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Running Processes ​

This page covers executing commands inside WebContainers.

Mental model ​

In WebContainers, processes are executed as follows:

js
webcontainerInstance.spawn('npm', ['install']);
webcontainerInstance.spawn('npm', ['install']);

The method spawn takes three arguments:

  • a string representing the command,
  • an array with the command options,
  • and, not required, spawn options (for example, specifying environment variables, disabling output, or specifying the size of the attached terminal).

In this case, we are running npm install. If our command had multiple arguments, like for example ls src -l, all the arguments would be added to the array like so:

js
webcontainerInstance.spawn('ls', ['src', '-l']);
webcontainerInstance.spawn('ls', ['src', '-l']);

Installing dependencies ​

Let's take a look at the following function that, when invoked, will install dependencies in a WebContainer instance:

js
async function installDependencies() {
  // Install dependencies
  const installProcess = await webcontainerInstance.spawn('npm', ['install']);
  // Wait for install command to exit
  return installProcess.exit;
}
async function installDependencies() {
  // Install dependencies
  const installProcess = await webcontainerInstance.spawn('npm', ['install']);
  // Wait for install command to exit
  return installProcess.exit;
}

Having installed the dependencies, this function will return the exit code (0 for a successful exit).

Reading process output ​

Calling spawn returns a WebContainerProcess. Every process has an output property of type ReadableStream<string> that receives all terminal output, including the stdout and stderr emitted by the spawned process and its descendants (for example, child processes).

Deep dive: ReadableStream

The output property is a ReadableStream. That’s because it is, in fact, a stream that can emit strings numerous times, just like the actual stdout or stderr from a process in Node.js. The advantage of streams is that they allow composition, meaning we can pipe data from one stream into another stream, for example source.pipeTo(destination). Furthermore, streams can be transferred via postMessage from one context to a different context, for example a web worker. A ReadableStream also keeps a buffer of the data which is only flushed once you start reading.

If you want to read data from output you can pipe it into a WriteableStream just like in the example below.

Output can be disabled by passing optional options to spawn and setting output: false. When set to false, the output stream will never receive any data. An example of a usage could be the following:

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

  installProcess.output.pipeTo(new WritableStream({
    write(data) {
      console.log(data);
    }
  }));
  const installProcess = await webcontainerInstance.spawn('npm', ['install']);

  installProcess.output.pipeTo(new WritableStream({
    write(data) {
      console.log(data);
    }
  }));

Running a dev server ​

WebContainers expose three events through the method on:

  • server-ready, which is emitted when the server is ready to accept requests,
  • port, which is emitted when a port is open or closed by a process,
  • error, emitted when an internal error is triggered.

The server-ready event can be used to, for example, run a dev server via npm run start:

js
async function startDevServer() {
  // Run `npm run start` to start the Express app
  await webcontainerInstance.spawn('npm', ['run', 'start']);

  // Wait for `server-ready` event
  webcontainerInstance.on('server-ready', (port, url) => {
    // ...
  });
}
async function startDevServer() {
  // Run `npm run start` to start the Express app
  await webcontainerInstance.spawn('npm', ['run', 'start']);

  // Wait for `server-ready` event
  webcontainerInstance.on('server-ready', (port, url) => {
    // ...
  });
}

Next steps ​

To further explore the WebContainer API in our WebContainer API starter, check the API Reference or follow our tutorial and build your first WebContainer app.