Keys concept in React - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to add a unique key to each list item in React.

React

const items = ['apple', 'banana', 'cherry'];
const list = items.map((item, index) => <li key=[1]>{item}</li>);

Drag options to blanks, or click blank then click option'

Aindex

Bitem

CMath.random()

Ditems

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-unique value like the whole array as key

Using Math.random() which changes on every render

Using index as the key ensures each list item has a unique identifier in this simple example.

2fill in blank

medium

Complete the code to correctly assign keys when rendering a list of objects.

React

const users = [{id: 1, name: 'Anna'}, {id: 2, name: 'Ben'}];
const list = users.map(user => <div key=[1]>{user.name}</div>);

Drag options to blanks, or click blank then click option'

Auser.name

BMath.random()

Cuser

Duser.id

Attempts:

3 left

💡 Hint

Common Mistakes

Using user object directly as key

Using random values that change on every render

Using user.id as key is best because it is unique and stable.

3fill in blank

hard

Fix the error in the code by choosing the correct key value.

React

const tasks = ['clean', 'cook', 'shop'];
return tasks.map(task => <p key=[1]>{task}</p>);

Drag options to blanks, or click blank then click option'

Atasks

Btask

CMath.random()

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Using the whole array as key

Using random values that change every render

Using task as key works here because task names are unique strings.

4fill in blank

hard

Fill both blanks to create a list with unique keys and display user names.

React

const users = [{id: 10, name: 'Lara'}, {id: 20, name: 'Mike'}];
const list = users.map(user => <li key=[1]>{user.[2]</li>);

Drag options to blanks, or click blank then click option'

Auser.id

Buser.name

Cname

Did

Attempts:

3 left

💡 Hint

Common Mistakes

Using name as key instead of id

Using incorrect property names

Use user.id for the key and user.name to display the name.

5fill in blank

hard

Fill all three blanks to create a React list with keys and display user emails.

React

const users = [{uid: 'a1', email: '[e-mail]'}, {uid: 'b2', email: '[e-mail]'}];
const list = users.map(user => <div key=[1]>{user.[2]</div>);

return <section>[3]</section>;

Drag options to blanks, or click blank then click option'

Auser.uid

Bemail

Clist

Duser.email

Attempts:

3 left

💡 Hint

Common Mistakes

Using email as key instead of uid

Not rendering the list inside the section

Use user.uid as key, access user.email to display emails, and render the list inside the section.

Practice

1. Why does React require keys when rendering lists with map()?

easy

A. To uniquely identify each list item for efficient updates

B. To style each list item differently

C. To sort the list items automatically

D. To prevent the list from rendering

Solution

1. Step 1: Understand React's list rendering

   React uses keys to track each item in a list so it can update only changed items instead of re-rendering the whole list.

2. Step 2: Identify the purpose of keys

   Keys must be unique and stable to help React identify which items changed, were added, or removed.

3. Final Answer:

   To uniquely identify each list item for efficient updates -> Option A

4. Quick Check:

   Keys = Unique IDs for list items [OK]

Hint: Keys uniquely identify list items for React updates [OK]

Common Mistakes:

• Thinking keys are for styling
• Believing keys sort the list
• Assuming keys prevent rendering

2. Which of the following is the correct way to assign keys in a React list?

easy

A. {items.map(item => <li key={index}>{item.name}</li>)}

B. {items.map(item => <li key={item.id}>{item.name}</li>)}

C. {items.map(item => <li key={Math.random()}>{item.name}</li>)}

D. {items.map(item => <li>{item.name}</li>)}

Solution

1. Step 1: Check key uniqueness and stability

   Using item.id ensures each key is unique and stable across renders.

2. Step 2: Evaluate other options

   Using index or Math.random() can cause unstable keys leading to rendering bugs. Omitting keys causes React warnings.

3. Final Answer:

   {items.map(item => <li key={item.id}>{item.name}</li>)} -> Option B

4. Quick Check:

   Use stable unique IDs as keys [OK]

Hint: Use unique stable IDs, not index or random, for keys [OK]

Common Mistakes:

• Using array index as key when list changes
• Using random values for keys
• Not providing any key

3. What will be the output in the console when this React code runs?

const items = [
  { id: 1, name: 'Apple' },
  { id: 2, name: 'Banana' },
  { id: 3, name: 'Cherry' }
];

const list = items.map(item => <li key={item.id}>{item.name}</li>);
console.log(list.length);

medium

A. 3

B. undefined

C. 0

D. Error

Solution

1. Step 1: Understand the map function output

   The map creates an array of 3 <li> React elements, one for each item.

2. Step 2: Check the length of the array

   The array list has 3 elements, so list.length is 3.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   Array length equals number of items mapped [OK]

Hint: map returns array same length as input [OK]

Common Mistakes:

• Confusing React elements with strings
• Expecting undefined from map
• Thinking keys affect array length

4. Identify the problem in this React code snippet:

const items = ['A', 'B', 'C'];

return (
  <ul>
    {items.map((item, index) => (
      <li key={index}>{item}</li>
    ))}
  </ul>
);

medium

A. map function syntax is incorrect

B. Missing unique key prop causes error

C. Using index as key can cause bugs if list changes

D. List items should be wrapped in a div

Solution

1. Step 1: Analyze key usage

   Using array index as key works only if list never changes order or items.

2. Step 2: Understand potential bugs

   If items are added, removed, or reordered, React may reuse wrong elements causing UI bugs.

3. Final Answer:

   Using index as key can cause bugs if list changes -> Option C

4. Quick Check:

   Index keys cause bugs on list changes [OK]

Hint: Avoid index keys if list can reorder or change [OK]

Common Mistakes:

• Thinking index keys are always safe
• Believing missing keys cause syntax errors
• Assuming map syntax is wrong

5. You have a list of users with possible duplicate names:

const users = [
  { id: 1, name: 'Sam' },
  { id: 2, name: 'Alex' },
  { id: 3, name: 'Sam' }
];

return (
  <ul>
    {users.map(user => <li key={user.name}>{user.name}</li>)}
  </ul>
);

What problem might occur and how to fix it?

hard

A. Wrap list in a fragment to fix duplicate keys

B. No problem; keys can be duplicate if names match

C. Keys must be numbers; convert names to numbers

D. Duplicate keys from same names cause rendering bugs; use user.id as key

Solution

1. Step 1: Identify duplicate keys

   Using user.name as key causes duplicates because 'Sam' appears twice.

2. Step 2: Fix by using unique stable keys

   Use user.id which is unique for each user to avoid React key conflicts.

3. Final Answer:

   Duplicate keys from same names cause rendering bugs; use user.id as key -> Option D

4. Quick Check:

   Keys must be unique; use unique IDs [OK]

Hint: Keys must be unique; avoid duplicates like same names [OK]

Common Mistakes:

• Assuming keys can be duplicated
• Thinking keys must be numeric only
• Trying to fix keys by wrapping in fragments