MCQs on Objects and Object-Oriented Basics | JavaScript

Master the essential concepts of objects and object-oriented programming with these multiple-choice questions. This set covers object declaration, properties, the this keyword, and methods—ideal for JavaScript and OOP learners.

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MCQs

1. Declaring Objects and Accessing Properties

1. Which syntax is correct for declaring an object in JavaScript?
   • A) var obj = {name: "John"}
   • B) var obj = "John";
   • C) var obj() = {name: "John"};
   • D) declare obj {name: "John"}
2. What is the output of console.log(obj.name); if obj = {name: "Alice"};?
   • A) “Alice”
   • B) undefined
   • C) null
   • D) obj.name
3. How do you access a property with a space in its name?
   • A) obj["first name"]
   • B) obj.first name
   • C) obj.first-name
   • D) obj{first name}
4. What is the syntax for dynamically accessing a property in an object?
   • A) obj[prop]
   • B) obj.prop
   • C) obj{prop}
   • D) prop(obj)
5. Which operator is used to access an object’s property?
   • A) +
   • B) -
   • C) .
   • D) &
6. What would console.log(person["age"]); display if var person = {age: 25};?
   • A) undefined
   • B) 25
   • C) null
   • D) error
7. Which syntax can be used to access a nested property in an object?
   • A) obj["property"]["nested"]
   • B) obj.property:nested
   • C) obj{property}.{nested}
   • D) obj+property.nested
8. If let car = {color: "blue"};, which of the following accesses the color property?
   • A) car[color]
   • B) car.color
   • C) car{color}
   • D) car.color[]

2. Adding, Updating, and Deleting Properties

9. How do you add a new property to an existing object?
   • A) object.property = value;
   • B) object{property} = value;
   • C) object->property = value;
   • D) object(property) = value;
10. What will be the result of delete person.age; if person = {age: 30};?
    • A) age = null
    • B) The age property is removed
    • C) age becomes undefined
    • D) Error is thrown
11. How would you update the color property of car to "red" if let car = {color: "blue"};?
    • A) car.color = "red";
    • B) car("color") = "red";
    • C) update car.color = "red";
    • D) color = "red";
12. Which method deletes a property from an object?
    • A) remove()
    • B) delete
    • C) clear()
    • D) cut()
13. What does car.model = "Sedan"; do if car initially has no model property?
    • A) Adds model property to car
    • B) Throws an error
    • C) Returns undefined
    • D) Removes model property
14. What would be the output of console.log(obj.height); after delete obj.height;?
    • A) The deleted value
    • B) null
    • C) undefined
    • D) error
15. Which is a valid way to add a property with a variable name?
    • A) obj[variable] = value;
    • B) obj(variable) = value;
    • C) obj.variable(value);
    • D) obj->variable = value;
16. How do you update an existing property value in an object?
    • A) object.property = newValue;
    • B) object->property = newValue;
    • C) object.updateProperty = newValue;
    • D) update.object.property(newValue);

3. this Keyword Basics

17. In an object, what does this refer to by default?
    • A) The object itself
    • B) Global scope
    • C) The DOM
    • D) Undefined
18. Which of these best describes the this keyword?
    • A) A variable that stores function arguments
    • B) A pointer to the calling context
    • C) A placeholder for strings
    • D) A syntax to create new variables
19. How is this used in a method?
    • A) this refers to the global object
    • B) this refers to the calling object
    • C) this is undefined in methods
    • D) this refers to the nested object
20. What does this.name represent inside an object method?
    • A) Accesses the name property of the object
    • B) Refers to a local variable name
    • C) Refers to a name property in global scope
    • D) Throws an error
21. What is this bound to in an arrow function?
    • A) The object itself
    • B) Its lexical scope
    • C) this is always undefined
    • D) The window object
22. Which scenario could make this undefined in an object?
    • A) Calling a method inside the object
    • B) Using strict mode with function expressions
    • C) Declaring a property within an object
    • D) Using this in a nested function without binding
23. What does this refer to when invoked outside any function or object?
    • A) Global object (e.g., window in browsers)
    • B) The DOM
    • C) An empty object
    • D) null
24. How do you bind this to a specific object in JavaScript?
    • A) .bind(this)
    • B) .bind(object)
    • C) object.this()
    • D) .apply(object, this)

4. Methods in Objects

25. Which syntax correctly defines a method within an object?
    • A) method: function() { /* code */ }
    • B) function.method() { /* code */ }
    • C) object.method() = function { /* code */ }
    • D) method() => { /* code */ }
26. What is the term for a function that is a property of an object?
    • A) Prototype
    • B) Method
    • C) Constructor
    • D) Object literal
27. How do you call a method named greet in an object person?
    • A) person.greet()
    • B) greet(person)
    • C) person["greet"]
    • D) call person.greet()
28. What is the return keyword used for in a method?
    • A) To end the function and send a value back
    • B) To create a new object property
    • C) To assign a property
    • D) To delete a method
29. If let obj = { add: function(x, y) { return x + y; }};, what does obj.add(2, 3); return?
    • A) 5
    • B) 23
    • C) undefined
    • D) Error
30. What is required for a method to access properties in the same object?
    • A) Use this.propertyName
    • B) Use obj.propertyName
    • C) Use return.propertyName
    • D) Use propertyName.this

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Answers

Qno	Answer
1	A) var obj = {name: "John"};
2	A) “Alice”
3	A) obj["first name"]
4	A) obj[prop]
5	C) .
6	B) 25
7	A) obj["property"]["nested"]
8	B) car.color
9	A) object.property = value;
10	B) The age property is removed
11	A) car.color = "red";
12	B) delete
13	A) Adds model property to car
14	C) undefined
15	A) obj[variable] = value;
16	A) object.property = newValue;
17	A) The object itself
18	B) A pointer to the calling context
19	B) this refers to the calling object
20	A) Accesses the name property of the object
21	B) Its lexical scope
22	B) Using strict mode with function expressions
23	A) Global object (e.g., window in browsers)
24	B) .bind(object)
25	A) method: function() { /* code */ }
26	B) Method
27	A) person.greet()
28	A) To end the function and send a value back
29	A) 5
30	A) Use this.propertyName