co Generator Async Flow Control Cheat Sheet

co is the classic generator-based async control flow library that paved the way for modern async/await syntax in JavaScript and Node.js.

Executing Generators

const co = require('co');

co(function* () {
  const result = yield Promise.resolve(42);
  return result * 2;
}).then(val => {
  console.log(val); // 84
}).catch(err => {
  console.error(err);
});

A generator function can yield Promises, thunks, arrays, or objects of promises. co() executes the generator to completion immediately.

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Generator to Reusable Promise Function (co.wrap)

Converts a generator into a standard function returning a regular Promise (identical to modern async function):

const getUser = co.wrap(function* (id) {
  const user = yield db.users.findById(id);
  const posts = yield db.posts.findByUser(user.id);
  return { user, posts };
});

// Calling returns a Promise:
getUser(123).then(data => {
  console.log(data);
});

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Working with Node.js Callback APIs

Using thunkify or modernized promisified wrappers (like mz or native fs.promises):

const thunkify = require('thunkify');
const fs = require('fs');

const readFile = thunkify(fs.readFile);

co(function* () {
  const content = yield readFile('config.json', 'utf8');
  console.log(JSON.parse(content));
});

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Parallel Execution (Arrays & Objects)

co(function* () {
  // Parallel resolution in array
  const [user, profile] = yield [fetchUser(), fetchProfile()];

  // Parallel resolution in object
  const data = yield {
    user: fetchUser(),
    profile: fetchProfile()
  };
  console.log(data.user, data.profile);
});