Difference between for...of and for...in loop in JavaScript.

Hello Everyone,

The difference between for-of and for-in loop really troubled me when I was learning JavaScript. And with this blog, I will try to clear the confusion once and for all.

Let's understand them one by one.

for...of Loop

The MDN Definition:

The for...of statement creates a loop iterating over iterable objects, including built-in String, Array, array-like objects (e.g., arguments or NodeList), TypedArray, Map, Set, and user-defined iterables.

I know that's not the explanation you came here for, So let me explain.

for...of loop works only with iterable objects. In JavaScript, iterables are objects which can be looped over.

String, Array, TypedArray, Map, and Set are all built-in iterables, because each of their prototype objects implements an @@iterator method. So, for...of loop works on the mentioned object types.

Object in JavaScript is not iterable by default. So, for...of loop does not work on objects.

In simple words, for...of works with strings and arrays but not with objects.

For instance:

cosnt str = "Hello World";

for(element of str) {
  console.log(element);
}
// H e l l o " " W o r l d

for...in Loop

The MDN Definition

The for...in statement iterates over all enumerable properties of an object that are keyed by strings (ignoring ones keyed by Symbols), including inherited enumerable properties.

Explanation:

So, for...of does not work with objects (non iterables), Then how do we loop over keys and values of an object? And the answer is for...in loop.

for...in works with those properties whose enumerable flag is set to true.

Enumerable flags for properties created via simple assignment or property initializer are by default true.
Enumerable flag for properties created via Object.defineProperty are by default false.

For instance:

const student = {
    registration: "123456",
    name: "Sandeep",
    age: 33,
}

for(key in student) {
  console.log(key, student[key]);
}
/*
registration "123465"

name "Sandeep"

age 33
*/

Now let's add a new property (marks) to the student object and set its enumerable flag to false. With enumerable flag set to false, marks key won't appear in the result of for...in loop.

const student = {
    registration: "123456",
    name: "Sandeep",
    age: 33,
}

Objec.defineProperty(student, "marks", {
  value: 98,
  enumerable: false,
})

console.log(student.marks);
// 98

for(key in student) {
  console.log(key, student[key]);
}
/*
registration "123465"

name "Sandeep"

age 33
*/

// marks key does not show up in the for...in loop result.

for...in also works with strings and arrays, because enumerable flags for string and array properties are also by default true.

In simple words use for...in to loop over objects. Although for...in works with strings and arrays, but it is not suggested to use that way.

Conclusion

for...of - Use to loop over strings and arrays.
for...in - Use to loop over objects.

That's it for this post.

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