Symbol.iterator

const iterable1 = {};
iterable1[Symbol.iterator] = function* () {
  yield 1;
  yield 2;
  yield 3;
};
console.log([...iterable1]);
// Expected output: Array [1, 2, 3]

const myIterable = {};
myIterable[Symbol.iterator] = function* () {
  yield 1;
  yield 2;
  yield 3;
};
[...myIterable]; // [1, 2, 3]

class Foo {
  *[Symbol.iterator]() {
    yield 1;
    yield 2;
    yield 3;
  }
}
const someObj = {
  *[Symbol.iterator]() {
    yield "a";
    yield "b";
  },
};
console.log(...new Foo()); // 1, 2, 3
console.log(...someObj); // 'a', 'b'

const nonWellFormedIterable = {};
nonWellFormedIterable[Symbol.iterator] = () => 1;
[...nonWellFormedIterable]; // TypeError: [Symbol.iterator]() returned a non-object value