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<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
​
</body>
</html>
 
const countdown = {
  [Symbol.iterator]() {
    const iterator = {
      value: 10,
      done: false,
      next() {
        this.done = this.done || this.value < 0;
​
        if (this.done) {
          return { done: true };
        } else {
          return { done: false, value: this.value-- };
        }
      },
      return(value) {
        console.log('Return to Forever')
        this.done = true;
        if (arguments.length === 1) {
          return { done: true, value };
        } else {
          return { done: true };
        }
      }
    };
​
    return iterator;
  }
};
​
function * mapWith (mapFn, iterable) {
  for (const value of iterable) {
    yield mapFn(value);
  }
}
​
const countdownInWords = mapWith(n => n, countdown);
​
for (const word of countdownInWords) {
  break;
}
​
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