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Quiz: ES6 Classes, Object-Oriented Programming & Async Data

Test your understanding of Classes, constructors, methods, inheritance, async/await, and fetch() API with these review questions.

1. Which special method is automatically executed when a new object instance is created with new MyClass()?

  1. constructor()
  2. init()
  3. create()
  4. setup()
Show Answer

The correct answer is A. The constructor() method is a special method for creating and initializing an object instance created with a class. Options B, C, and D are not the standard ES6 constructor keyword.

Concept Tested: Class Constructor Method

2. What keyword is used inside a class method to refer to the current object instance and its properties?

  1. this
  2. self
  3. me
  4. instance
Show Answer

The correct answer is A. In JavaScript classes, this refers to the specific instance of the class being created or operated upon. Options B, C, and D are used in other programming languages (like Python) but not JavaScript.

Concept Tested: This Instance Keyword

3. How does a child subclass inherit methods and properties from a parent superclass in ES6?

  1. class Car inherits Vehicle
  2. class Car extends Vehicle
  3. class Car implements Vehicle
  4. class Car derivedFrom Vehicle
Show Answer

The correct answer is B. The extends keyword is used in class declarations to create a class that is a child of another class. Options B, C, and D are invalid keywords in JavaScript class syntax.

Concept Tested: Class Inheritance Extends

4. What function must be called inside a derived subclass constructor before accessing this?

  1. parent()
  2. super()
  3. base()
  4. inherit()
Show Answer

The correct answer is B. In derived classes, super() must be called before using this to invoke the parent superclass constructor and initialize inherited properties. Options B, C, and D are incorrect.

Concept Tested: Super Constructor Call

5. What does the async keyword placed before a function declaration signify?

  1. The function executes in a separate Web Worker thread on the GPU
  2. The function operates asynchronously and automatically returns a Promise
  3. The function runs in an infinite background loop
  4. The function blocks the main UI thread until finished
Show Answer

The correct answer is B. An async function always returns a Promise. Inside an async function, the await keyword can pause execution until a Promise settles without blocking the browser. Options B, C, and D are incorrect.

Concept Tested: Async Function Promise

6. How do you asynchronously load and parse JSON data from an external web API using modern async/await?

  1. const data = fetch(url).parse();
  2. const data = loadJSONSync(url);
  3. const res = await fetch(url); const data = await res.json();
  4. const res = http.get(url); const data = res.body;
Show Answer

The correct answer is C. Modern JavaScript fetches network resources using await fetch(url) followed by await res.json() to parse the response payload into a JavaScript object. Options B, C, and D are invalid or outdated syntax.

Concept Tested: Async Await Fetch JSON

7. Which ES6 feature unpacks properties from an object into distinct variables (e.g. const { x, y } = particle;)?

  1. Variable Hoisting
  2. Array Slicing
  3. Object Destructuring
  4. Spread Syntax
Show Answer

The correct answer is C. Object destructuring is a JavaScript expression that makes it possible to unpack properties from objects into distinct variables concisely. Options B, C, and D refer to other language features.

Concept Tested: Object Destructuring Syntax

8. What does the Array method .map() return?

  1. The filtered subset of items matching a predicate
  2. A single accumulated scalar value
  3. A new array containing the results of calling a provided function on every element in the calling array
  4. An integer index of the first match
Show Answer

The correct answer is C. .map() transforms an array by applying a callback to each item, returning a brand-new array of equal length with the transformed values. Option B describes .reduce(). Option C describes .filter(). Option D describes .findIndex().

Concept Tested: Array Map Higher Order

9. Why is using try...catch blocks essential when fetching asynchronous external data across the internet?

  1. To prevent the browser from caching requests
  2. To speed up network download bandwidth
  3. To bypass cross-origin CORS security policies
  4. To gracefully handle network dropouts, HTTP errors (404/500), or invalid JSON parsing without crashing the application
Show Answer

The correct answer is D. Network requests can fail due to offline status, server outages, or corrupt payloads. Wrapping asynchronous calls in try...catch blocks catches runtime exceptions and allows fallback behavior. Options B, C, and D are false.

Concept Tested: Try Catch Async Error Handling

10. Which Array method removes elements that fail a test condition and returns a new array with only the elements that pass?

  1. .reduce()
  2. .map()
  3. .forEach()
  4. .filter()
Show Answer

The correct answer is D. .filter() evaluates a predicate function on every element, returning a new array containing only elements for which the callback returned true. Options B, C, and D perform different array operations.

Concept Tested: Array Filter Method