async-off

async-off: make asynchronous TypeScript code synchronous

Usage

async-off path/to/file.ts
cat path/to/file.ts | async-off

async-off --help
async-off -h

async-off converts asynchronous TypeScript code to synchronous code by:

• Removing the keyword await in expressions and in for-await-of loops.
• Removing the keyword async.
• Changing Promise<X> to X.
• Changing AsyncIterable<X> to Iterable<X> and AsyncIterator<X> to Iterator<X>.

More changes can be configured (see below).

What would I use this command for?

async-off’s approach only works for very simple code. Then you can avoid writing very similar code twice – once asynchronously and once synchronously. Examples of such code include:

• Helper functions for iterables: map(), filter(), etc. (see next section)
• A Markdown parser that can be used both asynchronously (with asynchronous iterables as input) and synchronously (with strings or normal iterables as input).

Example

Input:

async function* map<In, Out>(
  mapperFn: (x: In) => Out,
  iterable: AsyncOrSyncIterable<In>
): AsyncIterable<Out> {
  for await (const x of iterable) {
    yield mapperFn(x);
  }
}

//<async>
type AsyncOrSyncIterable<Item> = AsyncIterable<Item> | Iterable<Item>;
//</async>

//<async-off-config>
// {
//   "renameType": {
//     "AsyncOrSyncIterable": "Iterable"
//   }
// }
//</async-off-config>

Output:

function* map<In, Out>(
  mapperFn: (x: In) => Out,
  iterable: Iterable<In>
): Iterable<Out> {
  for (const x of iterable) {
    yield mapperFn(x);
  }
}

Configuration

async-off is configured via embedded JSON (see previous section). The following properties are supported:

interface ConfigJson {
  /**
   * Removes a function call wrapped around a value.
   * Use case – unit testing:
   * 
   * ```ts
   * processAsyncData(toAsyncIterable(['a', 'b', 'c']));
   * processSyncData(['a', 'b', 'c']);
   * ```
   * 
   * Note: (processAsyncData() is renamed separately.)
   */
  unwrapFunctionCall?: Array<string>;
  /**
   * Included by default: `"Promise"`
   */
  unwrapParamerizedType?: Array<string>;
  renameVariable?: Record<string, string>;
  /**
   * Included by default: `{"AsyncIterable": "Iterable", "AsyncIterator": "Iterator"}`
   */
  renameType?: Record<string, string>;
}

Tips

Different names for exports

const Functions = {
  map,
  filter,
};

//<async>
export {Functions as AsyncIterable};
//</async>

//<sync>
// export {Functions as Iterable};
//</sync>

Resources

• For syntax transformations, async-off uses the library ts-morph
  • The TypeScript AST Viewer helps with exploring how TypeScript code is represented as abstract syntax trees.