Prototype and Cloning

Copying instead of constructing

Intent: create new objects by copying an existing, already configured prototype rather than building from scratch. It helps when construction is expensive or complex and many objects differ only slightly: document templates, game entities, default request configurations. The key decision is shallow versus deep copy: a shallow copy shares nested objects, so mutating one copy can change another. JavaScript is itself prototype-based, and modern runtimes provide structuredClone() for deep copies of plain data (it does not copy functions or class prototypes, so check the docs for what it supports). In Python, the copy module offers copy.copy and copy.deepcopy. Avoid cloning objects that hold resources such as open connections or file handles.

Cloning a report template

Python, where the copy module is idiomatic.

import copy
from dataclasses import dataclass, field

@dataclass
class ReportTemplate:
    title: str
    sections: list[str] = field(default_factory=list)
    filters: dict[str, str] = field(default_factory=dict)

monthly_sales = ReportTemplate(
    title="Monthly sales",
    sections=["summary", "by-region", "top-products"],
    filters={"currency": "EUR"},
)

# Deep copy: the new report gets its own lists and dicts
q3_report = copy.deepcopy(monthly_sales)
q3_report.title = "Q3 sales"
q3_report.filters["quarter"] = "Q3"

# A shallow copy would share `filters`, so the edit above
# would also change monthly_sales.filters
shared = copy.copy(monthly_sales)

Spread is shallow

In JavaScript, { ...obj } and [...arr] copy only the top level; nested objects are still shared between the original and the copy.

त्वरित जाँच: What is the risk of a shallow copy?

• It cannot copy strings
• It deletes the original
• Nested objects are shared, so changing them in one copy affects the other
• It always throws for nested data