epam-short-track

Task 3. Deep-copy

Folder Name	Branch	Coefficient
/deep-copy	deep-copy	0.5

Task Description

Implement a custom copy function that creates a deep copy of a given object. Your implementation must handle all types listed below, support any level of nesting, and strictly follow the requirements.

Requirements

• Do not use rest/spread operators (...), structuredClone, JSON.stringify, JSON.parse, or any npm libraries.
• The function must perform a true deep copy: all nested objects and arrays must be recursively copied.
• All types must be handled:
  • Basic types: string, number, boolean, null, undefined, BigInt
  • Special types: Date, functions, arrays, objects, get/set accessors
• Keys must be strings. Keys and values must not be Symbols.
• Any property value can be null or undefined.
• The function must correctly copy objects with any nesting level.
• The solution must not mutate the original object.
• The code must be clean, readable, and follow best practices.

Example

const complexObject = {
  stringValue: "Hello, World!",
  numberValue: 42,
  booleanValue: true,
  nullValue: null,
  undefinedValue: undefined,
  bigIntValue: BigInt(9007199254740991),
  arrayValue: [1, 2, 3, "four", { nestedKey: "nestedValue" }],
  valueDate: new Date(),
  nestedObject: {
    level1: {
      level2: {
        level3: {
          stringValue: "Deep Nesting",
          numberValue: 3.14,
          booleanValue: false,
          arrayValue: [true, false, null],
          anotherNestedObject: {
            level4: "Final Level",
            functionValue: function() {
              return "I am a function!";
            }
          }
        }
      }
    }
  },
  functionValue: function() {
    return "I am a function!";
  }
};

const obj1 = copy(complexObject);

console.log(obj1 === complexObject); // false
console.log(obj1.nestedObject.level1.level2.level3 === complexObject.nestedObject.level1.level2.level3); // false
console.log(obj1.arrayValue === complexObject.arrayValue); // false

Implementation Details

• Your index.js file in the deep-copy folder must export the copy function as follows:

function copy(obj) {}

module.exports = { copy };

• Test your function locally with various objects, including the example above, before submitting your Pull Request.

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Pull Request Requirements

Your Pull Request must include:

• Task URL.
• Description of the types and structures of objects used for verification (with examples).
• Submission Date / Deadline Date.
• Your self-check of the task’s completion using checkboxes.

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Developer’s Diary

While working on this task, keep a developer’s diary. Write down the decisions you made, the approaches you considered, where you got stuck, and how you worked through it.

The diary is not graded. Its purpose is to help you understand your own work more deeply and to give your mentor a basis for a real conversation about the task.

The “Diary” folder can be placed in the root of the project.

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Mentor Checklist

Maximum Score: 100 points

Criteria	Points
The branch is named deep-copy	2
Commit messages follow RS School Git Convention	5
The deep-copy folder exists	2
The index.js file exists in the correct folder	2
The index.js file exports the copy function	1
The function correctly deep-copies the following types:
String	5
Number	5
Boolean	5
Null	5
Undefined	5
BigInt	5
Date	8
Function	8
Array	8
Object	8
get/set accessors	8
The solution is well-structured, readable, and follows best practices	10
Pull Request description includes all required elements:
Task URL is included in the PR	2
Description of test objects and types is included in the PR	2
Submission and Deadline Dates are included in the PR	2
Your self-check of the task’s completion using checkboxes is included in the PR	2
Penalty:
Less than 3 commits in the PR	-20
Commit after the deadline and before mentor review	-20
The solution includes any comments	-50
The solution includes console.log	-10
The solution includes rest/spread operators (...), structuredClone, JSON.stringify, JSON.parse, or any npm libraries.	-100

Mentor Interview Topics

After submitting the task, your mentor will ask 4–5 questions from the areas below. Answers account for ~80 points of the total score, so make sure you can explain the concepts in your own words — not just recite a definition.

Deep copy vs. shallow copy

• What is the difference between a shallow copy and a deep copy? Give a concrete example where shallow copy would cause a bug.
• Why can’t you use the assignment operator (=) to copy an object in JavaScript?
• What does Object.assign copy, and why is it not sufficient for a deep copy?

Recursion

• How does your implementation handle arbitrary nesting depth? Walk through the recursive calls for a two-level nested object.
• What is the base case of your recursion, and why is it important?
• What could go wrong with a deeply nested object in terms of call-stack limits?

Type detection and handling

• How do you distinguish between an Array, a Date, a plain Object, null, and a function at runtime in JavaScript?
• Why does typeof null === 'object' and how does your code handle that edge case?
• How does instanceof work, and when might it give unexpected results?
• Why must Date objects be copied differently from plain objects?
• How do you copy a function? Is a copied function truly independent from the original?

Property descriptors and accessors

• What is a property descriptor in JavaScript? What fields can it contain?
• What is the difference between a data descriptor and an accessor descriptor?
• How do you read and write property descriptors programmatically (Object.getOwnPropertyDescriptor, Object.defineProperty)?

JavaScript object model

• What is the difference between own properties and inherited properties? Which ones should copy handle?
• What does Object.getOwnPropertyNames return vs. for...in?
• What is the prototype chain, and should copy replicate it? Why or why not?

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Notes

• The solution must be well-structured, readable, and follow best practices.
• If any required element is missing or incorrect, points will be deducted accordingly.
• All requirements regarding forbidden methods and types must be strictly followed.

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