State Design Pattern

Structural: State Pattern

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What is the State Design Pattern in JavaScript?

View Answer:

Interview Response: The State Design Pattern allows an object to alter its behavior when its internal state changes, seemingly changing its class. It promotes loose coupling and increases manageability.

Technical Response: The State pattern provides state-specific logic to a limited set of objects in which each object represents a particular state. The State pattern is commonly used in JavaScript to convert massive switch-base state machines into objects.

Code Example:

This pattern's objects are as follows:

Context -- example code: TrafficLight

• exposes an interface that supports clients of the service
• keeps a reference to a state object that defines the current state.
• Allows State objects to change their current state to another state.

State -- example code: Red, Yellow, Green

• captures the state's values and associated behavior

Here's an example of a Traffic Light system using the State Design Pattern.

class TrafficLight {
  constructor() {
    this.states = [new GreenLight(), new YellowLight(), new RedLight()];
    this.current = this.states[0];
  }

  change() {
    const totalStates = this.states.length;
    let currentIndex = this.states.findIndex(light => light === this.current);

    if (currentIndex + 1 < totalStates) this.current = this.states[currentIndex + 1];
    else this.current = this.states[0];
  }

  sign() {
    return this.current.sign();
  }
}

class Light {
  constructor(light) {
    this.light = light;
  }
}

class GreenLight extends Light {
  constructor() {
    super('green');
  }

  sign() {
    return 'Go';
  }
}

class YellowLight extends Light {
  constructor() {
    super('yellow');
  }

  sign() {
    return 'Caution';
  }
}

class RedLight extends Light {
  constructor() {
    super('red');
  }

  sign() {
    return 'Stop';
  }
}

// usage
const trafficLight = new TrafficLight();

console.log(trafficLight.sign()); // Green: Go
trafficLight.change();

console.log(trafficLight.sign()); // Yellow: Caution
trafficLight.change();

console.log(trafficLight.sign()); // Red: Stop
trafficLight.change();

// Example output:
// Go
// Caution
// Stop

In this example, the TrafficLight class represents the Context, and it maintains a reference to a state object (current) which serves as the Current State. GreenLight, YellowLight, and RedLight classes represent Concrete States, each with different behaviors encapsulated in the sign() method.

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The State pattern belongs to which pattern category?

View Answer:

Interview Response: The State pattern belongs to the behavioral pattern category in JavaScript. It focuses on the behavior of objects and how they respond to changes in their internal state.

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When should you utilize the State Design Pattern in JavaScript?

View Answer:

Interview Response: You should utilize the JavaScript State Pattern when you have an object with multiple behaviors that depend on its internal state, leading to complex conditional statements.

Technical Response: In a real-world application, the State pattern could be handy for introducing new states that we haven't yet considered, possibly more simply than a switch case. Each state is contained and has its own set of internal functions.

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What are some of the advantages of employing the State pattern?

View Answer:

Interview Response: Advantages of employing the State pattern in JavaScript include improved code organization, flexibility, maintainability, and reduced conditional statements. It also enables easier testing.

Technical Response: Benefits of the State Pattern

• Singular Responsibility Principle -- Separate the code related to each state into separate classes.
• The Open/Closed Principle - Add new states without modifying existing state classes or the context.
• Simplify the contextual code by removing bulky state machine conditionals.

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What are some drawbacks of using the State Design Pattern?

View Answer:

Interview Response: It might increase complexity due to the creation of multiple new classes and also involve additional memory costs. Applying the pattern may be excessive if a state machine has only a few states or infrequently changes.

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Are there any alternatives to using the State pattern?

View Answer:

Interview Response: Yes, alternatives to using the State pattern in JavaScript include using conditional statements, using the Strategy pattern, or using polymorphism to represent different states.

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Why use the State Design Pattern in JavaScript?

View Answer:

Interview Response: It's used to manage complexity in code by allowing objects to change their behavior according to their current state, improving readability and maintainability.

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What kind of problems does the State Design Pattern solve?

View Answer:

Interview Response: It solves problems where an object's behavior needs to change based on its state, avoiding long conditionals and making state transitions explicit.

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How does the State Design Pattern differ from the Strategy Pattern?

View Answer:

Interview Response: While both involve encapsulated behaviors, State Pattern changes behavior based on internal state, while Strategy Pattern does so based on external control.

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Can you explain the concept of 'Context' and 'State' in the State Design Pattern?

View Answer:

Interview Response: The 'Context' is the entity having varying behavior, while 'State' encapsulates the behavior associated with a particular state of 'Context'.

Technical Response: In the State Design Pattern, "Context" is an object that can exhibit a variety of behaviors based on its current "State". The "State" is an interface that defines a common set of methods that Concrete States will implement.

Code Example:

Here's an example with a Water object that changes state between Solid, Liquid, and Gas.

class Water {
  constructor() {
    this.state = new SolidState(this);
  }

  heat() {
    this.state.heat();
  }

  cool() {
    this.state.cool();
  }

  changeState(state) {
    this.state = state;
  }

  printState() {
    this.state.printState();
  }
}

class State {
  constructor(water) {
    this.water = water;
  }

  heat() {
    throw new Error('This method must be overwritten!');
  }

  cool() {
    throw new Error('This method must be overwritten!');
  }

  printState() {
    throw new Error('This method must be overwritten!');
  }
}

class SolidState extends State {
  heat() {
    console.log('Heating ice. Turning to water.');
    this.water.changeState(new LiquidState(this.water));
  }

  cool() {
    console.log('Ice is already cool.');
  }

  printState() {
    console.log('The water is solid.');
  }
}

class LiquidState extends State {
  heat() {
    console.log('Heating water. Turning to gas.');
    this.water.changeState(new GasState(this.water));
  }

  cool() {
    console.log('Cooling water. Turning to ice.');
    this.water.changeState(new SolidState(this.water));
  }

  printState() {
    console.log('The water is liquid.');
  }
}

class GasState extends State {
  heat() {
    console.log('Gas is already hot.');
  }

  cool() {
    console.log('Cooling gas. Turning to water.');
    this.water.changeState(new LiquidState(this.water));
  }

  printState() {
    console.log('The water is gas.');
  }
}

const water = new Water();
water.printState(); // The water is solid.
water.heat(); // Heating ice. Turning to water.
water.printState(); // The water is liquid.
water.heat(); // Heating water. Turning to gas.
water.printState(); // The water is gas.
water.cool(); // Cooling gas. Turning to water.
water.printState(); // The water is liquid.

In this example, Water is the "Context". The "State" is an interface represented by the State class, and SolidState, LiquidState, and GasState are the "Concrete States". The heat() and cool() methods in the Water class delegate to the current state's corresponding methods.

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What's the benefit of encapsulating state-specific behavior in a State Design Pattern?

View Answer:

Interview Response: It makes the code more organized, easier to understand, and the states are easier to manage or extend in the future.

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When is the State Design Pattern not suitable?

View Answer:

Interview Response: When there are only a few states or transitions, or the complexity of different states doesn't justify the overhead of the pattern.

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Can State Design Pattern lead to performance issues?

View Answer:

Interview Response: Generally not, but if there are numerous complex states, it might potentially lead to performance considerations.

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