TypeScript State Pattern Example

In this article, we will learn how to use and implement the State Pattern in TypeScript with an example.
State is a behavioral design pattern that lets an object alter its behavior when its internal state changes. It appears as if the object changed its class.

TypeScript State Pattern Example

Let's create state.ts file and add the following code to it:
export interface State {
    handle(context: Context): void;
}

export class ConcreteStateA implements State {
    public handle(context: Context): void {
        console.log("`handle` method of ConcreteStateA is being called!");
        context.State = new ConcreteStateB();
    }
}

export class ConcreteStateB implements State {
    public handle(context: Context): void {
        console.log("`handle` method of ConcreteStateB is being called!");
        context.State = new ConcreteStateA();
    }
}

export class Context {
    private state: State;

    constructor(state: State) {
        this.state = state;
    }

    get State(): State {
        return this.state;
    }

    set State(state: State) {
        this.state = state;
    }

    public request(): void {
        console.log("request is being called!");
        this.state.handle(this);
    }
}

Usage

Let's create demo.ts file and add the following code to it:
import { ConcreteStateA, Context } from "./state";

export function show() : void {
 var context: Context = new Context(new ConcreteStateA());
 context.request();
 context.request();
 context.request();
 context.request();
 context.request();
 context.request();
 context.request();
 context.request();
}
show();
Run:
  • Compile the above code using the TypeScript compiler.
  • Above code is compiled to plan JavaScript code
  • Run Javascript code using node
design_patterns_in_typescript\state> tsc --target ES5 .\demo.ts
design_patterns_in_typescript\state> node .\demo.js
request is being called!
`handle` method of ConcreteStateA is being called!
request is being called!
`handle` method of ConcreteStateB is being called!
request is being called!
`handle` method of ConcreteStateA is being called!
request is being called!
`handle` method of ConcreteStateB is being called!
request is being called!
`handle` method of ConcreteStateA is being called!
request is being called!
`handle` method of ConcreteStateB is being called!
request is being called!
`handle` method of ConcreteStateA is being called!
request is being called!
`handle` method of ConcreteStateB is being called!

All TypeScript Design Patterns

1. Creational Design Patterns

Creational patterns provide various object creation mechanisms, which increase flexibility and reuse of existing code.
  1. TypeScript Singleton Pattern Example
  2. TypeScript Factory Design Pattern with Example
  3. TypeScript Abstract Factory Pattern Example
  4. TypeScript Builder Pattern Example
  5. TypeScript Prototype Pattern Example

2. Structural Design Patterns

Structural patterns explain how to assemble objects and classes into larger structures while keeping these structures flexible and efficient.
  1. TypeScript Bridge Pattern Example
  2. TypeScript Adapter Pattern Example
  3. TypeScript Decorator Pattern Example
  4. TypeScript Composite Pattern Example
  5. TypeScript Flyweight Design Pattern Example
  6. TypeScript Facade Pattern Example
  7. TypeScript Proxy Pattern Example

3. Behavioral Design Patterns

Behavioral design patterns are concerned with algorithms and the assignment of responsibilities between objects.
  1. TypeScript Command Pattern Example
  2. TypeScript Chain of Responsibility Pattern Example
  3. TypeScript Visitor Pattern Example
  4. TypeScript Template Method Pattern Example
  5. TypeScript Strategy Pattern Example
  6. TypeScript State Pattern Example
  7. TypeScript Observer Pattern Example
  8. TypeScript Memento Pattern Example
  9. TypeScript Mediator Pattern Example
  10. TypeScript Iterator Pattern Example
  11. TypeScript Interpreter Design Pattern Example

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