[javascript] What is the difference between ( for... in ) and ( for... of ) statements?

I know what is a for... in loop (it iterates over the keys), but I have heard about for... of for the first time (it iterates over values).

I am confused about for... of loop.

var arr = [3, 5, 7];
arr.foo = "hello";
    
for (var i in arr) {
  console.log(i); // logs "0", "1", "2", "foo"
}
    
for (var i of arr) {
  console.log(i); // logs "3", "5", "7"
  // it doesn't log "3", "5", "7", "hello"
}

I understand that for... of iterates over property values. Then why doesn't it log "3", "5", "7", "hello" instead of "3", "5", "7"?

Unlike for... in loop, which iterates over each key ("0", "1", "2", "foo") and also iterates over the foo key, the for... of does not iterate over the value of foo property, i.e., "hello". Why it is like that?

Here I console for... of loop. It should log "3", "5", "7","hello" but it logs "3", "5", "7". Why?

Example Link

Another difference between the two loops, which nobody has mentioned before:

Destructuring for...in is deprecated. Use for...of instead.

Source

So if we want to use destructuring in a loop, for get both index and value of each array element, we should to use the for...of loop with the Array method entries():

for (const [idx, el] of arr.entries()) {
    console.log( idx + ': ' + el );
}

The for...in statement iterates over the enumerable properties of an object, in an arbitrary order. Enumerable properties are those properties whose internal [[Enumerable]] flag is set to true, hence if there is any enumerable property in the prototype chain, the for...in loop will iterate on those as well.

The for...of statement iterates over data that iterable object defines to be iterated over.

Example:

Object.prototype.objCustom = function() {}; 
Array.prototype.arrCustom = function() {};

let iterable = [3, 5, 7];

for (let i in iterable) {
  console.log(i); // logs: 0, 1, 2, "arrCustom", "objCustom"
}

for (let i in iterable) {
  if (iterable.hasOwnProperty(i)) {
    console.log(i); // logs: 0, 1, 2,
  }
}

for (let i of iterable) {
  console.log(i); // logs: 3, 5, 7
}

Like earlier, you can skip adding hasOwnProperty in for...of loops.

Everybody did explain why this problem occurs, but it's still very easy to forget about it and then scratching your head why you got wrong results. Especially when you're working on big sets of data when the results seem to be fine at first glance.

Using Object.entries you ensure to go trough all properties:

var arr = [3, 5, 7];
arr.foo = "hello";

for ( var [key, val] of Object.entries( arr ) ) {
   console.log( val );
}

/* Result:

3
5
7
hello

*/

Here is a useful mnemonic for remembering the difference between for...in Loop and for...of Loop.

"index in, object of"

for...in Loop => iterates over the index in the array.

for...of Loop => iterates over the object of objects.

When I first started out learning the for in and of loop, I was confused with my output too, but with a couple of research and understanding you can think of the individual loop like the following : The

1. for...in loop returns the indexes of the individual property and has no effect of impact on the property's value, it loops and returns information on the property and not the value. E.g

let profile = { name : "Naphtali", age : 24, favCar : "Mustang", favDrink : "Baileys" }

The above code is just creating an object called profile, we'll use it for both our examples, so, don't be confused when you see the profile object on an example, just know it was created.

So now let us use the for...in loop below

for(let myIndex in profile){
    console.log(`The index of my object property is ${myIndex}`)
}
 // Outputs : 
        The index of my object property is 0
        The index of my object property is 1
        The index of my object property is 2
        The index of my object property is 3

Now Reason for the output being that we have Four(4) properties in our profile object and indexing as we all know starts from 0...n, so, we get the index of properties 0,1,2,3 since we are working with the for..in loop.

2. for...of loop* can return either the property, value or both, Let's take a look at how. In javaScript, we can't loop through objects normally as we would on arrays, so, there are a few elements we can use to access either of our choices from an object.

   • Object.keys(object-name-goes-here) >>> Returns the keys or properties of an object.

   • Object.values(object-name-goes-here) >>> Returns the values of an object.

   • Object.entries(object-name-goes-here) >>> Returns both the keys and values of an object.

Below are examples of their usage, pay attention to Object.entries() :

Step One: Convert the object to get either its key, value, or both.
Step Two: loop through.


// Getting the keys/property

   Step One: let myKeys = ***Object.keys(profile)***
   Step Two: for(let keys of myKeys){
             console.log(`The key of my object property is ${keys}`)
           }

// Getting the values of the property

    Step One: let myValues = ***Object.values(profile)***
    Step Two : for(let values of myValues){
                 console.log(`The value of my object property is ${values}`)
               }

When using Object.entries() have it that you are calling two entries on the object, i.e the keys and values. You can call both by either of the entry. Example Below.

Step One: Convert the object to entries, using ***Object.entries(object-name)***
Step Two: **Destructure** the ***entries object which carries the keys and values*** 
like so **[keys, values]**, by so doing, you have access to either or both content.


    // Getting the keys/property

       Step One: let myKeysEntry = ***Object.entries(profile)***
       Step Two: for(let [keys, values] of myKeysEntry){
                 console.log(`The key of my object property is ${keys}`)
               }

    // Getting the values of the property

        Step One: let myValuesEntry = ***Object.entries(profile)***
        Step Two : for(let [keys, values] of myValuesEntry){
                     console.log(`The value of my object property is ${values}`)
                   }

    // Getting both keys and values

        Step One: let myBothEntry = ***Object.entries(profile)***
        Step Two : for(let [keys, values] of myBothEntry){
                     console.log(`The keys of my object is ${keys} and its value 
is ${values}`)
                   }

Make comments on unclear parts section(s).

A see a lot of good answers, but I decide to put my 5 cents just to have good example:

For in loop

iterates over all enumerable props

let nodes = document.documentElement.childNodes;_x000D_
_x000D_
for (var key in nodes) {_x000D_
  console.log( key );_x000D_
}

For of loop

iterates over all iterable values

let nodes = document.documentElement.childNodes;_x000D_
_x000D_
for (var node of nodes) {_x000D_
  console.log( node.toString() );_x000D_
}

There are some already defined data types which allows us to iterate over them easily e.g Array, Map, String Objects

Normal for in iterates over the iterator and in response provides us with the keys that are in the order of insertion as shown in below example.

  const numbers = [1,2,3,4,5];
   for(let number in number) {
     console.log(number);
   }

   // result: 0, 1, 2, 3, 4

Now if we try same with for of, then in response it provides us with the values not the keys. e.g

  const numbers = [1,2,3,4,5];
   for(let numbers of numbers) {
    console.log(number);
  }

  // result: 1, 2, 3, 4, 5

So looking at both of the iterators we can easily differentiate the difference between both of them.

Note:- For of only works with the Symbol.iterator

So if we try to iterate over normal object, then it will give us an error e.g-

const Room = {
   area: 1000,
   height: 7,
   floor: 2
 }

for(let prop in Room) {
 console.log(prop);
 } 

// Result area, height, floor

for(let prop of Room) {
  console.log(prop);
 } 

Room is not iterable

Now for iterating over we need to define an ES6 Symbol.iterator e.g

  const Room= {
    area: 1000, height: 7, floor: 2,
   [Symbol.iterator]: function* (){
    yield this.area;
    yield this.height;
    yield this.floors;
  }
}


for(let prop of Room) {
  console.log(prop);
 } 

//Result 1000, 7, 2

This is the difference between For in and For of. Hope that it might clear the difference.

The for-in statement iterates over the enumerable properties of an object, in arbitrary order.

The loop will iterate over all enumerable properties of the object itself and those the object inherits from its constructor's prototype

You can think of it as "for in" basically iterates and list out all the keys.

var str = 'abc';
var arrForOf = [];
var arrForIn = [];

for(value of str){
  arrForOf.push(value);
}

for(value in str){
  arrForIn.push(value);
}

console.log(arrForOf); 
// ["a", "b", "c"]
console.log(arrForIn); 
// ["0", "1", "2", "formatUnicorn", "truncate", "splitOnLast", "contains"]

I found the following explanation from https://javascript.info/array very helpful:

One of the oldest ways to cycle array items is the for loop over indexes:

let arr = ["Apple", "Orange", "Pear"];

for (let i = 0; i < arr.length; i++) { alert( arr[i] ); } But for arrays there is another form of loop, for..of:

let fruits = ["Apple", "Orange", "Plum"];

// iterates over array elements for (let fruit of fruits) { alert( fruit ); } The for..of doesn’t give access to the number of the current element, just its value, but in most cases that’s enough. And it’s shorter.

Technically, because arrays are objects, it is also possible to use for..in:

let arr = ["Apple", "Orange", "Pear"];

for (let key in arr) { alert( arr[key] ); // Apple, Orange, Pear } But that’s actually a bad idea. There are potential problems with it:

The loop for..in iterates over all properties, not only the numeric ones.

There are so-called “array-like” objects in the browser and in other environments, that look like arrays. That is, they have length and indexes properties, but they may also have other non-numeric properties and methods, which we usually don’t need. The for..in loop will list them though. So if we need to work with array-like objects, then these “extra” properties can become a problem.

The for..in loop is optimized for generic objects, not arrays, and thus is 10-100 times slower. Of course, it’s still very fast. The speedup may only matter in bottlenecks. But still we should be aware of the difference.

Generally, we shouldn’t use for..in for arrays.

The for-in loop

for-in loop is used to traverse through enumerable properties of a collection, in an arbitrary order. A collection is a container type object whose items can be using an index or a key.

var myObject = {a: 1, b: 2, c: 3};_x000D_
var myArray = [1, 2, 3];_x000D_
var myString = "123";_x000D_
_x000D_
console.log( myObject[ 'a' ], myArray[ 1 ], myString[ 2 ] );

for-in loop extracts the enumerable properties (keys) of a collection all at once and iterates over it one at a time. An enumerable property is the property of a collection that can appear in for-in loop.

By default, all properties of an Array and Object appear in for-in loop. However, we can use Object.defineProperty method to manually configure the properties of a collection.

var myObject = {a: 1, b: 2, c: 3};_x000D_
var myArray = [1, 2, 3];_x000D_
_x000D_
Object.defineProperty( myObject, 'd', { value: 4, enumerable: false } );_x000D_
Object.defineProperty( myArray, 3, { value: 4, enumerable: false } );_x000D_
_x000D_
for( var i in myObject ){ console.log( 'myObject:i =>', i ); }_x000D_
for( var i in myArray ){ console.log( 'myArray:i  =>', i ); }

In the above example, the property d of the myObject and the index 3 of myArray does not appear in for-in loop because they are configured with enumerable: false.

There are few issues with for-in loops. In the case of Arrays, for-in loop will also consider methods added on the array using myArray.someMethod = f syntax, however, myArray.length remains 4.

The for-of loop

It is a misconception that for-of loop iterate over the values of a collection. for-of loop iterates over an Iterable object. An iterable is an object that has the method with the name Symbol.iterator directly on it one on one of its prototypes.

Symbol.iterator method should return an Iterator. An iterator is an object which has a next method. This method when called return value and done properties.

When we iterate an iterable object using for-of loop, the Symbol.iterator the method will be called once get an iterator object. For every iteration of for-of loop, next method of this iterator object will be called until done returned by the next() call returns false. The value received by the for-of loop for every iteration if the value property returned by the next() call.

var myObject = { a: 1, b: 2, c: 3, d: 4 };_x000D_
_x000D_
// make `myObject` iterable by adding `Symbol.iterator` function directlty on it_x000D_
myObject[ Symbol.iterator ] = function(){_x000D_
  console.log( `LOG: called 'Symbol.iterator' method` );_x000D_
  var _myObject = this; // `this` points to `myObject`_x000D_
  _x000D_
  // return an iterator object_x000D_
  return {_x000D_
    keys: Object.keys( _myObject ), _x000D_
    current: 0,_x000D_
    next: function() {_x000D_
      console.log( `LOG: called 'next' method: index ${ this.current }` );_x000D_
      _x000D_
      if( this.current === this.keys.length ){_x000D_
        return { done: true, value: null }; // Here, `value` is ignored by `for-of` loop_x000D_
      } else {_x000D_
        return { done: false, value: _myObject[ this.keys[ this.current++ ] ] };_x000D_
      }_x000D_
    }_x000D_
  };_x000D_
}_x000D_
_x000D_
// use `for-of` loop on `myObject` iterable_x000D_
for( let value of myObject ) {_x000D_
  console.log( 'myObject: value => ', value );_x000D_
}

The for-of loop is new in ES6 and so are the Iterable and Iterables. The Array constructor type has Symbol.iterator method on its prototype. The Object constructor sadly doesn't have it but Object.keys(), Object.values() and Object.entries() methods return an iterable (you can use console.dir(obj) to check prototype methods). The benefit of the for-of loop is that any object can be made iterable, even your custom Dog and Animal classes.

The easy way to make an object iterable is by implementing ES6 Generator instead of custom iterator implementation.

Unlike for-in, for-of loop can wait for an async task to complete in each iteration. This is achieved using await keyword after for statement documentation.

Another great thing about for-of loop is that it has Unicode support. According to ES6 specifications, strings are stored with UTF-16 encoding. Hence, each character can take either 16-bit or 32-bit. Traditionally, strings were stored with UCS-2 encoding which has supports for characters that can be stored within 16 bits only.

Hence, String.length returns number of 16-bit blocks in a string. Modern characters like an Emoji character takes 32 bits. Hence, this character would return length of 2. for-in loop iterates over 16-bit blocks and returns the wrong index. However, for-of loop iterates over the individual character based on UTF-16 specifications.

var emoji = "";_x000D_
_x000D_
console.log( 'emoji.length', emoji.length );_x000D_
_x000D_
for( var index in emoji ){ console.log( 'for-in: emoji.character', emoji[index] ); }_x000D_
for( var character of emoji ){ console.log( 'for-of: emoji.character', character ); }

Difference for..in and for..of:

Both for..in and for..of are looping constructs which are used to iterate over data structures. The only difference between them is the entities they iterate over:

1. for..in iterates over all enumerable property keys of an object
2. for..of iterates over the values of an iterable object. Examples of iterable objects are arrays, strings, and NodeLists.

Example:

let arr = ['el1', 'el2', 'el3'];

arr.addedProp = 'arrProp';

// elKey are the property keys
for (let elKey in arr) {
  console.log(elKey);
}

// elValue are the property values
for (let elValue of arr) {
  console.log(elValue)
}

In this example we can observe that the for..in loop iterates over the keys of the object, which is an array object in this example. The keys are 0, 1, 2 (which correspond to the array elements) and addedProp. This is how the arr array object looks in chrome devtools:

You see that our for..in loop does nothing more than simply iterating over these keys.

The for..of loop in our example iterates over the values of a data structure. The values in this specific example are 'el1', 'el2', 'el3'. The values which an iterable data structure will return using for..of is dependent on the type of iterable object. For example an array will return the values of all the array elements whereas a string returns every individual character of the string.

For...in loop

The for...in loop improves upon the weaknesses of the for loop by eliminating the counting logic and exit condition.

Example:

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

for (const index in digits) {
  console.log(digits[index]);
}

But, you still have to deal with the issue of using an index to access the values of the array, and that stinks; it almost makes it more confusing than before.

Also, the for...in loop can get you into big trouble when you need to add an extra method to an array (or another object). Because for...in loops loop over all enumerable properties, this means if you add any additional properties to the array's prototype, then those properties will also appear in the loop.

Array.prototype.decimalfy = function() {
  for (let i = 0; i < this.length; i++) {
    this[i] = this[i].toFixed(2);
  }
};

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

for (const index in digits) {
  console.log(digits[index]);
}

Prints:

0

1

2

3

4

5

6

7

8

9

function() { for (let i = 0; i < this.length; i++) { this[i] = this[i].toFixed(2); } }

This is why for...in loops are discouraged when looping over arrays.

NOTE: The forEach loop is another type of for loop in JavaScript. However, forEach() is actually an array method, so it can only be used exclusively with arrays. There is also no way to stop or break a forEach loop. If you need that type of behavior in your loop, you’ll have to use a basic for loop.

For...of loop

The for...of loop is used to loop over any type of data that is iterable.

Example:

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

for (const digit of digits) {
  console.log(digit);
}

Prints:

0

1

2

3

4

5

6

7

8

9

This makes the for...of loop the most concise version of all the for loops.

But wait, there’s more! The for...of loop also has some additional benefits that fix the weaknesses of the for and for...in loops.

You can stop or break a for...of loop at anytime.

const digits = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9];

for (const digit of digits) {
  if (digit % 2 === 0) {
    continue;
  }
  console.log(digit);
}

Prints:

1

3

5

7

9

And you don’t have to worry about adding new properties to objects. The for...of loop will only loop over the values in the object.

I found a complete answer at Iterators and Generators (Although it is for TypeScript, this is the same for JavaScript too)

Both for..of and for..in statements iterate over lists; the values iterated on are different though, for..in returns a list of keys on the object being iterated, whereas for..of returns a list of values of the numeric properties of the object being iterated.

Here is an example that demonstrates this distinction:

let list = [4, 5, 6];

for (let i in list) {
   console.log(i); // "0", "1", "2",
}

for (let i of list) {
   console.log(i); // "4", "5", "6"
}

Another distinction is that for..in operates on any object; it serves as a way to inspect properties on this object. for..of on the other hand, is mainly interested in values of iterable objects. Built-in objects like Map and Set implement Symbol.iterator property allowing access to stored values.

let pets = new Set(["Cat", "Dog", "Hamster"]);
pets["species"] = "mammals";

for (let pet in pets) {
   console.log(pet); // "species"
}

for (let pet of pets) {
    console.log(pet); // "Cat", "Dog", "Hamster"
}