Understanding Single Responsibility Principle in TypeScript: A Quick Guide

The main definition of Single Responsibility principle says that A class should have only one reason to change. Lets break down this statement for better understanding.

Suppose we have a class that manipulates the text that we pass to it, the single responsibility principle states that the class we create should only be responsible for manipulating the text, and any other action that it performs should not be the part of the class.

Lets take an example of such a class and see how we can refactor it :

class TextManipulator {
  text: string;

  constructor(text: string) {
    this.text = text;
  }

  appendText(newText: string) {
    return this.text.concat(newText);
  }

  findAndReplace(word: string, replacementWord: string) {
    if (this.text.includes(word)) {
      this.text.replace(word, replacementWord);
    }
    return this.text;
  }

  printText() {
    console.log("The text is ", this.text);
  }
}

In the above code one can see that the class is also performing the print action. This breaks the Single Responsibility principle. We can refactor the code by creating two new classes

class TextManipulator {
  private text: string;

  constructor(text: string) {
    this.text = text;
  }

  appendText(newText: string) {
    return this.text.concat(newText);
  }

  findAndReplace(word: string, replacementWord: string) {
    if (this.text.includes(word)) {
      this.text.replace(word, replacementWord);
    }
    return this.text;
  }

  getText() {
    return this.text;
  }
}

class PrintText {
  formattedText: TextManipulator;

  constructor(formattedText: TextManipulator) {
    this.formattedText = formattedText;
  }

  printText() {
    console.log("The text is ", this.formattedText.getText());
  }
}

In the refactored code we have two separate classes that are performing separate action.

Why Single Responsibility Principle

With Single Responsibility Principle in place we can achieve the following :-

1. Better code organization and maintainability
2. Higher code reusability
3. Improved readability
4. Easier debugging and testing

Things to take in account

The trick to implement single responsibility principle is to know what is the single responsibility of our class. However every developer has his/her vision for class responsibility, and since we don't have any instructions on how to implement, we are only left with our own interpretation.

There can be instances when we separate two classes that are in fact doing the same thing with business or architecture perspective. This can create a more complicated code with both the classes tightly coupled to each other, and thus reducing the sole purpose of SOLID principles

The key is not to overthink when creating new classes

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