@prgma/using

using

Bringing Python's with to JS/TS.

Rationale

Python's with syntax provides a concise, idiomatic, repeatable method of automatically performing setup and cleanup for an operation such as reading a file from disk, making a network request, or interacting with a database. In addition, it provides a single logical method of handling any errors that may arise.

Usage

To get started, create a context guard object with an enter method and (optionally) an exit method. The enter method should perform any required setup and return the value you'll be using:

import { using } from '@prgma/using';

const FileGuard = {
  enter() {
    // get handle
    const handle = fs.open('somefile');
    return handle;
  },
  exit(handle) {
    // close handle
    fs.close(handle);
  },
};

using(FileGuard, handle => {
  // do something with prepared handle
  console.log(handle);
});

Functional Form

If you prefer a more functional approach, you can instead provide a function that returns the value to use and (optionally) a cleanup function. If you're returning both, make sure to wrap them in an array:

import { using } from '@prgma/using';

function open(path: string) {
  return function() {
    // get handle
    const handle = fs.open(path);
    return [
      handle,
      handle => {
        // close handle
        fs.close(handle);
      },
    ];
  };
}

using(open('foo.txt'), handle => {
  // do something with prepared handle
  console.log(handle);
});

Asynchronous Setup/Cleanup

Setup and cleanup processes generally require some form of interaction with external services, which is often made simpler by using an async function. Object context guards may have enter return a Promise that resolves to the value to use, which will work as you would expect:

import { using } from '@prgma/using';

async function getApiMessageForUser(userId: number): Promise<string> {
  return `Hello, user ${userId}!`;
}

function getMessage(userId: number): ContextGuards<string> {
  return {
    async enter() {
      const message = await getApiMessageForUser(userId);
      return message;
    },
    exit() {
      // cleanup
    },
  };
}

using(getMessage(1), message => {
  // do something with fetched message
  console.log(message);
});

exit may also be async - if so, you can await the entire using call, which will resolve when the cleanup has finished.

(async () => {
  await using(withAsyncCleanup, () => {/* ... */});
  console.log('cleanup finished!');
})();

Included Guards

Filesystem (using/fs)

The included open guard works just like Python's open - provide a file path and access flags ('r' by default) to get a file descriptor ready to use and automatically cleaned up upon completion:

import { using } from '@prgma/using';
import { open } from '@prgma/using/fs';
import * as fs from 'fs';

using(open('foo.txt', 'rw'), fd => {
  fs.write(fd, 'Hello, world!');
});