Iterators

The Iterator Protocol

An object is iterable if it implements Symbol.iterator. The method must return an object with a next() method yielding { value, done }.

const range = {
  start: 1,
  end: 5,
  [Symbol.iterator]() {
    let current = this.start;
    const end = this.end;
    return {
      next() {
        if (current <= end) {
          return { value: current++, done: false };
        }
        return { value: undefined, done: true };
      }
    };
  }
};

for (const num of range) {
  console.log(num);
}

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Custom Iterables

Create iterable objects representing any sequence — like Fibonacci numbers, custom collections, or infinite sequences.

const fibonacci = {
  [Symbol.iterator]() {
    let a = 0, b = 1;
    return {
      next() {
        const value = a;
        a = b;
        b = value + b;
        if (value > 50) return { done: true };
        return { value, done: false };
      }
    };
  }
};

for (const num of fibonacci) {
  console.log(num);
}

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Built-in Iterables

Arrays, Strings, Maps, Sets, and NodeLists are built-in iterables. You can use for...of on all of them.

const str = "Hello";
for (const char of str) {
  console.log(char);
}

const map = new Map([["a", 1], ["b", 2]]);
for (const [key, value] of map) {
  console.log(key + ":", value);
}

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