React JS App: conditional render

ReactJS conditional render

description

1 Overview

📘 conditional render

React conditional rendering refers to the technique of rendering different components or elements in a React application based on certain conditions. This allows you to control the rendering of components based on the values of variables or the state of your application.

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2 Basic conditional render

Your components will often need to display different things depending on different conditions.

Here’s an example of how you might use conditional rendering in a React component:

App.js

function MyComponent(props) {
  if (props.isLoading) {
    return <div>Loading...</div>;
  } else {
    return <div>Hello, world!</div>;
  }
}

In this example, the MyComponent component uses an if statement to determine whether to render a “Loading…” message or a “Hello, world!” message, based on the value of the isLoading prop.

3 Ternary operator

You can also use the ternary operator ?: to conditionally render elements in a more concise way:

App.js

function MyComponent(props) {
  return props.isLoading ? <div>Loading...</div> : <div>Hello, world!</div>;
}

In this example, the ternary operator checks the value of the isLoading prop and returns a “Loading…” message if it is true, or a “Hello, world!” message if it is false.

Conditional rendering is a useful technique for controlling the rendering of components in a React application. It allows you to tailor the rendering of your application based on the state of your application or the values of variables.

4 Conditionally returning JSX

Let’s say you have a PackingList component rendering several Items, which can be marked as packed or not.

App.js

export default function PackingList() {
  return (
    <section>
      <h1>Sally Ride's Packing List</h1>
      <ul>
        <Item 
          isPacked={true} 
          name="Space suit" 
        />
        <Item 
          isPacked={true} 
          name="Helmet with a golden leaf" 
        />
        <Item 
          isPacked={false} 
          name="Photo of Tam" 
        />
      </ul>
    </section>
  );
}

Notice that some of the Item components have their isPacked prop set to true instead of false. You want to add a checkmark (✔) to packed items if isPacked={true}.

If the isPacked prop is true, this code returns a different JSX tree. With this change, some of the items get a checkmark at the end:

App.js

function Item({ name, isPacked }) {
  if (isPacked) {
    return <li className="item">{name} ✔</li>;
  }
  return <li className="item">{name}</li>;
}

export default function PackingList() {
  return (
    <section>
      <h1>Sally Ride's Packing List</h1>
      <ul>
        <Item 
          isPacked={true} 
          name="Space suit" 
        />
        <Item 
          isPacked={true} 
          name="Helmet with a golden leaf" 
        />
        <Item 
          isPacked={false} 
          name="Photo of Tam" 
        />
      </ul>
    </section>
  );
}

Executing App.js, without conditional rendering

Executing App.js, with conditional rendering

5 Example 1:

In this example, when you press send, setIsSent(true) tells React to re-render the UI:

App.js

import { useState } from 'react';

export default function Form() {
  const [isSent, setIsSent] = useState(false);
  const [message, setMessage] = useState('Hi!');
  if (isSent) {
    return <h1>Your message is on its way!</h1>
  }
  return (
    <form onSubmit={(e) => {
      e.preventDefault();
      setIsSent(true);
      sendMessage(message);
    }}>
      <textarea
        placeholder="Message"
        value={message}
        onChange={e => setMessage(e.target.value)}
      />
      <button type="submit">Send</button>
    </form>
  );
}

function sendMessage(message) {
  // ...
}

Executing App.js, inital render

Executing App.js, just filling textarea

Executing App.js, re-render, isSent is true, so the first return is executed

Here’s what happens when you click the button Send:

1. The onSubmit event handler executes.
2. setIsSent(true) sets isSent to true and queues a new render.
3. React re-renders the component according to the new isSent value.
4. The new rendering with isSent to true prints the first return (#7)

6 Example 2: crosswalk

Here is a crosswalk light component that toggles on when the button is pressed:

App.js

import { useState } from 'react';

export default function TrafficLight() {
  const [walk, setWalk] = useState(true);

  function handleClick() {
    setWalk(!walk);
    alert(walk ? 'Stop is next' : 'Walk is next');
  }

  return (
    <>
      <button onClick={handleClick}>
        Change to {walk ? 'Stop' : 'Walk'}
      </button>
      <h1 style={{
        color: walk ? 'darkgreen' : 'darkred'
      }}>
        {walk ? 'Walk' : 'Stop'}
      </h1>
    </>
  );
}

Executing App.js, inital render

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