Adding and removing properties in Javascript - Cheat Sheet & Quick Revision

Recall & Review

beginner

How do you add a new property to an existing JavaScript object?

You add a new property by assigning a value to it using dot notation or bracket notation. For example: <br>obj.newProp = value; or obj['newProp'] = value;

beginner

How do you remove a property from a JavaScript object?

You remove a property using the delete operator. For example: <br>delete obj.propName; removes the property propName from obj.

beginner

What happens if you try to access a property that does not exist on an object?

If a property does not exist, accessing it returns undefined. It does not cause an error.

intermediate

Can you add properties to objects created from classes or functions in JavaScript?

Yes, JavaScript objects are flexible. You can add properties to any object, including those created from classes or constructor functions, by assigning new properties.

intermediate

What is the difference between dot notation and bracket notation when adding or removing properties?

Dot notation uses a fixed property name (like obj.prop). Bracket notation uses a string or variable (like obj['prop'] or obj[varName]). Bracket notation is useful when property names have spaces or special characters or are dynamic.

Which of the following adds a property 'age' with value 30 to the object 'person'?

A Both B and C

B person['age'] = 30;

C person.age = 30;

D person.add('age', 30);

Both dot notation and bracket notation can add properties to an object.

How do you remove the property 'name' from the object 'user'?

A user.remove('name');

B user.name = null;

C delete user.name;

D user['name'] = undefined;

The 'delete' operator removes a property from an object.

What will be the output of console.log(obj.nonExistentProp); if 'nonExistentProp' does not exist on 'obj'?

A null

B Error

C 0

D undefined

Accessing a non-existing property returns 'undefined' in JavaScript.

Which notation allows you to use a variable as the property name when adding a property?

A Bracket notation

B Neither

C Both dot and bracket notation

D Dot notation

Bracket notation allows dynamic property names using variables.

If you set a property to undefined, does it remove the property from the object?

A Yes, it removes the property

B No, the property still exists with value undefined

C It depends on the object type

D It throws an error

Setting a property to undefined does not remove it; the property still exists.

Explain how to add and remove properties from a JavaScript object with examples.

Think about how you assign values to new keys and how you delete keys.

Describe the difference between setting a property to undefined and deleting a property from an object.

Consider what happens to the property key in each case.