Iterators in JavaScript

An iterator is an object that provides a sequence of values, one at a time, through a next() method.

• The next() method returns an object:

{ value: any, done: boolean }

• value: The current value in the sequence.
• done: true if the iteration is complete, otherwise false.
• Iterator Protocol: Any object with a next() method that returns {value, done} is an iterator.

Basic Example

const array = [1, 2, 3];
const iterator = array[Symbol.iterator](); // Built-in iterator

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

Characteristics of Iterators

• Sequential Access: Provides values one at a time.
• Single-use: Once consumed, you need a new iterator for a new iteration.
• Built-in Support: Arrays, strings, Set, Map, and generator objects.

Creating Custom Iterators

You can manually define an iterator by creating an object with a next() method:

const myIterable = {
  items: [10, 20, 30],
  [Symbol.iterator]() {
    let index = 0;
    return {
      next: () => index < this.items.length
        ? { value: this.items[index++], done: false }
        : { done: true }
    };
  }
};

const iterator = myIterable[Symbol.iterator]();
console.log(iterator.next()); // { value: 10, done: false }
console.log(iterator.next()); // { value: 20, done: false }
console.log(iterator.next()); // { value: 30, done: false }
console.log(iterator.next()); // { done: true }

Iterators with Set and Map

Set Iterators

• .values(): Iterator for values
• .keys(): Same as .values() (for Map compatibility)
• .entries(): Iterator for [value, value] pairs

const set = new Set([1, 2, 3]);
const values = set.values();
console.log(values.next()); // { value: 1, done: false }

Map Iterators

• .entries(): [key, value] iterator
• .keys(): Iterator for keys
• .values(): Iterator for values

const map = new Map([["apples", 500], ["bananas", 300]]);
const entries = map.entries();
console.log(entries.next()); // { value: ["apples", 500], done: false }

Iterators and References

Iterators yield references to objects in collections. Modifying them affects the original:

const map = new Map([[{ id: 1 }, 500]]);
const key = map.keys().next().value;
key.id = 2;
console.log([...map][0][0].id); // 2

Use Cases

• Custom Iteration: Define specific iteration behavior for objects/classes.
• Lazy Evaluation: Combine with generators for on-demand value production.
• Data Processing: Iterate Sets or Maps for filtering, mapping, or transforming values.
• Integration with for...of: Use iterators under the hood for looping.

Limitations

• Single-use: Cannot rewind; create a new iterator to iterate again.
• Non-Iterable Types: WeakSet and WeakMap do not provide iterators.
• Manual Management: Custom iterators need careful state tracking.