Template Method

Two exporters, one copy-pasted skeleton

CSV and JSON exporters both do open → header → rows → close — the skeleton is duplicated, only the formatting differs.

class CsvExporter {
  export(rows) {
    open(file);                 // 1
    write("id,name\n");      // 2
    rows.forEach(r =>
      write(`${r.id},${r.name}\n`)); // 3
    close(file);                // 4
  }
}

class JsonExporter {
  export(rows) {
    open(file);                 // 1
    write("[\n");            // 2
    rows.forEach(r =>
      write(JSON.stringify(r))); // 3
    close(file);                // 4
  }
}

Fix the skeleton once, override the steps

The base class owns export() — the template method — and calls hooks that subclasses override.

class Exporter {
  export(rows) {       // skeleton
    this.open();
    this.header();
    rows.forEach(r =>
      this.row(r));
    this.close();
  }
  header() {}  row(r) {}  // hooks
}

Same skeleton, different steps

Pick an exporter and run it. All four steps fire in the same order — only header() and row() change what they produce.

// click "Run export()" to see the output

One skeleton, many fillings

• Define the algorithm's skeleton once in the base class; let subclasses fill in specific steps — removes duplication (DRY).
• Hollywood principle: "don't call us, we'll call you" — the base class calls your hooks, not the other way around.
• vs. Strategy: Template Method swaps steps via inheritance at compile time; Strategy swaps a whole algorithm via composition at runtime.
• Careful: inheritance is rigid — one base class, one chosen shape. Prefer Strategy when callers need to swap behavior at runtime.