Q219- What are generators and iterators?

Generators and Iterators are features in JavaScript that allow you to control the flow of iteration — they let you pause and resume function execution, producing values one at a time instead of all at once.

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1️⃣ Iterators

An iterator is an object that defines a sequence and returns one value at a time when its next() method is called.

Iterator Protocol

An object is an iterator if it has a next() method that returns:

{ value: <any>, done: <boolean> }

Example:

function createIterator(arr) {
  let index = 0;
  return {
    next: function () {
      return index < arr.length
        ? { value: arr[index++], done: false }
        : { done: true };
    },
  };
}

const iterator = createIterator(["a", "b", "c"]);
console.log(iterator.next()); // { value: "a", done: false }
console.log(iterator.next()); // { value: "b", done: false }
console.log(iterator.next()); // { value: "c", done: false }
console.log(iterator.next()); // { done: true }

✅ Iterators are the foundation for for...of loops and iterable objects like arrays, maps, and sets.

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2️⃣ Generators

A generator is a special function that automatically creates an iterator.
Defined using function*, it can pause execution (yield) and resume later (next()).

Example:

function* generatorExample() {
  yield 1;
  yield 2;
  yield 3;
}

const gen = generatorExample();
console.log(gen.next()); // { value: 1, done: false }
console.log(gen.next()); // { value: 2, done: false }
console.log(gen.next()); // { value: 3, done: false }
console.log(gen.next()); // { value: undefined, done: true }

✅ The generator function yields one value at a time — it pauses between yields until next() is called again.

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3️⃣ Passing Values into Generators

You can also send values back into a generator:

function* greet() {
  const name = yield "What's your name?";
  yield `Hello, ${name}!`;
}

const gen = greet();
console.log(gen.next().value); // "What's your name?"
console.log(gen.next("Teekam").value); // "Hello, Teekam!"

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4️⃣ Real Use Cases

• Lazy data generation
• Asynchronous control flow (before async/await)
• Managing complex iterations (e.g., custom sequence generators)

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💡 In Short:

• Iterator: An object that returns values using .next().
• Generator: A function (function*) that automatically creates an iterator using yield.
  Together, they provide a powerful way to manage sequential data and asynchronous operations in JavaScript.