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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.

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;
}
interface BootOptions {
  coep?: 'require-corp' | 'credentialless' | 'none';
  workdirName?: string;
}

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.

▸ 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', listener: PortListener | ErrorListener | ServerReadyListener): () => 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: '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

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;
}
interface Options {
  encoding?: BufferEncoding | null;
}

encoding?: BufferEncoding | null ​

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

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

SpawnOptions Properties ​


▸ 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';