JavaScript Object Fundamentals Quiz — Questions & Answers

Sharpen your understanding of JavaScript objects with these easy questions, ideal for beginners and anyone reviewing the basics. Each question covers a different foundational aspect of JavaScript objects.

This quiz contains 10 questions. Below is a complete reference of all questions, answer choices, and correct answers. You can use this section to review after taking the interactive quiz above.

1. Question 1: JavaScript Object Syntax

   Which of the following correctly defines a JavaScript object with properties 'name' and 'age'?

   • var person = { name: 'Alice', age: 25 };
   • var person = [ name: 'Alice', age: 25 ];
   • var person = 'name: Alice, age: 25';
   • var person = ( name: 'Alice', age: 25 );
   Show correct answer

   Correct answer: var person = { name: 'Alice', age: 25 };

   Explanation: JavaScript objects are defined using curly braces and key-value pairs, as in the first option. Square brackets are for arrays, parentheses are not used for objects, and the last option defines a string, not an object.

2. Question 2: Accessing Object Properties

   Given var car = { model: 'Toyota', year: 2020 }, which code accesses the value 'Toyota'?

   • car.model
   • car['model']
   • car-model
   • car(model)
   Show correct answer

   Correct answer: car.model

   Explanation: Object properties can be accessed with dot notation (car.model). Bracket notation is also valid, but 'car.model' is direct and correct. 'car(model)' is a function call, and 'car-model' is invalid syntax.

3. Question 3: Adding Properties to Objects

   How can you add a new property 'color' with value 'red' to an existing object bike?

   • bike = { color: 'red' };
   • bike.color = 'red';
   • color.bike = 'red';
   • bike('color') = 'red';
   Show correct answer

   Correct answer: bike.color = 'red';

   Explanation: Adding a property is done with dot notation as in the correct answer. The second option reverses the object/property relationship. The third is not valid JavaScript, and the fourth option replaces the whole object instead of adding a property.

4. Question 4: Object Property Types

   Which type of values can a JavaScript object property NOT hold?

   • A number
   • A function
   • An undefined value
   • JavaScript object properties can hold any type
   Show correct answer

   Correct answer: JavaScript object properties can hold any type

   Explanation: JavaScript object properties are flexible and can hold any data type, including numbers, undefined, and even functions. Therefore, there is no type that they cannot hold.

5. Question 5: Deleting an Object Property

   What JavaScript keyword removes a property from an object?

   • unset
   • remove
   • delete
   • discard
   Show correct answer

   Correct answer: delete

   Explanation: The 'delete' keyword is the correct way to remove a property from a JavaScript object. The other options are not valid JavaScript keywords for this action.

6. Question 6: Checking Property Existence

   Which syntax checks if an object user has a property named 'email'?

   • user->email
   • user.has('email')
   • 'email' in user
   • email in user
   Show correct answer

   Correct answer: 'email' in user

   Explanation: The 'in' operator checks if a property exists in an object. The second and third options are not valid JavaScript, and the fourth option has the operands reversed.

7. Question 7: Iterating Properties

   Which loop iterates over all enumerable properties of a JavaScript object?

   • for (key of object)
   • foreach key in object
   • forEach(object)
   • for (var key in object)
   Show correct answer

   Correct answer: for (var key in object)

   Explanation: The 'for...in' loop is used to iterate over object properties. 'forEach' works with arrays, 'for...of' is designed for iterable objects like arrays, and 'foreach key in object' is not valid syntax.

8. Question 8: Object Methods

   How do you define a function inside a JavaScript object so it becomes a method?

   • As a number value
   • As a string value
   • You cannot define functions in objects
   • As a property with a function value
   Show correct answer

   Correct answer: As a property with a function value

   Explanation: A method is a property whose value is a function. Defining it as a string or number won't make it a method, and JavaScript does allow defining functions inside objects.

9. Question 9: Nested Objects

   Given an object student with a property address that is also an object, how do you access the city inside address?

   • student.address.city
   • student(city)
   • student.address->city
   • student['address'.city]
   Show correct answer

   Correct answer: student.address.city

   Explanation: You access nested object properties with dot notation as in 'student.address.city'. The other options use incorrect syntax for accessing nested properties.

10. Question 10: Object Comparison

    What happens if you compare two different objects in JavaScript that have identical properties and values using '=='?

    • They are equal only if they have the same properties
    • They are always equal
    • JavaScript throws an error
    • They are not equal
    Show correct answer

    Correct answer: They are not equal

    Explanation: In JavaScript, objects are compared by reference, not by their content. Two separately created objects with identical properties and values are still considered different, so the comparison returns false. The other options misrepresent how JavaScript handles object comparison or suggest errors that do not occur.