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How Do You Create Components In React.js?

6 minutes read

In React.js, components are the building blocks of the user interface. They are reusable code pieces that encapsulate a part of the UI and its behavior.


To create a component in React.js, you can start by defining a JavaScript function or class that represents the component. The function or class should return the desired JSX (JavaScript XML) for the component's UI.


For functional components, you can define a function that takes props as a parameter and returns the JSX for the component. For class components, you can define a class that extends React.Component and implements a render method that returns the JSX.


Once you have defined the component, you can use it in other parts of your application by importing it and including it in the JSX of other components. Components can also accept props, which are used to pass data down to child components and customize their behavior.


Overall, creating components in React.js involves defining a function or class that represents the UI and behavior of a specific part of the application, and then using that component in other parts of the application as needed.


How do you use portals in React.js?

In React.js, portals are used to render a child component into a different part of the DOM hierarchy. This can be useful in cases where you want a child component to be rendered outside of its parent component, such as displaying a modal dialog or a tooltip.


To use portals in React.js, you can create a portal by using the ReactDOM.createPortal method. This method takes two parameters: the child component that you want to render and the target DOM element where you want to render the child component.


Here's an example of how you can use portals in React.js:

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import React from 'react';
import ReactDOM from 'react-dom';

const Modal = ({ children }) => {
  const modalRoot = document.getElementById('modal-root');

  return ReactDOM.createPortal(
    children,
    modalRoot
  );
};

const App = () => {
  return (
    <div>
      <h1>My App</h1>
      <Modal>
        <div>Modal Content</div>
      </Modal>
    </div>
  );
};

ReactDOM.render(<App />, document.getElementById('root'));


In this example, we have a Modal component that uses a portal to render its children into a separate DOM element with the id 'modal-root'. This allows the Modal component to be rendered outside of its parent component.


You would need to define the 'modal-root' element in your HTML file for this to work:

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<div id="root"></div>
<div id="modal-root"></div>


By using portals in React.js, you can easily render child components in different parts of the DOM hierarchy without having to worry about CSS styles or z-index conflicts.


How do you conditionally render components in React.js?

Conditional rendering in React.js is typically done using JavaScript expressions within curly braces in the JSX code. Here are some common ways to conditionally render components in React.js:

  1. Using the conditional (ternary) operator:
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{ condition ? <Component /> : null }


  1. Using an if-else statement outside of the return statement:
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let componentToRender;
if (condition) {
  componentToRender = <Component />;
} else {
  componentToRender = null;
}

return componentToRender;


  1. Using a logical && operator:
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{ condition && <Component /> }


  1. Using a switch statement:
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switch(condition) {
  case true:
    return <Component />;
  case false:
    return null;
  default:
    return null;
}


  1. Using inline functions and variables:
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const renderComponent = () => {
  if (condition) {
    return <Component />;
  } else {
    return null;
  }
}

return renderComponent();


These are just a few examples of how you can conditionally render components in React.js. The method you choose will depend on your specific use case and coding style.


How do you create a functional component in React.js?

To create a functional component in React.js, you need to follow these steps:

  1. Import the necessary React library:
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import React from 'react';


  1. Define the functional component by creating a function that returns JSX (JavaScript XML) elements:
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const MyComponent = () => {
  return <div>Hello, World!</div>;
}


  1. Export the component so that it can be imported and used in other parts of your application:
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export default MyComponent;


Once you have followed these steps, you can import and use your functional component in another component or in your main application file. To use the functional component, simply include it as a JSX element in the desired location, like this:

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import MyComponent from './MyComponent';

function App() {
  return (
    <div>
      <MyComponent />
    </div>
  );
}


And that's it! You have successfully created and used a functional component in React.js.


How do you create a custom hook in React.js?

To create a custom hook in React.js, you can follow these steps:

  1. Create a new file for your custom hook, for example, useCustomHook.js.
  2. Import the necessary React hooks that you want to use in your custom hook, for example, useState, useEffect, useContext, etc.
  3. Define your custom hook function, following the convention of starting the function name with "use", for example, useCustomHook.
  4. Write your custom hook logic inside the function. You can use any of the existing React hooks or create custom logic specific to your needs.
  5. Export the custom hook function at the end of the file.


Here is an example of a custom hook that fetches data from an API using the useEffect and useState hooks:

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import { useState, useEffect } from 'react';

const useDataApi = (initialUrl, initialData) => {
  const [data, setData] = useState(initialData);
  const [url, setUrl] = useState(initialUrl);
  const [isLoading, setIsLoading] = useState(false);
  const [error, setError] = useState();

  useEffect(() => {
    const fetchData = async () => {
      setIsLoading(true);
      setError(null);

      try {
        const response = await fetch(url);
        if (!response.ok) {
          throw new Error('Failed to fetch data');
        }
        const json = await response.json();
        setData(json);
      } catch (error) {
        setError(error);
      }

      setIsLoading(false);
    };

    fetchData();
  }, [url]);

  return { data, isLoading, error, setUrl };
};

export default useDataApi;


You can then use this custom hook in your React components like this:

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import React from 'react';
import useDataApi from './useDataApi';

const App = () => {
  const { data, isLoading, error, setUrl } = useDataApi('https://api.example.com/data', []);

  if (isLoading) {
    return <div>Loading...</div>;
  }

  if (error) {
    return <div>Error: {error.message}</div>;
  }

  return (
    <div>
      {data.map(item => (
        <div key={item.id}>{item.name}</div>
      ))}
      <button onClick={() => setUrl('https://api.example.com/other-data')}>
        Load More Data
      </button>
    </div>
  );
};

export default App;


By creating custom hooks, you can encapsulate complex logic and state management and reuse it across multiple components in your React application.


What is the useCallback hook used for in React.js?

The useCallback hook is used in React.js to memoize functions so that they are only redefined when their dependencies change. This can be useful for optimizing performance by preventing unnecessary re-renders of components that use the callback functions. useCallback is typically used to pass callbacks to child components that rely on references not changing between renders.

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