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Prototype

Overview​

Create new objects by copying an existing instance (the prototype).

When to use​

  • Object creation is expensive or complex.
  • You need many similar instances with small variations.

Java example​

class Report implements Cloneable {
    private String title;
    private List<String> sections;

    Report(String title, List<String> sections) {
        this.title = title;
        this.sections = new ArrayList<>(sections);
    }

    public Report copy() {
        return new Report(title, sections);
    }
}

TypeScript example​

type Report = {
  title: string;
  sections: string[];
};

const prototype: Report = { title: "Monthly", sections: ["Summary", "KPIs"] };
const copy: Report = { ...prototype, sections: [...prototype.sections] };

Pros and cons​

Pros:

  • Fast creation via cloning.
  • Avoids complex constructor logic.

Cons:

  • Deep copy can be tricky.
  • Clones may share mutable state if not careful.

Common pitfalls​

  • Shallow copy of nested objects.
  • Mutating shared references after cloning.