Adding and removing properties in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output after adding a property?

Consider the following JavaScript code. What will be logged to the console?

Javascript

const obj = {a: 1};
obj.b = 2;
console.log(obj);

A TypeError

B {"a":1}

C undefined

D {"a":1,"b":2}

💡 Hint

Adding a property to an object uses dot notation and changes the object.

The property 'b' is added with value 2, so the object now has keys 'a' and 'b'.

❓ Predict Output

intermediate

What happens after deleting a property?

What will be the output of this code snippet?

Javascript

const obj = {x: 10, y: 20};
delete obj.x;
console.log(obj);

A {"x":10,"y":20}

B {"y":20}

C undefined

D SyntaxError

💡 Hint

The delete operator removes a property from an object.

Deleting 'x' removes it from the object, leaving only 'y'.

❓ Predict Output

advanced

What is the output after adding a property with a variable key?

What will this code print to the console?

Javascript

const key = 'color';
const obj = {shape: 'circle'};
obj[key] = 'red';
console.log(obj);

A {"shape":"circle","key":"red"}

B {"color":"red"}

C {"shape":"circle","color":"red"}

D ReferenceError

💡 Hint

Bracket notation allows using variables as property keys.

The variable 'key' holds 'color', so obj['color'] = 'red' adds that property.

❓ Predict Output

advanced

What error occurs when deleting a non-configurable property?

What will happen when running this code?

Javascript

const obj = {};
Object.defineProperty(obj, 'fixed', {
  value: 42,
  configurable: false
});
delete obj.fixed;
console.log(obj.fixed);

A 42

B undefined

C TypeError

D SyntaxError

💡 Hint

Non-configurable properties cannot be deleted.

Deleting a non-configurable property fails silently in non-strict mode, so the property remains.

🧠 Conceptual

expert

Which option causes a runtime error when adding/removing properties?

Which of the following code snippets will cause a runtime error when executed?

A const obj = Object.freeze({a: 1}); obj.b = 2;

B const obj = {a: 1}; delete obj.a;

C const obj = {}; obj['b'] = 3;

D const obj = {a: 1}; obj.a = 5;

💡 Hint

Frozen objects cannot be changed; adding properties throws error in strict mode.

Object.freeze makes the object immutable. Adding properties throws a TypeError in strict mode.

Practice

1.

Which of the following is the correct way to add a new property age with value 25 to an existing object person?

easy

A. person.age = 25;

B. add person.age = 25;

C. person->age = 25;

D. person(age) = 25;

Solution

1. Step 1: Understand how to add properties in JavaScript objects

   Properties are added by assigning a value to object.property.

2. Step 2: Check each option's syntax

   Only person.age = 25; uses correct JavaScript syntax to add a property.

3. Final Answer:

   person.age = 25; -> Option A

4. Quick Check:

   Adding property = person.age = 25 [OK]

Hint: Use dot notation: object.property = value to add [OK]

Common Mistakes:

• Using invalid syntax like 'add' keyword
• Using arrow or parentheses incorrectly
• Trying to call property as a function

2.

Which of the following lines correctly removes the property color from the object car?

easy

A. car.color = null;

B. remove car.color;

C. car.delete(color);

D. delete car.color;

Solution

1. Step 1: Recall how to remove properties in JavaScript

   The delete keyword removes a property from an object.

2. Step 2: Verify each option

   Only delete car.color; correctly removes the property.

3. Final Answer:

   delete car.color; -> Option D

4. Quick Check:

   Remove property = delete object.property [OK]

Hint: Use 'delete object.property;' to remove properties [OK]

Common Mistakes:

• Using 'remove' keyword which doesn't exist
• Assigning null instead of deleting
• Calling delete as a method

3.

What will be the output of the following code?

const obj = {a: 1};
obj.b = 2;
delete obj.a;
console.log(obj);

medium

A. {a: 1, b: 2}

B. {b: 2}

C. {a: undefined, b: 2}

D. {}

Solution

1. Step 1: Add property 'b' to object

   After obj.b = 2;, object is {a:1, b:2}.

2. Step 2: Delete property 'a'

   delete obj.a; removes property 'a', so object becomes {b:2}.

3. Final Answer:

   {b: 2} -> Option B

4. Quick Check:

   Delete removes property completely [OK]

Hint: Deleting removes property; it doesn't set undefined [OK]

Common Mistakes:

• Thinking delete sets property to undefined
• Expecting original property to remain
• Confusing object output format

4.

Find the error in the following code that tries to remove the name property from user:

const user = {name: 'Alice', age: 30};
remove user.name;

medium

A. The keyword remove is invalid in JavaScript.

B. The property name cannot be deleted.

C. The object user is not defined.

D. The property should be set to null instead.

Solution

1. Step 1: Check the keyword used for deletion

   JavaScript uses delete, not remove, to delete properties.

2. Step 2: Confirm object and property validity

   user is defined and name is a valid property.

3. Final Answer:

   The keyword remove is invalid in JavaScript. -> Option A

4. Quick Check:

   Use 'delete' keyword, not 'remove' [OK]

Hint: Use 'delete' keyword to remove properties, not 'remove' [OK]

Common Mistakes:

• Using 'remove' instead of 'delete'
• Trying to set property to null to delete
• Assuming property can't be deleted

5.

You have an object settings with properties {theme: 'dark', fontSize: 14, showSidebar: true}. You want to remove showSidebar only if it exists, then add a new property language with value 'en'. Which code correctly does this?

hard

A. settings.showSidebar = undefined;
settings.language = 'en';

B. delete settings.showSidebar;
settings.language == 'en';

C. if ('showSidebar' in settings) {
delete settings.showSidebar;
}
settings.language = 'en';

D. if (settings.showSidebar) {
settings.showSidebar = null;
}
settings.language = 'en';

Solution

1. Step 1: Check if property exists before deleting

   Using 'showSidebar' in settings ensures property exists before deletion.

2. Step 2: Add new property correctly

   settings.language = 'en'; adds the new property properly.

3. Step 3: Verify other options

   delete settings.showSidebar;
   settings.language == 'en';

   uses wrong comparison operator == instead of assignment =.

   settings.showSidebar = undefined;
   settings.language = 'en';

   sets property to undefined but does not remove it.

   if (settings.showSidebar) {
     settings.showSidebar = null;
   }
   settings.language = 'en';

   sets property to null but does not remove it.

4. Final Answer:

   if ('showSidebar' in settings) { delete settings.showSidebar; } settings.language = 'en'; -> Option C

5. Quick Check:

   Check existence, delete, then add property [OK]

Hint: Check property with 'in' before delete; assign new property with '=' [OK]

Common Mistakes:

• Using '==' instead of '=' for assignment
• Setting property to undefined or null instead of deleting
• Not checking if property exists before deleting