What is the purpose of Symbol.iterator and how do you use it?

Symbol.iterator is a well-known symbol that specifies the default iterator for an object, making it iterable with for...of loops and spread operators.

     // Built-in iterables use Symbol.iterator
     const arr = [1, 2, 3];
     const iterator = arr[Symbol.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 }
     

Creating custom iterables:

     class Range {
       constructor(start, end) {
         this.start = start;
         this.end = end;
       }

       [Symbol.iterator]() {
         let current = this.start;
         const end = this.end;

         return {
           next() {
             if (current <= end) {
               return { value: current++, done: false };
             }
             return { done: true };
           }
         };
       }
     }

     const range = new Range(1, 5);
     for (const num of range) {
       console.log(num); // 1, 2, 3, 4, 5
     }

     console.log([...range]); // [1, 2, 3, 4, 5]
     

Using generator function:

     class Countdown {
       constructor(start) {
         this.start = start;
       }

       *[Symbol.iterator]() {
         for (let i = this.start; i >= 0; i--) {
           yield i;
         }
       }
     }

     const countdown = new Countdown(5);
     console.log([...countdown]); // [5, 4, 3, 2, 1, 0]
     

Practical example - infinite sequence:

     const fibonacci = {
       [Symbol.iterator]() {
         let prev = 0, curr = 1;
         return {
           next() {
             [prev, curr] = [curr, prev + curr];
             return { value: prev, done: false };
           }
         };
       }
     };

     // Get first 10 fibonacci numbers
     const fib10 = [];
     for (const num of fibonacci) {
       fib10.push(num);
       if (fib10.length === 10) break;
     }
     console.log(fib10); // [1, 1, 2, 3, 5, 8, 13, 21, 34, 55]