Embedding expressions in JSX in React

Introduction

Embedding expressions in JSX lets you show dynamic values inside your React components. It helps make your UI change based on data or user actions.

Showing a user's name inside a greeting message.

Displaying the current date or time that updates.

Calculating and showing a total price in a shopping cart.

Conditionally showing text or elements based on a value.

Syntax

React

const element = <tag>{expression}</tag>;

Use curly braces {} to embed any JavaScript expression inside JSX.

Expressions can be variables, math, function calls, or ternary conditions.

Examples

Shows a variable inside JSX to greet the user by name.

React

const name = 'Alex';
const greeting = <h1>Hello, {name}!</h1>;

Embeds a math expression result inside a paragraph.

React

const sum = 5 + 3;
const result = <p>5 + 3 = {sum}</p>;

Uses a ternary expression to show different text based on a condition.

React

const isLoggedIn = true;
const message = <p>{isLoggedIn ? 'Welcome back!' : 'Please sign in.'}</p>;

Sample Program

This React component shows a greeting that changes based on the time of day. It uses expressions inside JSX to embed variables and a condition.

React

import React from 'react';

function Greeting() {
  const userName = 'Jamie';
  const hour = new Date().getHours();
  const greetingMessage = hour < 12 ? 'Good morning' : 'Good afternoon';

  return (
    <main>
      <h1>{greetingMessage}, {userName}!</h1>
      <p>Current hour: {hour}</p>
    </main>
  );
}

export default Greeting;

Important Notes

Only JavaScript expressions (not statements) can go inside curly braces.

Expressions inside JSX are automatically converted to strings or elements.

Use parentheses to wrap complex expressions for better readability.

Summary

Use curly braces {} to embed JavaScript expressions inside JSX.

Expressions can be variables, calculations, or conditional logic.

This makes your React UI dynamic and responsive to data changes.

Practice

1. In React JSX, how do you embed a JavaScript variable name inside a <p> tag to display it?

easy

A. name

B. {name}

C. ${name}

D. \{name\}

Solution

1. Step 1: Understand JSX embedding syntax

   In JSX, JavaScript expressions must be wrapped in curly braces {} to be evaluated and rendered.

2. Step 2: Apply embedding to variable

   To display the variable name, write it inside curly braces within the JSX tag: {name}.

3. Final Answer:

   <p>{name}</p> -> Option B

4. Quick Check:

   Use curly braces {} to embed variables [OK]

Hint: Use curly braces {} to embed variables in JSX [OK]

Common Mistakes:

• Using quotes instead of curly braces
• Trying to use template literals inside JSX directly
• Escaping curly braces incorrectly

2. Which of the following is the correct way to embed a JavaScript expression that adds two numbers inside JSX?

easy

A. {2 + 3}

B. 2 + 3

C. ${2 + 3}

D. {'2 + 3'}

Solution

1. Step 1: Recognize expression embedding

   JSX requires expressions inside curly braces to evaluate them, so {2 + 3} will calculate the sum.

2. Step 2: Identify correct syntax

   {2 + 3} correctly uses curly braces with the expression 2 + 3. Other options treat it as plain text or incorrect syntax.

3. Final Answer:

   <div>{2 + 3}</div> -> Option A

4. Quick Check:

   Curly braces evaluate expressions in JSX [OK]

Hint: Wrap expressions in {} to evaluate inside JSX [OK]

Common Mistakes:

• Putting expressions as plain text without braces
• Using template literals without braces
• Wrapping expressions in quotes

3. What will be rendered by this React component?

function Greeting() {
  const hour = 10;
  return <h1>Good {hour < 12 ? 'Morning' : 'Evening'}!</h1>;
}

medium

A. Good {hour < 12 ? 'Morning' : 'Evening'}!

B. Good Evening!

C. SyntaxError

D. Good Morning!

Solution

1. Step 1: Evaluate the conditional expression

   The variable hour is 10, which is less than 12, so the condition hour < 12 is true.

2. Step 2: Determine the rendered string

   Because the condition is true, the expression returns 'Morning', so the rendered output is <h1>Good Morning!</h1>.

3. Final Answer:

   <h1>Good Morning!</h1> -> Option D

4. Quick Check:

   Conditional expression inside {} renders correct text [OK]

Hint: Evaluate expressions inside {} before rendering [OK]

Common Mistakes:

• Thinking JSX shows raw expressions
• Confusing true/false in ternary
• Missing curly braces for expressions

4. Identify the error in this JSX snippet:

const count = 5;
return <div>You have {count} new messages</div>;

medium

A. No error, code is correct

B. Missing curly braces around count

C. JSX must use parentheses around return value

D. Curly braces should be escaped

Solution

1. Step 1: Check expression embedding

   The variable count is correctly embedded inside curly braces {count} within JSX.

2. Step 2: Verify JSX syntax

   The JSX syntax is valid; parentheses around return are optional for single-line JSX.

3. Final Answer:

   No error, code is correct -> Option A

4. Quick Check:

   Curly braces correctly embed variables in JSX [OK]

Hint: Curly braces embed variables; parentheses optional for single JSX line [OK]

Common Mistakes:

• Thinking parentheses are mandatory for return
• Trying to escape curly braces unnecessarily
• Omitting curly braces around variables

5. Given this React component, what will be the output?

function ListItems() {
  const items = ['Apple', 'Banana', 'Cherry'];
  return (
    <ul>
      {items.map(item => <li key={item}>{item.length > 5 ? item : 'Short'}</li>)}
    </ul>
  );
}

hard

A. ShortShortShort

B. AppleBananaCherry

C. ShortBananaCherry

D. Runtime error due to missing keys

Solution

1. Step 1: Evaluate each item length

   Items are 'Apple'(5), 'Banana'(6), 'Cherry'(6). The condition item.length > 5 is true for 'Banana' and 'Cherry', false for 'Apple'.

2. Step 2: Apply conditional rendering

   For 'Apple', length is 5 (not > 5), so it renders 'Short'. For 'Banana' and 'Cherry', it renders the item name.

3. Step 3: Check keys and JSX structure

   Each <li> has a unique key from the item string, so no error occurs.

4. Final Answer:

   <ul><li>Short</li><li>Banana</li><li>Cherry</li></ul> -> Option C

5. Quick Check:

   Conditional expression inside map renders correct list [OK]

Hint: Check condition inside map for each item before rendering [OK]

Common Mistakes:

• Assuming all items render their names
• Forgetting to add keys in list items
• Confusing length comparison operator