JavaScript Design Patterns - Behavioral - Template

The template pattern allows defining the skeleton of an algorithm in the superclass but lets subclasses override specific algorithm steps without changing its structure.

In this example, We are creating a simple template method to calculate taxes and extending this template to VAT and GST (type of taxes). In this way, we can reuse the same structure in several tax classes.

class Tax {
  calc(value) {
    if (value >= 1000) {
      value = this.overThousand(value);
    }

    return this.complementaryFee(value);
  }

  complementaryFee(value) {
    return value + 10;
  }
}

class VAT extends Tax {
  constructor() {
    super();
  }

  overThousand(value) {
    return value * 1.1;
  }
}

class GST extends Tax {
  constructor() {
    super();
  }

  overThousand(value) {
    return value * 1.2;
  }
}

export { VAT, GST };

A complete example is here 👉 https://stackblitz.com/edit/vitejs-vite-ccbqh8?file=template.js

Conclusion

Use this pattern when you want to let clients extend only particular steps of an algorithm but not the whole algorithm or its structure.

I hope you found it helpful. Thanks for reading. 🙏

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