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

By default, all properties of an array or object will appear in for ... in. However, this behavior is avoidable. Using Object.defineProperty can decide whether a property is enumerable or not.
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'
}
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