1. Primitives Are Immutable, But Are Variables?

In JavaScript, primitive values (such as numbers, strings, and booleans) are immutable, meaning their actual value cannot be altered once created. However, this does not mean that variables holding primitive values are immutable. Variables are simply references, and unless declared with const, they are free to point to new values in memory.

For example:

let x = 10;

x = 20; // The variable' x' is reassigned, but the original value 10 remains unchanged in memory.

Here, 10 was immutable but x was free to reference a new value (20). The previous value (10) is discarded if no other reference exists.

2. Are Object Properties Like Variables?

Object properties behave similarly to variables in that they hold references to values. When an object is considered immutable, its properties cannot be changed, added, or removed, i.e., the memory addresses where its properties reference cannot be altered. However, this doesn't mean the values themselves are inherently immutable—only the reference stored in the property is locked; if the values are primitives, it will, of course, remain immutable, but if it is another object, it will have to be made immutable if needed else the nested object's property can be changed.

Example:

// "use strict"; // results in TypeError during reassignment

const person = {

firstName: "John",

lastName: "Doe",

age: 30,

email: "john.doe@example.com",

address: {

street: "123 Main St",

city: "Anytown",

state: "CA",

zip: "91234",

country: "USA",

},

};

person.age = 31; // Allowed because person although the variable is a constant i.e., it cannot be reassigned, the object's properties can be changed.

Object.freeze(person); // Freezes the object and its properties

person.age = 32; // Cannot change the property because the object is frozen

console.log(person.age); // 31

person.address.city = "New City"; // Allowed because the object is frozen, but the properties of the object are not frozen

console.log(person.address.city); // New City

NOTE: Attempting to alter the fields of a frozen object in JavaScript does not raise an error by default. Instead, the operation fails silently. However, if you are running in strict mode, it will raise a TypeError.

3. How Does Immutability Affect Memory?

When an object is immutable, its properties hold a fixed reference to memory. However, this does not prevent the memory itself from being modified externally. Primitives inside immutable objects also remain immutable, but this doesn't prevent them from being removed from memory if no remaining references exist.

Example:

let a = "hello";

let obj = { greeting: a };

a = "world"; //'obj.greeting' still holds "hello"

Here, even though a was reassigned to "world", the object obj still references "hello", preventing it from being garbage collected. If obj is also reassigned or deleted, then "hello" may eventually be removed from memory.

Does Immutability Prevent Memory Cleanup?

In the example below, following the freeze operation, the code attempts to delete the age property from the person object using the delete operator. However, because the object is frozen, this operation will not succeed, and the age property will remain intact.

//"use strict";

const person = {

firstName: "John",

lastName: "Doe",

age: 30,

};

Object.freeze(person);

delete person.age; // will not delete

console.log(person); // { firstName: 'John', lastName: 'Doe', age: 30 }

However, this does not mean immutability prevents garbage collection. JavaScript's garbage collector will free up memory if any variable or object no longer references a value. For example, if person is deleted, the memory the properties hold will be freed if there are no other references. Immutability only ensures that certain references do not change; it does not make the underlying data persist forever.

In conclusion, making an object immutable makes its properties immutable (their references become immutable) but does not prevent the referenced value from being altered if it's mutable.