JS Template Method Pattern

Define the skeleton of an algorithm in a base class and let subclasses override specific steps

When to Use

• Multiple classes share the same algorithmic structure but differ in specific steps
• You want to enforce a fixed sequence of operations while allowing step customization
• Building frameworks or libraries where users extend base behavior

Instructions

1. Create a base class with a "template method" that calls a series of step methods in order.
2. Implement default behavior for shared steps; leave varying steps abstract or as no-ops.
3. Subclasses override only the steps they need to customize.
4. Never let subclasses change the template method itself — keep it final via convention or documentation.

class DataProcessor {
  process(data) {
    const validated = this.validate(data);
    const transformed = this.transform(validated);
    return this.format(transformed);
  }
  validate(data) {
    return data;
  } // default: no-op
  transform(data) {
    throw new Error('Subclass must implement transform()');
  }
  format(data) {
    return JSON.stringify(data);
  } // default: JSON
}

class CSVProcessor extends DataProcessor {
  transform(data) {
    return data.map((row) => row.join(','));
  }
  format(data) {
    return data.join('\n');
  }
}

Details

The Template Method pattern uses inheritance to vary parts of an algorithm. The base class defines the algorithm's skeleton, and subclasses fill in the blanks. This is the inverse of the Strategy pattern — Template Method uses inheritance, Strategy uses composition.

Trade-offs:

• Tight coupling through inheritance — subclasses are bound to the base class structure
• Hard to compose — you cannot mix steps from different base classes
• JavaScript's prototype chain makes "final" methods hard to enforce

When NOT to use:

• When steps vary independently and need to be composed freely — use Strategy pattern instead
• When there are only one or two implementations — inheritance overhead is not worth it
• When functional composition (pipe/compose) achieves the same result more flexibly

Source

https://patterns.dev/javascript/template-method-pattern

Process

1. Read the instructions and examples in this document.
2. Apply the patterns to your implementation, adapting to your specific context.
3. Verify your implementation against the details and edge cases listed above.

Harness Integration

• Type: knowledge — this skill is a reference document, not a procedural workflow.
• No tools or state — consumed as context by other skills and agents.

Success Criteria

• The patterns described in this document are applied correctly in the implementation.
• Edge cases and anti-patterns listed in this document are avoided.