for_in.js

const person = {
    firstName: "..",
    lastName: '..',
    age: 30
}
console.log(person['age']);
// We want to modify the age
for(var nameOfKey in person) {
    if(nameOfKey == "age") { // If the name of key is age
        // If the key is age,
        // then change the value of 
        // this key
        person[nameOfKey] = 35;
    }
}
console.log(person['age']);