coroutine.js

coroutine.js

NOTE: This is an experimental library that expects the client code to use ES6 features that are not yet widely supported (generator functions, yield). Check the browser compatibility or consider transpiling the client code that uses the library to ES5.

Co-routine implementation for JavaScript. More background https://en.wikipedia.org/wiki/Coroutine

Useful tool for asynchronous programming in case some functions are asynchronous and we have to wait for their completion before proceeding further. Example below explains this in more detail.

For newer proposed versions of JavaScript checkout the async/await mechanism which is very similar to the present coroutine implementation https://jakearchibald.com/2014/es7-async-functions/

Installation

The library requires support of ES5, generator functions and the yield keyword.

Bower

bower install coroutine.js

NPM

npm install coroutine.js

Direct download

• Minified version (ES5)

  https://raw.githubusercontent.com/antivanov/coroutine.js/master/coroutine.min.js

• Full version (ES5)

  https://raw.githubusercontent.com/antivanov/coroutine.js/master/coroutine.js

• Original ES6 version

  The built library can be loaded both as an AMD and CommonJS module.

  https://github.com/antivanov/coroutine.js/blob/master/src/coroutine.js

Quick example

Very simple and contrived example of summing up values and outputting the sum, more detailed examples can be found under the examples directory. Note how the code inside the coroutine invocation looks as if it was synchronous although every get returns a Promise object.

Browser

var coroutine = coroutine.default;

var values = {
  'a': 1,
  'b': 2,
  'c': 3
};

function defer(func) {
  setTimeout(func, 100);
}

function get(key) {
  return new Promise(function(resolve, reject) {
    defer(() => {
      resolve(values[key]);
    });
  });
}

coroutine(function* () {
  var x = yield get('a');
  var y = yield get('b');

  return x + y + (yield get('c'));
}).then((sumOfValues) => {
  console.log(sumOfValues);
});

Node.js

Same code as before, only the library is imported differently.

var coroutine = require('coroutine.js').default;
...

Why

Asynchronous programming in JavaScript can be quite confusing and lead to code that is hard to understand and support. Let's consider the case of interacting with some hypothetical data store via asynchronous method invocations.

Callbacks

  store.insert(newItem, () => {
    store.find(items => {
      //do something with items
    });
  });

When the store method completes its execution the callback passed as the last argument is invoked. In more complex examples we might end up with deeply nested structure in which it will be hard to track the order of execution and implement proper error handling, i.e. we will experience the well-known "callback hell" http://callbackhell.com/

Using callbacks in this manner provides a workaround for older JavaScript versions lacking language constructs that deal with asynchronous execution.

However, with modern JavaScript we can do better.

Promises

Promises somewhat aleviate the pains of using callbacks by providing the asbtraction of asynchronous computation.

  store.insert(newItem).then(() => {
    return store.find();
  }).then(items => {
    //do something with items
  });

Every store method now returns a promise https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise. Promises can be chained to each other with using then and we can also handle errors in a more unified and orderly manner.

However the code still remains quite low-level and requires wrapping every invocation of an asynchronous method into quite wordy promise handling code.

We can still do better and write cleaner asynchronous code.

Coroutines

The latest versions of JavaScript introduce generator https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function* functions that allow to define a computation that can be suspended and resumed. From here there is one step to implementing the coroutine mechanism that the present library provides.

  coroutine(function* () {
    yield store.insert(newItem);
    var items = yield store.find();

    //do something with items
  });

Store methods still return promises, and these are yielded https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield from the generator function to the coroutine implementation that waits for their completion and passes the results further the execution flow of the generator.

Usage

In order to use the library require it in Node.js or import it as an AMD module with Require.js or import it globally. Examples can be found in the examples folder.

Browser and JavaScript engine support

Target JavaScript engine has to support:

• Generators, and yield keyword http://kangax.github.io/compat-table/es6/#test-generators
• Promises http://caniuse.com/#search=promise

The library has been tested with:

Chrome 46.0.2490.86 Firefox 44.0

Node.js 5.0.0

License

MIT License (c) Anton Ivanov