for ... in vs for ... of

#Differences

1. The values iterated on the `for ... in` and `for ... of` statements are different.
   `for ... in` iterates over the enumerable property keys of object. Whereas `for ... of` iterates over the values of the numeric properties of object.
   const list = ['a', 'b', 'c'];
   
   for (let i in list) {
       console.log(i); // '0', '1', '2'
   }
   
   for (let i of list) {
       console.log(i); // 'a', 'b', 'c'
   }

2. Unlike `for ... in`, `for ... of` does not support plain objects:
   const person = {
       firstName: 'Foo',
       lastName: 'Bar',
       age: 42,
   };
   
   // TypeError: `person` is not iterable
   for (let k of person) {
       ...
   }

   It is because a plain object is not iterable. To fix that, we can use the `Object.keys()` method to iterate on the object properties:
   for (let k of Object.keys(person)) {
       console.log(k, ':', person[k]);
   }
   
   // firstName: Foo
   // lastName: Bar
   // age: 42

3. `for ... of` supports iterating over a Unicode string.
   const msg = 'Hell😀 W😀rld';
   
   // for ... in
   for (let i in msg) {
       console.log(msg[i]);
   }
   
   // Output:
   // 'H', 'e', 'l', 'l', '�', ' ', 'W', '�', '�', 'r', 'l', 'd'
   
   // for ... of
   for (let c of msg) {
       console.log(c);
   }
   
   // Output:
   // 'H', 'e', 'l', 'l', '😀', ' ', 'W', '😀', 'r', 'l', 'd'

4. `for ... of` loop can wait for an async task to complete in each iteration via the `await` keyword:
   for await (... of ...) {
       ...
   }

#Good practices

1. It is not recommended to add a custom method to primitive objects such as `Array`, `Boolean`, `Number`, `String`, etc. Since `for ... in` statement loops over the enumerable properties, it will include new methods which are added to the prototype.
   Array.prototype.isEmpty = function () {
       return (this.length = 0);
   };
   
   const a = ['cat', 'dog', 'mouse'];
   for (let i in a) {
       console.log(i); // '0', '1', '2', 'isEmpty'
   }

2. Destructing `for ... in` is deprecated. Instead, use `for ... of`:
   const addressBook = new Map();
   addressBook.set('Foo', '111-222-333');
   addressBook.set('Bar', '444-555-666');
   
   for (const [name, phone] of addressBook) {
       console.log(name, ':', phone);
   }
   
   // Foo: 111-222-333
   // Bar: 444-555-666

#Good to know

let person = {
    firstName: 'Foo',
    lastName: 'Bar',
};

// The 'age' property is not enumerable
Object.defineProperty(person, 'age', {
    value: 42,
    enumerable: false,
});

for (let i in person) {
    console.log(i); // 'firstName', 'lastName'
}