Loop over objects

for..in statement

for..in statement is used for iterating keys of an object.

js

const person = {
    name: 'john',
    age: 18
};

for (let key in person) {
    console.log(key)
}
// name
// age

key of Array

Array is also object. But its keys are are indices.

js

const array = [1, 12, 123];
for (let key in array) {
    console.log(key); 
}
// 0
// 1
// 2

for..of statement

Use for..of statement to iterating the values of properties.

the object to be iterated must be iterable

js

const person = {
    name: 'john',
    age: 18
};

for (const prop of person) {
    console.log(prop);
}
// 'john'
// 18

You might notice the occurrences of const and let in for..in and for..of loop.

let allows the mutation of variable in scope.
const forced the variable in scope immutable.