRevokeError is set and the revocation failed, the locally-saved credentials are discarded nonetheless.

Signature

logout(options?: { ignoreRevokeError?: boolean }): Promise<void>

▸ on ​

Listens for an event. The listener is called every time the event gets emitted.

Signature


on(event: 'logged-out' | 'auth-failed', listener: () => void | (reason: { error: string, description: string }) => void): () => void


Returns

Returns a function to unsubscribe from the events. Once unsubscribed, the listener will no longer be called.


Overloads


▸ on(event: 'logged-out', listener: () => void): () => void

Listens for logged-out events, which are emitted when the credentials are revoked, meaning the user needs to re-authenticate.


▸ on(event: 'auth-failed', listener: (reason: { error: string, description: string }) => void): () => void

Listens for auth-failed events, which are emitted when the user declines authorization in another tab / popup.

The property error corresponds to a constant that your code can match against, while description is a human readable error that can be useful for development. The possible values of error are:

  • access_denied: The user denied the authorization request.
  • invalid_scope: The scope is invalid, unknown or malformed.

More might be addded in the future, which is reason behind having error typed as a string instead of an union.

DirEnt ​

A representation of a directory entry, see the Node.js API.

DirEnt Properties ​


▸ name ​

The name of the file or directory.

Signature

name: string | Uint8Array

DirEnt Methods ​

▸ isDirectory ​

Whether the entry is a directory.

Signature

isDirectory(): boolean

▸ isFile ​

Whether the entry is a file.

Signature

isFile(): boolean

FileSystemAPI ​

Interface to interact directly with the WebContainer file system. Modeled after fs.promises in Node.

FileSystemAPI Methods ​

▸ mkdir ​

Creates a new directory. If the directory already exists, it will throw an error.

Signature

mkdir(path: String, options?: Options): Promise<void>

Options

When recursive is set to true, it will create any missing folders in the path.

ts
interface Options {
  recursive?: boolean;
}
interface Options {
  recursive?: boolean;
}

▸ readdir ​

Reads a given directory and returns an array of its files and directories.

Signature

readdir(path: string, options?: Options): Promise<Uint8Array[]> | Promise<string[]> | Promise<DirEnt<Uint8Array>[]> | Promise<DirEnt<string>[]>

Options

ts
interface Options {
  encoding?: BufferEncoding;
  withFileTypes?: boolean;
}
interface Options {
  encoding?: BufferEncoding;
  withFileTypes?: boolean;
}

encoding: BufferEncoding ​

The encoding (see BufferEncoding) can be any one of those accepted by Buffer.


withFileTypes: boolean ​

When set to true, the return value is an array of Dirent objects.

▸ readFile ​

Reads the file at the given path. If the file does not exist, it will throw an error.

Signature

readFile(path: string, encoding?: BufferEncoding | null): Promise<Uint8Array> | Promise<string>

By default, it returns a UInt8Array. A second argument can be passed as the encoding.

Example

Without encoding:

ts
const bytes = await webcontainerInstance.fs.readFile('/package.json');
const bytes = await webcontainerInstance.fs.readFile('/package.json');

With a specified encoding:

ts
const content = await webcontainerInstance.fs.readFile('/package.json', 'utf-8');
const content = await webcontainerInstance.fs.readFile('/package.json', 'utf-8');

Returns

Promise<Uint8Array> or Promise<string>

▸ rename ​

Rename the file from oldPath to newPath. The parent folders in newPath must all exist.

Signature

rename(oldPath: string, newPath: string): Promise<void>

Example

Renaming a file:

js
await webcontainerInstance.fs.rename('/src/index.js', '/src/main.js');
await webcontainerInstance.fs.rename('/src/index.js', '/src/main.js');

▸ rm ​

Deletes a file or a directory. If the path is a file, it will delete the file. If the path is a directory, a second argument is needed with option recursive set to true to delete the directory and everything inside it, including nested folders.

Signature

rm(path: string, options?: Options): Promise<void>

Options

ts
interface Options {
  force?: boolean;
  recursive?: boolean;
}
interface Options {
  force?: boolean;
  recursive?: boolean;
}

force?: boolean ​

When true, exceptions will be ignored if the path does not exist.

recursive?: boolean ​

If true, it will recursively remove directories, including nested directories.

Example

Deleting a file:

js
await webcontainerInstance.fs.rm('/src/main.js');
await webcontainerInstance.fs.rm('/src/main.js');

Deleting a directory:

js
await webcontainerInstance.fs.rm('/src', { recursive: true });
await webcontainerInstance.fs.rm('/src', { recursive: true });

▸ writeFile ​

Writes a file to the given path. If the file does not exist, it will create a new one. If the file exists, it will overwrite the file.

Signature

writeFile(path: string, data: string | Uint8Array, options?: string { encoding?: null | BufferEncoding } | null): Promise<void>

Example

Default:

js
await webcontainerInstance.fs.writeFile('/src/main.js', 'console.log("Hello from WebContainers!")');
await webcontainerInstance.fs.writeFile('/src/main.js', 'console.log("Hello from WebContainers!")');

With encoding:

js
await webcontainerInstance.fs.writeFile(
  '/src/main.js',
  '\xE5\x8D\x83\xE8\x91\x89\xE5\xB8\x82\xE3\x83\x96\xE3\x83\xAB\xE3\x83\xBC\xE3\x82\xB9',
  { encoding: 'latin1' }
);
await webcontainerInstance.fs.writeFile(
  '/src/main.js',
  '\xE5\x8D\x83\xE8\x91\x89\xE5\xB8\x82\xE3\x83\x96\xE3\x83\xAB\xE3\x83\xBC\xE3\x82\xB9',
  { encoding: 'latin1' }
);

▸ watch ​

Watch for changes to a given file or directory.

Signature

watch(path: string, options: Options, listener: Listener): Watcher

watch(path: string, listener: Listener): Watcher

Options

ts
interface Options {
  encoding?: BufferEncoding | null;
  recursive?: boolean;
}
interface Options {
  encoding?: BufferEncoding | null;
  recursive?: boolean;
}

encoding?: BufferEncoding | null ​

Specifies the character encoding to be used for the filename passed to the listener. Default: 'utf8'.

recursive?: boolean ​

Indicates whether all subdirectories should be watched, or only the current directory. This applies when a directory is specified. Default: false.

Listener

ts
type Listener = (event: 'rename' | 'change', filename: string | Buffer) => void;
type Listener = (event: 'rename' | 'change', filename: string | Buffer) => void;

Watcher

ts
interface Watcher {
  close(): void;
}
interface Watcher {
  close(): void;
}

close(): void ​

Stop watching for changes on the given Watcher.

Example

Watching a file:

js
webcontainerInstance.fs.watch('/src/main.js', (event) => {
    console.log(`action: ${event}`);
});
webcontainerInstance.fs.watch('/src/main.js', (event) => {
    console.log(`action: ${event}`);
});

Watching a directory:

js
webcontainerInstance.fs.watch('/src', { recursive: true }, (event, filename) => {
    console.log(`file: ${filename} action: ${event}`);
});
webcontainerInstance.fs.watch('/src', { recursive: true }, (event, filename) => {
    console.log(`file: ${filename} action: ${event}`);
});

FileSystemTree ​

A tree-like structure to describe the contents of a folder to be mounted.

ts
interface FileSystemTree {
  [name: string]: FileNode | DirectoryNode;
}
interface FileSystemTree {
  [name: string]: FileNode | DirectoryNode;
}

Also see FileNode and DirectoryNode.

Example

js
const tree = {
  myproject: {
    directory: {
      'foo.js': {
        file: {
          contents: 'const x = 1;',
        },
      },
      '.envrc': {
        file: {
          contents: 'ENVIRONMENT=staging'
        }
      },
    },
  },
  emptyFolder: {
    directory: {}
  },
};
const tree = {
  myproject: {
    directory: {
      'foo.js': {
        file: {
          contents: 'const x = 1;',
        },
      },
      '.envrc': {
        file: {
          contents: 'ENVIRONMENT=staging'
        }
      },
    },
  },
  emptyFolder: {
    directory: {}
  },
};

FileNode ​

ts
interface FileNode {
  file: {
    contents: string | Uint8Array;
  };
}
interface FileNode {
  file: {
    contents: string | Uint8Array;
  };
}

FileNode Properties ​


▸ file: { contents: string | Uint8Array } ​

Represents a file with contents. Also see FileSystemTree.

DirectoryNode ​

ts
interface DirectoryNode {
  directory: FileSystemTree;
}
interface DirectoryNode {
  directory: FileSystemTree;
}

DirectoryNode Properties ​


▸ directory: FileSystemTree ​

Represents a directory node. Also see FileSystemTree.

SpawnOptions ​

Options that control spawning a process.

ts
export interface SpawnOptions {
  cwd?: string;
  env?: Record<string, string | number | boolean>;
  output?: boolean;
  terminal?: { cols: number; rows: number };
}
export interface SpawnOptions {
  cwd?: string;
  env?: Record<string, string | number | boolean>;
  output?: boolean;
  terminal?: { cols: number; rows: number };
}

SpawnOptions Properties ​


▸ cwd?: string ​

Current working directory for the process, relative to workdir this instance (which you can change when booting WebContainer).

By default, the working directory of the spawned process is workdir.


▸ env?: Record<string, string | number | boolean> ​

Environment variables to set for the process.


▸ output?: boolean ​

When set to false, no terminal output is sent back to the process, and the output stream will never produce any chunks.


▸ terminal?: { cols: number; rows: number } ​

The size of the attached terminal.

WebContainerProcess ​

A running process spawned in a WebContainer instance.

WebContainerProcess Properties ​


▸ exit: Promise<number> ​

A promise for the exit code of the process.


▸ input: WritableStream<string> ​

An input stream for the attached pseudoterminal device.


▸ output: ReadableStream<string> ​

A stream that receives all terminal output, including the stdout and stderr emitted by the spawned process and its descendants.

Can be disabled by setting output to false via spawn.

Signature

output: ReadableStream<string>

WebContainerProcess Methods ​

▸ kill ​

Kills the process.

Signature

kill(): void

▸ resize ​

Resizes the attached terminal.

Signature

resize(dimensions: { cols: number, rows: number }): void

BufferEncoding ​

ts
type BufferEncoding =
  | 'ascii'
  | 'utf8'
  | 'utf-8'
  | 'utf16le'
  | 'ucs2'
  | 'ucs-2'
  | 'base64'
  | 'base64url'
  | 'latin1'
  | 'binary'
  | 'hex';
type BufferEncoding =
  | 'ascii'
  | 'utf8'
  | 'utf-8'
  | 'utf16le'
  | 'ucs2'
  | 'ucs-2'
  | 'base64'
  | 'base64url'
  | 'latin1'
  | 'binary'
  | 'hex';