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.