ES2015 roadtrip to awesomeness: Generators

generators.js

/**
 * ES2015 Generators
 */
// think of ES2015 generators as a new kind of function,
// which may be paused in the middle, one or many times,
// and resumed later, allowing other code to run during these
// periods of pause, and be passed different information
// each time it is started or paused, and respond accordingly

// to create a generator function,
// append an * to the "function" keyword
function* listPeople() {
  // use "yield" expression every time
  // you need to send something "out" when
  // pausing the generator
  yield 'Andrew';
  yield 'Cesar';
  yield 'Linis';

  // yield can be used without any expression at the right.
  // in these cases, it will just put out "undefined"
}

// generator functions do not get called immediately
// like normal functions. Instead, they return a
// generator object.
const people = listPeople();

// now, you can access the values the generator yields using the .next() method.
// This method returns an object with the requested value
// and a boolean that indicates if the generator function
// is done with all its values (and therefore, it not return anything else)
console.log(people.next()); // { value: 'Andrew', done: false }
console.log(people.next().value); // -> "Cesar"
console.log(people.next().done); // -> false
console.log(people.next().value); // -> "Linis"
console.log(people.next().value); // -> undefined
console.log(people.next().done); // -> true

//-------------------------------------------------------------------------------
// scope in generator functions
// when declaring variables inside a generator,
// they retain their values until all the
// yield expressions are complete.

function* listScientists() {
  const scientists = [
    { name: 'Isaac Newton', profession: "physicist" },
    { name: 'Galileo Galilei', profession: "astronomist" },
    { name: 'Alan Turing', profession: "computer scientist" },
    { name: 'Gottfried Wilhelm Leibniz', profession: "mathematician" },
    { name: 'Michel Foucault', profession: "philosopher" },
    { name: 'Amedeo Avogadro', profession: "chemist" }
  ];

  for(const scientist of scientists) {
    yield scientist;
  }
}

const scientistList = listScientists();
console.log(scientistList.next().value.name); // -> Isaac Newtown
// NOTE: the "scientist" constant inside the generator
// is not being re-declared here
console.log(scientistList.next().value.name); // -> Galileo Galilei

// ----------------------------------------------------------
// the next() method can receive a value. This value is passed in
// to the generator and returned by the current yield expression
function *complicatedMath(x) {
    var y = 2 * (yield (x + 1));  //(1)
    var z = yield (y / 3); // (2)
    yield (x + y + z); //(3)
}

const algebra = complicatedMath(5);

console.log(algebra.next().value); // -> 6 (1)
console.log(algebra.next(12).value); // -> 8 (2)
console.log(algebra.next(13).value); // -> 42 (3)


// ----------------------------------------------------------
// generators return a data type that is iterable
// (i.e: implements Symbol.iterator). Therefore, we can
// use for..of to loop through a generator
for(const scientist of listScientists()) {
  console.log(scientist.name);
}