immutability-patterns

Immutability Patterns in TypeScript:

• Understanding Immutability:

  Immutability is a design principle that ensures data cannot be altered after creation. It leads to safer code by preventing unexpected changes, making the application state more predictable and easier to debug.

• Readonly Keyword:

  interface Person {
    readonly name: string;
    readonly age: number;
  }
  
  let person: Person = { name: 'John', age: 30 };
  // Trying to change the name or age will result in a TypeScript error.

• Const Assertions:

  let immutableObject = { x: 10, y: 20 } as const;
  // immutableObject.x = 25; // This line would cause a TypeScript error.

• Using ReadonlyArray and ReadonlyMap:

  let readonlyArray: ReadonlyArray<number> = [1, 2, 3];
  // readonlyArray[0] = 10; // Error, cannot modify an index of a readonly array.
  
  let readonlyMap: ReadonlyMap<string, number> = new Map();
  // readonlyMap.set('key', 1); // Error, cannot add entries to a readonly map.

• Immutable Data Libraries:

  Use Immutable.js to manage collections like List, Stack, Map, etc., which offer methods that return new instances rather than modifying the original instance, ensuring immutability.

• Object.freeze:

  let frozenObject = Object.freeze({ name: 'John' });
  // frozenObject.name = 'Jane'; // Error, cannot assign to 'name' because it is a read-only property.

• Spread Operator for Copies:

  let originalArray = [1, 2, 3];
  let copiedArray = [...originalArray];
  // Now, copiedArray is a shallow copy of originalArray.

• Immer Library for Immutable State:

  import produce from 'immer';
  
  let baseState = { counter: 0 };
  let nextState = produce(baseState, draftState => {
    draftState.counter++;
  });
  // baseState is untouched, nextState is a new object with counter incremented.

  Structural sharing means that when a new version of an object is created after a change, it reuses most of the existing structure. This efficient practice is common in functional programming and helps in managing immutable states.