SOLID design principle
Tags: programming, oop, softwaredevelopment, javascript
Published: 2024-01-14 13:25:38


![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/wj4nrr0r6zy4o4fvvn5i.jpg)

**S.O.L.I.D** stands for  
- **S** stands for Single responsibility principle
- **O** stands for the open-closed principle
- **L** stands for the Liskov substitution principle
- **I** stands for Interface segregation principle
- **D** stands for Dependency Inversion principle

---
### Single Responsibility Principle


**Single responsibility principle** is a simple principle, and it tells us that a class should have a single primary responsibility and as a consequence, it should only have one reason to change, That reason is somehow related to its responsibility in other words it's a bat idea to add more than one responsibility to a class.

<!--bad code example img-->
```
// Bad Practice
class Journal {
    constructor() {
        this.entries = []
    }
    addEntry(text) {
        //
    }
    removeEntry(index) {
        //
    }
    save(filename) {
        //
    }
    loadFromUrl(url) {
        //
    }
}
```

In the above code, we made a journal that is going to keep records. The bad practice signifies that you add a lot of code and give a multiple responsibility for a single class for journal management as well as file handling. 

<!--better code example img-->
```
// Better Practice
class Journal {
    constructor() {
        this.entries = []
    }
    addEntry(text) {
        //
    }
    removeEntry(index) {
        //
    }
}
class PersistenceManager {
    preprocessor() {
        //
    }
    saveToFile(journal, filename) {
        //
    }
}
```
In the above example, it can be seen that it is easy to understand and implement the code by grouping the functionalities by the class instead of sticking all the functionality into a single class

There is an anti-pattern that shows up during a coding practice which is rather bad as opposed to a good. There is also a pattern called a god object pattern which is a huge massive class that has lots of responsibility and spaghetti code which is very difficult to figure out

And so the single responsibility is the exact opposite that basically lets us have just one responsibility and if we need additional responsibility then we just take other classes as they don't cost us anything. 

Another term we used is called separation of concerns. So separation of concern is what we do when we refactor for example let's suppose you find a complicated algorithm and what we are trying to do is split it into different parts that are somehow related. 

Seperation of the concern is also widely used in one of the most popular JS libraries react js. So in React, we solved this issue by creating container components that contain all the logic, which would then pass down the data via props to the presentational component. And with the introduction of React hooks, there's a new approach to this: using custom hooks. 

The code is available to preview on *"singleResponsibilityPrincipale.js"*

---

### Open-Closed Principle


**Objects-Closed Principle(OCP)** states that objects or entities should be open for extension but closed for modification which means you never jump into an existing class and start to modify it unless you have to.

```
class Product {
constructor(name, color, size) {
    //
    }
}
class ProductFilter {
    filterByColor(products, color) {
        //
    }
    filterBySize(products, size) {
        //
    }
    filterBySizeAndColor(products, size, color) {
        //
    }
    // state space explosion
    // 3 criteria (+weight) = 7 methods

    // OCP = open for open for extension but closed for modification
}
```
In the above code extending the functionality is not a good thing because if the product filter class is something that other people might have just copied your class, and then you modified it, but it doesn't they just took a copy having something that has already been tested and put into production modifying it explicitly might not be a good idea

```
// !Better Practice

class Product {
    constructor(name, color, size) {
        //
    }
}
class ColorSpecification {
    constructor(color) {
        //
    }
    isSatisfied(product) {
        //
    }
}
class SizeSpecification {
    constructor(size) {
        //
    }
    isSatisfied(product) {
        //
    }
}
class BetterSpecification {
    filter(items, spec) {
        //
    }
}
class AndSpecification {
    constructor(...spec) {
        //
    }
    isSatisfied(product) {
        //
    }
}

```
But generally, it's not such a great practice because it affects the scalability it affects maintenance ability of our code, So a better approach is to use inheritance or some sort of way of extending functionality

Now typically an object-oriented programming language these specification classes would have some sort of base class you would have sort of maybe you have some kind of abstract class called specification but in javascript, there is no abstract class, but we could just force it into the constructor by making sure that if somebody calls the constructor of the specification then they get an error. 

Generally, the idea is that you use inheritance of some kind or seperation of concerns effectively to separate each criterion by which you want to filter into separate specification 

The implementation can be previewed in *"openClosed.js"*

---

### Liskov Substitution Principle

The third principle of solid design is the Liskov Substitution principle, and it is named after Barbra Liskov. The Liskov substitution principle states that any subclass object should be substituted for the superclass object from which it is derived.

```
// BAD

class Rectangle {
    constructor() {
        this.width = 0
        this.height = 0
    }

    setColor(color) {
        // ...
    }

    render(area) {
        // ...
    }

    setWidth(width) {
        this.width = width
    }

    setHeight(height) {
        this.height = height
    }

    getArea() {
        return this.width * this.height
    }
}

class Square extends Rectangle {
    setWidth(width) {
        this.width = width
        this.height = width
    }

    setHeight(height) {
        this.width = height
        this.height = height
    }
}

function renderLargeRectangles(rectangles) {
    //
}
```
According to the code above rectangle class should substitute for the rectangle class but in the code above it only works for rectangles not for square

```
class Shape {
    setColor(color) {
        // ...
    }

    render(area) {
        // ...
    }
}

class Rectangle extends Shape {
    constructor(width, height) {
        super()
        this.width = width
        this.height = height
    }

    getArea() {
        return this.width * this.height
    }
}

class Square extends Shape {
    constructor(length) {
        super()
        this.length = length
    }

    getArea() {
        return this.length * this.length
    }
}

function renderLargeShapes(shapes) {
    //
}

```

So we are going to create a shape class that will be extended both to a rectangle and a square. The rectangle class has the constructor both with and height, and we can get the area by multiplying width and height, while the square class has a constructor with length only, and we render the area through the renderLargeShapes function 

The code implementation can be previewed in *"lsp.js"*

---

### Interface Segregation Principle

Interface segregation principle states 'A client should never be forced to implement an interface that it doesn’t use, or clients shouldn’t be forced to depend on methods they do not use'.

### Simple Approach

Suppose you enter a restaurant, and you are a pure vegetarian. The waiter in that restaurant gave you the menu card which includes vegetarian items, non-vegetarian items, drinks, and sweets. In this case, as a customer, you should have a menu card that includes only vegetarian items, not everything that you don’t eat in your food.

Here the menu should be different for different types of customers. The common or general menu card for everyone can be divided into multiple cards instead of just one. Using this principle helps in reducing the side effects and frequency of required changes.

In JavaScript, we don't have an interface by default. But we all would have faced situations where we want to do so many things on the constructor of a class.

```
class DOMTraverser {
    constructor(settings) {
        this.settings = settings
        this.setup()
    }

    setup() {
        this.rootNode = this.settings.rootNode
        this.animationModule.setup()
    }

    traverse() {
        // ...
    }
}

const $ = new DOMTraverser({
    rootNode: document.getElementsByTagName("body"),
    animationModule() {}, // Most of the time, we won't need to animate when traversing.
    // ...
})
```
We create a dom traverser class awe pass a settings it gets a root node and we pass an animation module always but we may not need to animate 

```
// GOOD

class DOMTraverser {
    constructor(settings) {
        this.settings = settings
        this.options = settings.options
        this.setup()
    }

    setup() {
        this.rootNode = this.settings.rootNode
        this.setupOptions()
    }

    setupOptions() {
        if (this.options.animationModule) {
            // ...
        }
    }

    traverse() {
        // ...
    }
}

const $ = new DOMTraverser({
    rootNode: document.getElementsByTagName("body"),
    options: {
        animationModule() {},
    },
})

```
So in the code above we make the passing of the animation module optional by making animation module objects optional so if passed in the animation module it is implemented if not its is not implemented

The implementation of this principle can be previewed in *"isp.js"*

## Dependency inversion principle

The dependency inversion principle states that the high-level module must not depend on the low-level module, but they should depend on abstractions states that the high-level module must not depend on the low-level module, but they should depend on abstractions

The dependency inversion principle defines a relationship that you should have between low-level modules and high-level modules.

### Simple approach
You can consider a real-life example of a TV remote battery. Your remote needs a battery but it’s not dependent on the battery brand. You can use any XYZ brand that you want and it will work. So we can say that the TV remote is loosely coupled with the brand name. Dependency Inversion makes your code more reusable

```
// BAD

class InventoryRequester {
    constructor() {
        this.REQ_METHODS = ["HTTP"]
    }

    requestItem(item) {
        // ...
    }
}

class InventoryTracker {
    constructor(items) {
        this.items = items

        // BAD: 
        // We should just have requestItems depend on a request method: `request`
        this.requester = new InventoryRequester()
    }

    requestItems() {
        this.items.forEach((item) => {
            this.requester.requestItem(item)
        })
    }
}

const inventoryTracker = new InventoryTracker(["apples", "bananas"])
inventoryTracker.requestItems()
```
In the above code, we create an inventory tracker class which is always going to use an inventory requester class that is always going to use the http method. We have created a dependency on a specific request implementation.
```
// GOOD

class InventoryTracker {
    constructor(items, requester) {
        this.items = items
        this.requester = requester
    }

    requestItems() {
        this.items.forEach((item) => {
            this.requester.requestItem(item)
        })
    }
}

class InventoryRequesterV1 {
    constructor() {
        this.REQ_METHODS = ["HTTP"]
    }

    requestItem(item) {
        // ...
    }
}

class InventoryRequesterV2 {
    constructor() {
        this.REQ_METHODS = ["WS"]
    }

    requestItem(item) {
        // ...
    }
}


const inventoryTracker = new InventoryTracker(
    ["apples", "bananas"],
    new InventoryRequesterV2()
)
inventoryTracker.requestItems()

```
By constructing our dependencies externally and injecting them, we can easily
substitute our request module for a fancy new one that uses WebSockets.

## Additional Content
The above content resources are in the GitHub link below

[Resources](https://github.com/adarshmaharjan/solid_principale/tree/main)