@expo/spawn-async

What is @expo/spawn-async?

The @expo/spawn-async npm package is a utility for spawning child processes asynchronously in Node.js. It is designed to make it easier to work with child processes by providing a promise-based API. This allows developers to use async/await syntax for cleaner and more readable code when dealing with processes that need to be executed outside of the main Node.js process.

What are @expo/spawn-async's main functionalities?

Executing shell commands

This feature allows you to execute shell commands asynchronously. The code sample demonstrates how to use @expo/spawn-async to run the 'echo' command with 'hello world' as an argument. The output of the command is then logged to the console.

const spawnAsync = require('@expo/spawn-async');

async function runCommand() {
  try {
    const result = await spawnAsync('echo', ['hello world']);
    console.log(result.stdout); // Output: hello world
  } catch (error) {
    console.error(error.stderr);
  }
}

runCommand();

Handling errors

This feature demonstrates error handling when executing commands that might fail or do not exist. The code sample attempts to run a non-existent command, which results in an error being caught and logged.

const spawnAsync = require('@expo/spawn-async');

async function runCommand() {
  try {
    await spawnAsync('some-nonexistent-command');
  } catch (error) {
    console.error('Failed to execute command:', error.stderr);
  }
}

runCommand();

Other packages similar to @expo/spawn-async

child_process

The 'child_process' module is a core Node.js module that provides the ability to spawn child processes. While it offers more control and flexibility compared to @expo/spawn-async, it does not provide a promise-based API out of the box, making @expo/spawn-async more convenient for use with async/await syntax.

execa

Execa is a popular npm package that provides a similar functionality to @expo/spawn-async. It offers a promise-based API for executing shell commands, making it easy to use with async/await. Execa also provides additional features such as better Windows support and the ability to run shell scripts, which might make it a more suitable choice for certain projects.

README

spawn-async

A cross-platform version of Node's child_process.spawn as an async function that returns a promise. Supports Node 12 LTS and up.

Usage:

import spawnAsync from '@expo/spawn-async';

(async function () {
  let resultPromise = spawnAsync('echo', ['hello', 'world']);
  let spawnedChildProcess = resultPromise.child;
  try {
    let {
      pid,
      output: [stdout, stderr],
      stdout,
      stderr,
      status,
      signal,
    } = await resultPromise;
  } catch (e) {
    console.error(e.stack);
    // The error object also has the same properties as the result object
  }
})();

API

spawnAsync takes the same arguments as child_process.spawn. Its options are the same as those of child_process.spawn plus:

• ignoreStdio: whether to ignore waiting for the child process's stdio streams to close before resolving the result promise. When ignoring stdio, the returned values for stdout and stderr will be empty strings. The default value of this option is false.

It returns a promise whose result is an object with these properties:

• pid: the process ID of the spawned child process
• output: an array with stdout and stderr's output
• stdout: a string of what the child process wrote to stdout
• stderr: a string of what the child process wrote to stderr
• status: the exit code of the child process
• signal: the signal (ex: SIGTERM) used to stop the child process if it did not exit on its own

If there's an error running the child process or it exits with a non-zero status code, spawnAsync rejects the returned promise. The Error object also has the properties listed above.

Accessing the child process

Sometimes you may want to access the child process object--for example, if you wanted to attach event handlers to stdio or stderr and process data as it is available instead of waiting for the process to be resolved.

You can do this by accessing .child on the Promise that is returned by spawnAsync.

Here is an example:

(async () => {
  let ffmpeg$ = spawnAsync('ffmpeg', ['-i', 'path/to/source.flac', '-codec:a', 'libmp3lame', '-b:a', '320k', '-ar', '44100', 'path/to/output.mp3']);
  let childProcess = ffmpeg$.child;
  childProcess.stdout.on('data', (data) => {
    console.log(`ffmpeg stdout: ${data}`);
  });
  childProcess.stderr.on('data', (data) => {
    console.error(`ffmpeg stderr: ${data}`);
  });
  let result = await ffmpeg$;
  console.log(`ffmpeg pid ${result.pid} exited with code ${result.code}`);
})();