Generator and Iterator ⚙️

Questions 🤔

What are generator and iterator
How generator function is different from regular function
What denotes a generator function
What are function and function
Is the below syntax is right?

function* myGenerator() {
  yield 'Hey there!!';
}

What is the role of yield
How yield is different than yield*
What does generator object will contain
What is generator composition and how can we achieve it?
How to break a generator

So far we have seen the Javascript function can return an only single value, multiple values in a single object or nothing but have you ever thought that how can one function return multiple values? Is it really possible?

Yes, it is possible with the help of Generator Functions

A generator function can return multiple values one after other using its generator object

Generator functions

To create a generator function we need to suffix a function keyword with * something like this function* which will make a function as a generator

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

// We can also write a generator function by adding the suffix to the function name.
// But function* is the recommended one which describe the kind of function is not same  
function *myGenerator() {
  yield 'Hello Hackers';  
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

Generator functions behave differently from regular ones. When such a function is called, it doesn’t run its code. Instead, it returns a special object, called generator object, to manage the execution.

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

let gen = myGenerator();
console.log(gen);   // myGenerator {<suspended>}
                    // __proto__: Generator
                    // [[GeneratorLocation]]: VM1873:1
                    // [[GeneratorStatus]]: "suspended"
                    // [[GeneratorFunction]]: ƒ* myGenerator()
                    // [[GeneratorReceiver]]: Window
                    // [[Scopes]]: Scopes[3]

The main method in the generator object is next(). When called, it runs the execution until the nearest yield <value> statement (value can be omitted, then it’s undefined). Then the function execution pauses, and the yielded value is returned to the outer code.

The result of next() is always an object with two properties

value: the yielded value
done: true if the function code has finished, otherwise false

Generator Object Methods and States

next() Returns the next value in a generator
return() Returns a value in a generator and finishes the generator
throw() Throws an error and finishes the generator
suspended Generator has halted execution but has not terminated
closed Generator has terminated by either encountering an error, returning, or iterating through all values

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

let gen = myGenerator();
console.log(gen.next());  // {value: "Hello Hackers", done: false}
console.log(gen.next());  // {value: "Namaste Developers", done: false}
console.log(gen.next());  // {value: "Hi Beautiful People", done: true}

In the above code, the generator myGenerator returns 3 different results highlighted one, and the in the third log value of done is true which means generator notifies that there is no other value left to return. This is the real beauty of the generator

Remember

A generator will remember the last returned value. When we call next time it will not return the previously returned value instead it will return next value if any value present
We can't make anonymous function and arrow function as a generator function if we do so Javascript will throw an error Function statements require a function name / Unexpected token '*'

`yield` Operators

Generators introduce a new keyword to JavaScript: yield. yield can pause a generator function and return the value that follows yield, providing a lightweight way to iterate through values.

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

Now, when we call next() on the generator function, it will pause every time it encounters yield. done will be set to false after each yield, indicating that the generator has not finished. Once it encounters a return, or there are no more yields encountered in the function, done will flip to true, and the generator will be finished.

Iterating Over a Generator

Generators are iterators which means we can loop over the generator methods

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

let gen = myGenerator();
for (let value of gen) {
  console.log(value); // Hello Hackers, Namaste Developers
}

If you see the above code result it will return only first two value but it hasn't returned third value that is Hi Beautiful People

It’s because for..of iteration ignores the last value, when done: true. So, if we want all results to be shown by for..of, we must return them with yield

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  yield 'Hi Beautiful People';
}

let gen = myGenerator();
for (let value of gen) {
  console.log(value); // Hello Hackers, Namaste Developers, Hi Beautiful People
}

As generators are iterable, we can call all related functionality, e.g. the spread syntax ...

function* myGenerator() {
  yield 'Hackers';
  yield 'Developers';
  yield 'Beautiful People';
}

let result = ['Hi', ...myGenerator()];
console.log(result); // ["Hi", "Hackers", "Developers", "Beautiful People"]

In the code above, ...myGenerator() turns the iterable generator object into an array of items. to know more how that works read about spread and rest syntax

Using generators for iterables

Let's see with one sample code of generator for iterables

let range = {
  from: 1,
  to: 5,

  // for..of range calls this method once in the very beginning
  [Symbol.iterator]() {
    // ...it returns the iterator object:
    // onward, for..of works only with that object, asking it for next values
    return {
      current: this.from,
      last: this.to,

      // next() is called on each iteration by the for..of loop
      next() {
        // it should return the value as an object {done:.., value :...}
        if (this.current <= this.last) {
          return { done: false, value: this.current++ };
        } else {
          return { done: true };
        }
      },
    };
  },
};

// iteration over range returns numbers from range.from to range.to
console.log([...range]); // 1,2,3,4,5

We can use a generator function for iteration by providing it as Symbol.iterator.

We still do have a scope to reduce no of lines

let range = {
  from: 1,
  to: 5,

  *[Symbol.iterator]() {
    // a shorthand for [Symbol.iterator]: function*()
    for (let value = this.from; value <= this.to; value++) {
      yield value;
    }
  },
};

console.log([...range]);// 1,2,3,4,5

That works, because range[Symbol.iterator]() now returns a generator, and generator methods are exactly what for..of expects. it has a .next() method that returns values in the form {value: ..., done: true/false}

Generator composition

Generator composition is a special feature of generators that allows to transparently embed generators in each other

For instance, we have a function that generates a sequence of numbers:

function* generateSequence(start, end) {
  for (let i = start; i <= end; i++) yield i;
}

Now we’d like to reuse it to generate a more complex sequence

First, digits 0..9 (with character codes 48…57),
followed by uppercase alphabet letters A..Z (character codes 65…90)
followed by lowercase alphabet letters a..z (character codes 97…122

function* generateSequence(start, end) {
  for (let i = start; i <= end; i++) yield i;
}

function* generatePasswordCodes() {

  // 0..9
  yield* generateSequence(48, 57);

  // A..Z
  yield* generateSequence(65, 90);

  // a..z
  yield* generateSequence(97, 122);

}

let str = '';

for(let code of generatePasswordCodes()) {
  str += String.fromCharCode(code);
}

console.log(str); // 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

The yield* directive delegates the execution to another generator. This term means that yield* gen iterates over the generator gen and transparently forwards its yields outside. As if the values were yielded by the outer generator.

The result is the same as if we inlined the code from nested generators

function* generateSequence(start, end) {
  for (let i = start; i <= end; i++) yield i;
}

function* generatePasswordCodes() {

  // yield* generateSequence(48, 57);
  for (let i = 48; i <= 57; i++) yield i;

  // yield* generateSequence(65, 90);
  for (let i = 65; i <= 90; i++) yield i;

  // yield* generateSequence(97, 122);
  for (let i = 97; i <= 122; i++) yield i;

}

let str = '';

for(let code of generatePasswordCodes()) {
  str += String.fromCharCode(code);
}

console.log(str); // 0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz

Note: The above code snippet is taken from Javascript info

Closing a Generator

As we've seen, a generator can have its done property set to true and it's status set to closed by iterating through all its values. There are two additional ways to immediately cancel a generator: with the return() method, and with the throw() method.

With return(), the generator can be terminated at any point, just as if a return statement had been in the function body. You can pass an argument into return(), or leave it blank for an undefined value.

function* myGenerator() {
  yield 'Hello Hackers';
  yield 'Namaste Developers';
  return 'Hi Beautiful People';
}

let gen = myGenerator();
console.log(gen.next());  // {value: "Hello Hackers", done: false}
console.log(gen.return ());  // {value: undefined, done: true}
console.log(gen.next());  // {value: undefined, done: true}

The first next() will give us 'Hello Hackers', with done set to false. If we invoke a return() method on the Generator object right after that, we'll now get the passed value and done set to true. Any additional call to next() will give the default completed generator response with an undefined value.

Reference

Questions 🤔

Generator functions#

Generator Object Methods and States#

Remember

yield Operators#

Iterating Over a Generator#

Using generators for iterables#

Generator composition#

Closing a Generator#