Iterator Pattern

Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.

What is it?

The Iterator pattern allows sequential traversal of a collection without revealing its internal structure. It abstracts the logic of iteration into a separate object.

Example

interface Iterator<T> {
  hasNext(): boolean;
  next(): T;
}

interface IterableCollection<T> {
  createIterator(): Iterator<T>;
}

class NameRepository implements IterableCollection<string> {
  private names: string[] = ['Alice', 'Bob', 'Charlie', 'Diana'];

  createIterator(): Iterator<string> {
    let index = 0;
    const names = this.names;

    return {
      hasNext: () => index < names.length,
      next: () => names[index++],
    };
  }
}

// Usage
const nameRepo = new NameRepository();
const iterator = nameRepo.createIterator();

while (iterator.hasNext()) {
  console.log(iterator.next());
}

Common Uses

• Traversing custom data structures
• Abstracting away iteration logic
• Supporting multiple iteration strategies

When to Use

• To iterate without exposing internal structure
• To have multiple iterators over the same collection

Caution

• Overhead for small/simple collections
• Not ideal for performance-critical tight loops