Add or change properties by assignment:
let user = { name: "Alice" }
user.age = 30 // Add new property
user.name = "Alicia" // Modify existing
user["email"] = "a@b.c" // Bracket notation works too
console.log(user)
// { name: "Alicia", age: 30, email: "a@b.c" }
Objects are mutable. Even with const, you can change properties. const only prevents reassigning the variable itself.