Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.

As per MDN The value of a constant cannot change through re-assignment, and it can't be redeclared, so inside of for...in and for...of how is working?

const data = ['A', 'B', 'C', 'D'];

//Here const key is changed
for (const key in data) {
  console.log('key ',key);
}

//Here const value is changed
for (const value of data) {
  console.log('value ',value);
}

Constants are block-scoped, much like variables defined using the let statement. The value of a constant cannot change through re-assignment, and it can't be redeclared.

As per MDN The value of a constant cannot change through re-assignment, and it can't be redeclared, so inside of for...in and for...of how is working?

const data = ['A', 'B', 'C', 'D'];

//Here const key is changed
for (const key in data) {
  console.log('key ',key);
}

//Here const value is changed
for (const value of data) {
  console.log('value ',value);
}

• javascript
• ecmascript-6
• constants

• possible duplicate of scope rules of variables in for, for-in and for-of loops – Bergi Commented Apr 6, 2018 at 14:12

3 Answers 3

Every iteration of a loop has its own block scope.

 for(let i = 0; i < 10; i++)
   setTimeout(() => console.log(i), 1);

That creates 10 seperate scopes, thats why it logs 10 different numbers. Therefore you can also declare constants in these different scopes.

The first three words of the material you quoted explains.

Constants are block-scoped

Each time you go around the for loop, you go to the top of a clean block. The first thing you do to it is to create a constant inside it.

In for-of-loop those constans are being declared for every iteration (in independent scopes), whereas in a for-loop you're re-using the variable, so you won't be able to re-assign values after its declaration.

Example with for-loop

const data = ['A', 'B', 'C', 'D'];

for (const i = 0; i < data.length; i++) {
  console.log('value ',data[i]);
}