Strings are immutable in JavaScript. We cannot, for example, change a character within a string. 
let name = 'Akshay';
name[1] = 'd'; // error
Any manipulation on strings is either done by creating/returning new string objects. For example, when we convert a string to upper case, the original string remains unchanged, and a new string is returned. 
let name = 'Akshay';
let newName = name.toUpperCase();
console.log(name); // Akshay
console.log(newName); // AKSHAY