🔥 JavaScript Interview Series(8): Mutable vs Immutable Data in JavaScript

Hey everyone, and welcome to our JavaScript Interview Series! Today, we're diving deep into a fundamental concept that often trips up developers in interviews: mutable versus immutable data. Understanding this distinction is crucial for writing predictable, bug-free code. Let's get started!

What's the Big Deal with Mutable vs. Immutable?

In simple terms, mutable means something can be changed after it's created. Immutable means it cannot be changed.

In JavaScript, this concept is tied to data types.

• Primitive data types are immutable. These include string, number, boolean, null, undefined, symbol, and BigInt. Once you create a primitive value, you can't alter that specific value in memory. When it seems like you're changing it, you're actually creating a new value and assigning it to the variable.

• Objects (including arrays and functions) are mutable. This means you can change their properties or elements after they've been created. This is because variables that hold objects actually store a reference—a pointer to the location in memory where the object lives.

Now, let's get to the questions you're likely to face.

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1. What is the difference between mutable and immutable data types in JavaScript?

Assessment of: Core understanding of data types and their characteristics.
Standard Answer: The key difference lies in whether the value can be changed after it's created. Immutable data types, like primitives (strings, numbers, booleans), cannot be altered. Any operation that appears to modify an immutable value actually creates a new value. Mutable data types, which are objects (including arrays), can be modified in place after their creation. Variables holding objects store a reference to the object's location in memory, so changes to the object are reflected through all references to it.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can you give an example of trying to "change" a string and explain what's happening under the hood?
2. If an object is mutable, what does a variable assigned to that object actually hold?
3. What are the practical implications of this difference when writing functions?

2. Is JavaScript a "pass-by-value" or "pass-by-reference" language?

Assessment of: Understanding how data is passed to functions.
Standard Answer: This is a bit of a trick question. JavaScript is technically always pass-by-value. However, the "value" that is passed depends on the data type. For primitive types, a copy of the actual value is passed to the function. For objects, a copy of the reference (the memory address) is passed. So, while you can't reassign the original object variable from within the function, you can modify the properties of the object that the reference points to. This behavior can make it seem like it's pass-by-reference for objects.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. If you pass an object to a function and change one of its properties, will the original object be affected? Why?
2. If you pass an object to a function and try to reassign the parameter to a new object, will the original object variable be affected? Why not?
3. How does this behavior impact the way you handle state in applications, for example, in a Redux-like architecture?

3. What is the difference between const and Object.freeze()?

Assessment of: Knowledge of immutability enforcement.
Standard Answer: const and Object.freeze() serve different purposes. const creates an immutable binding, meaning you cannot reassign the variable to a new value. However, if the const variable holds a mutable object, the object's properties can still be changed. Object.freeze(), on the other hand, makes an object's properties immutable. You cannot add, remove, or change the properties of a frozen object.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. If I have a const object, can I add a new property to it? Why?
2. Does Object.freeze() work on nested objects? What is this concept called?
3. How would you create a "deeply" frozen object?

4. Can you explain the difference between a shallow copy and a deep copy of an object?

Assessment of: Understanding object copying techniques.
Standard Answer: A shallow copy creates a new object and copies the top-level properties of the original object. If a property is a primitive type, the value is copied. If a property is a reference to another object, the reference is copied, not the nested object itself. This means both the original and the copy will point to the same nested objects. A deep copy, on the other hand, creates a completely independent duplicate of the original object, including all nested objects and arrays. Changes to the deep copy will not affect the original.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. How can you create a shallow copy of an object in JavaScript? (e.g., spread syntax, Object.assign())
2. What is a common way to create a deep copy, and what are its limitations? (e.g., JSON.stringify() and JSON.parse())
3. When would you prefer a shallow copy over a deep copy?

5. What are the benefits of using immutable data structures?

Assessment of: Understanding the "why" behind immutability.
Standard Answer: The main benefits of immutability are:

• Predictability: It's easier to reason about your code when you know that data cannot be changed unexpectedly. This helps to prevent bugs caused by unintended side effects.
• Performance Optimization: Immutability allows for easy and cheap change detection. If the reference to an object hasn't changed, you know the object itself hasn't changed. This is heavily used in libraries like React for optimizing re-renders.
• Concurrency: Immutable data is inherently thread-safe, which is beneficial in concurrent and parallel programming environments.
• Easier Debugging and State Management: With immutable data, you can keep a history of state changes, which simplifies debugging and implementing features like undo/redo. Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)
• How does React's state management rely on the concept of immutability?
• Can you explain how immutability can lead to performance gains in certain scenarios?
• Are there any downsides to using immutable data structures?

6. Which array methods are mutable and which are immutable?

Assessment of: Knowledge of the JavaScript standard library.
Standard Answer: Several common array methods mutate the original array. These include push(), pop(), shift(), unshift(), splice(), sort(), and reverse(). On the other hand, many methods are immutable and return a new array, leaving the original unchanged. Examples include map(), filter(), reduce(), slice(), and concat(). The spread syntax (...) is also a popular way to create a new array immutably.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. If you need to add an element to the beginning of an array without mutating it, how would you do it?
2. How would you remove an element from an array by its index in an immutable way?
3. Why is it often recommended to use the immutable versions of these methods in modern JavaScript development?

7. How can you achieve immutability for objects and arrays in your code?

Assessment of: Practical application of immutability principles.
Standard Answer: To achieve immutability, you should avoid methods that mutate the original data. For objects, instead of directly modifying properties, you can create a new object using the spread syntax ({...original, propertyToChange: newValue}) or Object.assign({}, original, { propertyToChange: newValue }). For arrays, use methods that return a new array like map() or filter(), and the spread syntax for adding or removing elements. For deep immutability, you might need to write a recursive function to freeze all nested objects or use a library.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Show me how you would update a property in a nested object without mutating the original object.
2. When might the spread operator not be sufficient for achieving immutability?
3. What are some popular libraries that help enforce immutability in JavaScript?

8. What is a "side effect" in the context of functions, and how does immutability relate to it?

Assessment of: Understanding functional programming concepts.
Standard Answer: A side effect is any effect of a function that is observable outside of the function itself, other than its return value. Modifying a global variable or mutating an object that was passed in as an argument are common examples of side effects. Immutability is a key principle in reducing side effects. By working with immutable data, your functions don't change external state. Instead, they take data as input and produce new data as output, leading to what are called pure functions. Pure functions are easier to test, reason about, and debug.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. Can you give an example of a function with a side effect and then refactor it to be a pure function?
2. Why are pure functions with immutable data so important in functional programming?
3. How can side effects make code harder to debug?

9. What are libraries like Immutable.js, and why would you use them?

Assessment of: Awareness of the broader JavaScript ecosystem.
Standard Answer: Immutable.js is a library by Facebook that provides a set of persistent immutable data structures like List and Map. These data structures are completely separate from JavaScript's native objects and arrays. The primary reason to use a library like this is for performance and safety in large applications. It uses advanced techniques like structural sharing to optimize memory usage when creating new versions of data structures. This can be more efficient than manually deep copying large, complex objects. It also provides a rich, chainable API for data manipulation.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What is "structural sharing" and how does it improve performance?
2. What are some potential downsides of introducing a library like Immutable.js into a project? (e.g., learning curve, interoperability with native JS)
3. Are there other libraries that help with immutability, and how do they differ? (e.g., Immer)

10. How would you design a function that updates a user's profile information immutably?

Assessment of: Problem-solving and applying concepts to a practical scenario.
Standard Answer: I would design a function that takes two arguments: the original user object and an object containing the updates. The function would then return a new user object with the updates applied, leaving the original object untouched. I would use the spread syntax for this because it's concise and clearly expresses the intent to create a new object.

function updateUserProfile(user, updates) {
  return {
    ...user,
    ...updates
  };
}

const currentUser = {
  id: 1,
  name: 'John Doe',
  email: '[e-mail]'
};

const newProfileInfo = {
  email: '[e-mail]',
  lastLogin: new Date()
};

const updatedUser = updateUserProfile(currentUser, newProfileInfo);

// currentUser remains unchanged
// updatedUser is a new object with the merged information

This approach ensures that the function is pure and has no side effects.
Possible 3 follow-up questions: 👉 (Want to test your skills? Try a Mock Interview — each question comes with real-time voice insights)

1. What if the user object had a nested object, like preferences? How would you update a property within preferences immutably?
2. How would you handle removing a property from the user profile immutably?
3. If this function were part of a larger application state, why is returning a new object crucial?