gensync

What is gensync?

The gensync package allows for the creation and management of generator functions or functions that return promises, enabling them to be used in a synchronous-like manner without blocking the event loop. It provides a way to write asynchronous code that looks and behaves like synchronous code, making it easier to read and maintain. This can be particularly useful in scenarios where asynchronous operations need to be performed in a specific sequence or when dealing with complex control flow.

What are gensync's main functionalities?

Handling asynchronous operations synchronously

This feature allows for the execution of asynchronous operations in a way that resembles synchronous code execution. The code sample demonstrates how to define an asynchronous operation using gensync and execute it, handling the result with a promise.

const gensync = require('gensync');

const asyncOperation = gensync(function* () {
  const result = yield Promise.resolve('Result of async operation');
  return result;
});

asyncOperation().then(console.log); // Logs: 'Result of async operation'

Error handling in asynchronous operations

This feature demonstrates how gensync can be used to handle errors in asynchronous operations in a synchronous-like manner. The code sample shows how to catch and handle errors using try/catch blocks within a gensync function.

const gensync = require('gensync');

const asyncOperationWithError = gensync(function* () {
  try {
    const result = yield Promise.reject(new Error('Error occurred'));
    return result;
  } catch (error) {
    return 'Error handled';
  }
});

asyncOperationWithError().then(console.log); // Logs: 'Error handled'

Other packages similar to gensync

co

The 'co' package is similar to gensync in that it also allows for managing and handling asynchronous operations using generator functions. However, 'co' focuses more on leveraging generators to simplify asynchronous code, whereas gensync provides a broader set of utilities for working with both generator functions and functions returning promises.

bluebird

While 'bluebird' is primarily known as a promise library offering enhanced performance and additional features compared to native Promises, it also includes utilities for converting generator functions into functions that return promises, similar to what gensync offers. However, gensync is more focused on the seamless integration of synchronous and asynchronous code through generators and promises.

README

gensync

This module allows for developers to write common code that can share implementation details, hiding whether an underlying request happens synchronously or asynchronously. This is in contrast with many current Node APIs which explicitly implement the same API twice, once with calls to synchronous functions, and once with asynchronous functions.

Take for example fs.readFile and fs.readFileSync, if you're writing an API that loads a file and then performs a synchronous operation on the data, it can be frustrating to maintain two parallel functions.

Example

const fs = require("fs");
const gensync = require("gensync");

const readFile = gensync({
  sync: fs.readFileSync,
  errback: fs.readFile,
});

const myOperation = gensync(function* (filename) {
  const code = yield* readFile(filename, "utf8");

  return "// some custom prefix\n" + code;
});

// Load and add the prefix synchronously:
const result = myOperation.sync("./some-file.js");

// Load and add the prefix asynchronously with promises:
myOperation.async("./some-file.js").then(result => {

});

// Load and add the prefix asynchronously with promises:
myOperation.errback("./some-file.js", (err, result) => {

});

This could even be exposed as your official API by doing

// Using the common 'Sync' suffix for sync functions, and 'Async' suffix for
// promise-returning versions.
exports.myOperationSync = myOperation.sync;
exports.myOperationAsync = myOperation.async;
exports.myOperation = myOperation.errback;

or potentially expose one of the async versions as the default, with a .sync property on the function to expose the synchronous version.

module.exports = myOperation.errback;
module.exports.sync = myOperation.sync;

API

gensync(generatorFnOrOptions)

Returns a function that can be "await"-ed in another gensync generator function, or executed via

• .sync(...args) - Returns the computed value, or throws.
• .async(...args) - Returns a promise for the computed value.
• .errback(...args, (err, result) => {}) - Calls the callback with the computed value, or error.

Passed a generator

Wraps the generator to populate the .sync/.async/.errback helpers above to allow for evaluation of the generator for the final value.

Example

const readFile = function* () {
  return 42;
};

const readFileAndMore = gensync(function* (){
  const val = yield* readFile();
  return 42 + val;
});

// In general cases
const code = readFileAndMore.sync("./file.js", "utf8");
readFileAndMore.async("./file.js", "utf8").then(code => {})
readFileAndMore.errback("./file.js", "utf8", (err, code) => {});

// In a generator being called indirectly with .sync/.async/.errback
const code = yield* readFileAndMore("./file.js", "utf8");

Passed an options object

• opts.sync

  Example: (...args) => 4

  A function that will be called when .sync() is called on the gensync() result, or when the result is passed to yield* in another generator that is being run synchronously.

  Also called for .async() calls if no async handlers are provided.

• opts.async

  Example: async (...args) => 4

  A function that will be called when .async() or .errback() is called on the gensync() result, or when the result is passed to yield* in another generator that is being run asynchronously.

• opts.errback

  Example: (...args, cb) => cb(null, 4)

  A function that will be called when .async() or .errback() is called on the gensync() result, or when the result is passed to yield* in another generator that is being run asynchronously.

  This option allows for simpler compatibility with many existing Node APIs, and also avoids introducing the extra even loop turns that promises introduce to access the result value.

• opts.name

  Example: "readFile"

  A string name to apply to the returned function. If no value is provided, the name of errback/async/sync functions will be used, with any Sync or Async suffix stripped off. If the callback is simply named with ES6 inference (same name as the options property), the name is ignored.

• opts.arity

  Example: 4

  A number for the length to set on the returned function. If no value is provided, the length will be carried over from the sync function's length value.

Example

const readFile = gensync({
  sync: fs.readFileSync,
  errback: fs.readFile,
});

const code = readFile.sync("./file.js", "utf8");
readFile.async("./file.js", "utf8").then(code => {})
readFile.errback("./file.js", "utf8", (err, code) => {});

gensync.all(iterable)

Promise.all-like combinator that works with an iterable of generator objects that could be passed to yield* within a gensync generator.

Example

const loadFiles = gensync(function* () {
  return yield* gensync.all([
    readFile("./one.js"),
    readFile("./two.js"),
    readFile("./three.js"),
  ]);
});

gensync.race(iterable)

Promise.race-like combinator that works with an iterable of generator objects that could be passed to yield* within a gensync generator.

Example

const loadFiles = gensync(function* () {
  return yield* gensync.race([
    readFile("./one.js"),
    readFile("./two.js"),
    readFile("./three.js"),
  ]);
});