co

Generator functions can also be used to control code execution flow. By using yield expressions, we can control when the execution of a generator object should be suspended. When the execution of a generator object is suspended, other code can have the chance to run and choose the best time to resume the execution. yield* expressions allow the delegation to other generator objects or iterable objects, which can create complicated nested or recursive execution flows.

Generator functions are most useful when combining with Promises. As described in MDN,

The Promise object is used for asynchronous computations. A Promise represents a value which may be available now, or in the future, or never.

If the value of a yield expression is a Promise object, then we can suspend the execution of the generator object when waiting for the Promise to be resolved. When the Promise is fulfilled, we can resume the execution of the generator object with the fulfilled value as the value of the yield expression. Otherwise, we can finish the generator with the rejected error.

To support this kind of scenarios, we need to use the library co. In the code below, timeoutToPromise is a helper method that creates a Promise object using setTimeout. Generator function calculate uses yield expression and the Promise object created by timeoutToPromise. co(calculate, 1, 2) turns the generator function calculate into a Promise object.

const co = require('co');

function timeoutToPromise(action, timeout) {
  return new Promise(function(resolve, reject) {
    setTimeout(function() {
      resolve(action());
    }, timeout);
  });
}

function *calculate(v1, v2) {
  return yield timeoutToPromise(function() {
    return v1 + v2;
  }, 1000);
}

co(calculate, 1, 2).then(function (value) {
  console.log(value);
}, function (err) {
  console.error(err);
});
// -> Output 3 after about 1s delay

Below is an example of using co with generator functions which have yield expressions with other generator objects. value is a generator function which takes the argument v as the seed of generating two random values v1 and v2. yield value(1) in calculate uses a generator object value(1) as the target of yield expression.

const co = require('co');

function *value(v) {
  return yield {
    v1: v + Math.random() * 100,
    v2: v + Math.random() * 500
  };
}

function *calculate() {
  const values = yield value(1);
  return values.v1 + values.v2;
}

co(calculate).then(function (value) {
  console.log(value);
}, function (err) {
  console.error(err);
});
// -> Output random number