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User Defined Iterators

Hard

In plain terms

Iterators allow custom iteration via the iterator protocol. An object with a next() method that returns { value, done }. Used by for...of. Implement Symbol.iterator for custom iteration.

What you need to know

  • •Iterator protocol: next() returns { value, done }
  • •Symbol.iterator
  • •Enables for...of

Try it yourself

Copy the code below and run it in your browser console or a code editor:

const range = {
  from: 1,
  to: 3,
  [Symbol.iterator]() {
    let current = this.from;
    return {
      next: () => ({
        value: current,
        done: current++ > this.to
      })
    };
  }
};

for (const n of range) {
  console.log(n);  // 1, 2, 3
}

Learn more

Dive deeper with these trusted resources:

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