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Logical AND (&&)

Learn the condition && element pattern and its pitfalls.

Logical AND (&&)

The logical AND pattern looks like { 'condition && element' }. If the condition is true, element renders; otherwise nothing happens. It's ideal for one-sided show when situations.

Example: { 'hasError && <Error />' }. If hasError is true, Error renders; otherwise nothing.

Beware: if the condition returns a falsy value like 0, React may print 0 on screen. So make sure the condition is boolean: { 'hasError === true && <Error />' } is safer.

&& pitfall

When you write { 'count && <List />' }, if count is 0 React may print 0 on screen. This is because 0 is rendered as a numeric value.

Two common fixes: turn the condition into a boolean (count > 0) or use !!count to make it an explicit boolean.

Ternary also works: { 'count > 0 ? <List /> : null' }. This removes the 0 problem entirely.

Practice Show error message

Complete the skeleton. Write a component called ErrorBox that takes ({ '{ hasError }' }) as parameter. If hasError === true, return <p className="error">An error occurred</p>; otherwise null (with &&). Don't forget to export.

Starter code

App.jsx
// Write the ErrorBox component here

Sample solution

App.jsx
function ErrorBox({ hasError }) {
  return hasError === true && <p className="error">An error occurred</p>;
}

export default ErrorBox;

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 When is the && pattern used?

  • To store state
  • To choose between two options
  • To show only when a condition holds Correct answer
  • To update DOM

Explanation: && is ideal for one-sided conditional render.

2 && operator

Question 
The && pattern: ____ condition && element.
Answer 
The && pattern: condition condition && element.

Explanation: condition && element is the pattern.

3 Match the && pattern

Term Meaning
condition Question
&& Operator
element Result
0 Risk

All lessons in Beginner

1 React intro

  1. What is React?
  2. Setup and Vite
  3. Your first component
  4. Summary - React intro

2 JSX basics

  1. What is JSX?
  2. Expressions and curly braces
  3. Attributes and className
  4. Summary - JSX basics

3 Components

  1. Component logic
  2. Component tree
  3. Naming rules
  4. Summary - Components

4 Profile card

  1. Project: Profile card
  2. Avatar component
  3. Info list
  4. Assemble the card

5 Props

  1. What are props?
  2. Reading props
  3. Children prop
  4. Default props
  5. Summary - Props

6 Conditional rendering

  1. What is conditional rendering?
  2. Ternary operator
  3. Logical AND (&&)
  4. Early return
  5. Summary - Conditional rendering

7 List rendering

  1. What is list rendering?
  2. The key attribute
  3. Array to components
  4. Filtering and sorting
  5. Summary - List rendering

8 Shopping list

  1. Project: Shopping list
  2. Item component
  3. List body
  4. Empty state

9 Events

  1. Events
  2. The event object
  3. Form events
  4. Summary - Events

10 useState

  1. What is state?
  2. The useState call
  3. Updating state
  4. Multiple state values
  5. Summary - useState

11 Counter app

  1. Project: Counter app
  2. Number and buttons
  3. Decrement and reset
  4. Boundary control

12 Forms

  1. Controlled input
  2. Form submission
  3. Multiple fields
  4. Summary - Forms

13 Styling

  1. Styling with CSS
  2. CSS Modules
  3. Inline styles
  4. Summary - Styling

14 styled-components

  1. What is styled-components?
  2. Styling with props
  3. Using themes
  4. Summary - styled-components

15 useReducer

  1. What is useReducer?
  2. Action and state
  3. useState vs useReducer
  4. Summary - useReducer

16 useEffect

  1. What is useEffect?
  2. Dependency array
  3. Cleanup function
  4. Summary - useEffect

17 Context

  1. What is context?
  2. createContext
  3. Provider and useContext
  4. Summary - Context

18 useRef

  1. What is useRef?
  2. DOM access
  3. Timer example
  4. Summary - useRef

19 Todo app

  1. Project: Todo app
  2. Todo entry form
  3. Todo list
  4. Filter and summary

20 Beginner final

  1. Challenge: Weather card
  2. Challenge: Cart summary
  3. Challenge: Counter and reset
  4. Challenge: Dropdown menu