====== Ensuring immutability in Java Script ======

Ensuring immutability in JavaScript means **creating objects and values that cannot be changed once they are created.** \\  While JavaScript does not have built-in support for immutability in the way that some other languages do, there are several practices and libraries that you can use to achieve this goal.

===== 1. Using const for Variables =====

Declaring variables with const ensures that the variable itself cannot be reassigned, but if the variable is an object or an array, its contents can still be changed: <code javascript>const a = [1, 2, 3];
a.push(4); // This will work because the array itself is mutable.
a = [4, 5, 6]; // This will cause an error because `a` cannot be reassigned.
</code>

===== 2. Object.freeze =====

You can use **Object.freeze()** to make an object immutable. \\ It prevents new properties from being added to the object, existing properties from being removed, and it prevents changing the enumerability, configurability, and writability of existing properties: <code javascript>const obj = { x: 5, y: 6 };
Object.freeze(obj);
obj.x = 10; // Will have no effect </code>
<note>Please note that Object.freeze() is shallow, meaning that it only affects the immediate properties of the object. If the object contains other objects, those objects will not be frozen. \\ \\ 

In **strict mode**, **obj.x = 10;** will throw a **TypeError**. In non-strict mode, the error is ignored.
</note>

===== 3. Immutable.js =====

Libraries such as Immutable.js provide persistent immutable data structures. These are collections that, once created, are never changed. Any modifications to the collection return a new collection that shares as much of the structure with the original as possible, minimizing unnecessary copying.

<note>**Object.freeze** only makes the object itself immutable, not any nested objects. You would have to recursively freeze nested objects to achieve deep immutability. **Immutable.js**, on the other hand, provides deep immutability out of the box.</note>

<note>
**Object.freeze** can be slow, especially if used recursively on large objects. **Immutable.js** is optimized for performance and can handle large data structures efficiently.
</note>

===== 4. Functional Programming Techniques =====

Using functional programming practices such as avoiding side effects and using pure functions can help ensure immutability. By always returning new objects and never modifying input parameters, you can create a more predictable and maintainable codebase.

===== 5. Deep Copying =====

Creating deep copies of objects ensures that you're working with a completely separate copy and not a reference to the original object. Be cautious, though, as deep copying can be computationally expensive.

===== 6. Using TypeScript =====

TypeScript can allow you to define read-only properties and provide compile-time checks for immutability: <code javascript>type Point = Readonly<{
  x: number;
  y: number;
}>
</code>

===== 7. Using ESLint =====

ESLint can help ensure immutability in JavaScript by providing linting rules that enforce certain coding practices related to immutability. \\ Here are some ways ESLint can be used for this purpose.

==== 7.1 Using Built-in Rules ====

  * **prefer-const**: This rule flags variables that are never reassigned, signaling that they should be declared using the **const** keyword.
  * **no-var**: This rule requires **let** or **const** instead of **var**, encouraging block scoping and potentially more predictable behavior.

==== 7.2 Plugins and Custom Rules ====

There are ESLint plugins specifically designed to enforce immutability, such as **eslint-plugin-immutable**. \\ This plugin provides several rules like:
  * **immutable/no-mutation**: This rule forbids the mutation of variables and object properties.
  * **immutable/no-this**: This rule forbids the use of **this**, helping to promote functional programming patterns.

==== 7.3 Combining with Other Tools ====

ESLint can be used in combination with tools like TypeScript, which can enforce immutability through type annotations. ESLint has good integration with TypeScript, allowing you to lint TypeScript code and enforce practices like using **readonly** for properties.

==== 7.4 Custom Configuration ====

You can customize ESLint rules and create your own rules to enforce specific immutability patterns within your project.