How to embed Javascript expressions in JSX?

Using JavaScript Expressions In JSX

You can embed JavaScript expressions inside JSX elements by wrapping them in curly braces . This allows you to inject dynamic content or evaluate expressions within your JSX elements.

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1. Embedding JavaScript Variables

You can embed JavaScript variables inside JSX elements to dynamically set values. The below example will display "Hello, John Doe!" on the screen.

const name = "John Doe";
return <h1>Hello, {name}!</h1>;

Evaluating Expressions: Any valid JavaScript expression can be used within the curly braces. For example, mathematical operations or function calls can be embedded:

const a = 5;
const b = 3;
return <p>{a + b}</p>; // Outputs: 8

How do you print falsy values in JSX? The falsy values such as false, null, undefined, and true are valid children but they don't render anything. If you still want to display them then you need to convert it to string. Let's take an example on how to convert to a string,

<div>My JavaScript variable is {String(myVariable)}.</div>

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2. Function Calls In JSX

You can also call JavaScript functions inside JSX elements and render the returned value

// Define a JavaScript function that returns a greeting message
function generateGreeting(name) {
  return `Hello, ${name}!`;
}

// Call the generateGreeting function within JSX
function GreetingComponent() {
  return <p>{generateGreeting("Alice")}</p>;
}

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3. Conditional Rendering In JSX

Mentiond below are ways to conditionally Render in JSX

1. Using if Statements

   You can use traditional if statements to conditionally render elements or execute logic. However, if statements are not inline and require more verbose code since JSX cannot directly accommodate them without wrapping them in a function.

   function Greeting(isLoggedIn) {
     // Using if statement directly inside return
     if (isLoggedIn) {
       return <p>Welcome back, User!</p>;
     } else {
       return <p>Please log in.</p>;
     }
   }

2. Using the Ternary Operator (condition ? trueValue : falseValue)

   The ternary operator is a compact way to conditionally render different JSX elements or values based on a condition. It’s an inline conditional expression.

   <h1>Hello!</h1>;
   {
     messages.length > 0 ? (
       <h2>You have {messages.length} unread messages.</h2>
     ) : (
       <h2>You don't have unread messages.</h2>
     );
   }

3. Using the Logical && Operator (condition && value)

   The logical && operator is a shorthand for rendering something only if a condition is true. This is useful when you only need to display something for the true case without handling the false case.

   <h1>Hello!</h1>;
   {
     messages.length > 0 && <h2>You have {messages.length} unread messages.</h2>;
   }

   This works because:

   • If messages.length > 0 evaluates to true, the second expression (the JSX) is evaluated and rendered.
   • If messages.length > 0 evaluates to false, the entire expression short-circuits, and nothing is rendered.

info

JavaScript's ternary and logical operators, as mentioned in the examples above, are inline conditional expressions. Inline conditional expressions are ways to conditionally render elements or execute code directly within your JSX (JavaScript XML) without using separate if blocks or function calls.

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4. How to Loop Inside JSX?

You can simply use Array.prototype.map with ES6 arrow function syntax. For example, the items array of objects is mapped into an array of components.

<tbody>
  {items.map((item) => (
    <SomeComponent key={item.id} name={item.name} />
  ))}
</tbody>

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Why Can't You Use Statements In JSX?

JSX allows you to embed expressions inside curly braces {} and an expression evaluates to a value that can be rendered.

A for loop, however, is a control flow statement, which doesn’t return a value but instead performs an action (e.g., iterating over an array).

// This is not allowed
<tbody>
  for (let i = 0; i < items.length; i++) {
    <SomeComponent key={items[i].id} name={items[i].name} />
  }
</tbody>

JavaScript has a distinction between statements (like for, if, while) and expressions (like function calls, arithmetic operations). JSX can only handle expressions that produce a value, so using a for loop directly in JSX would not be valid.