Template Method

Defines an algorithm's structure (skeleton) in a base class. Invariant steps are implemented there; variant steps are left abstract or provide default implementations that subclasses override.

When to Use

• Multiple classes share the same algorithm structure but differ in specific steps
• Duplication of the algorithm skeleton across subclasses should be eliminated
• Extension points (hooks) should be provided without allowing the overall structure to be changed
• Framework/library code provides the algorithm; application code fills in steps

How to Apply

1. Identify invariant parts (always the same) and variant parts (differ by subclass) of the algorithm
2. Create an abstract base class with the templateMethod() that calls the steps in order
3. Invariant steps are fully implemented in the base class
4. Variant steps are declared abstract or given a no-op/default implementation (hooks)
5. Subclasses override only the variant steps

Example

abstract class DataMigration {
  // Template method — defines the skeleton
  run() {
    this.readData();
    const processed = this.processData();
    this.writeData(processed);
    this.sendReport();  // hook — optional override
  }

  abstract readData(): void;
  abstract processData(): object[];
  abstract writeData(data: object[]): void;

  protected sendReport() { /* default: do nothing */ }
}

class CsvMigration extends DataMigration {
  readData()                { console.log('Read CSV'); }
  processData(): object[]   { return []; }
  writeData(d: object[])    { console.log('Write to DB'); }
  override sendReport()     { console.log('Email report'); }
}

References

• refactoring.guru/design-patterns/template-method