Syntax

1const [stateVariable, setStateFunction] = useState(initialValue);

Usage

1. Setting Up State
   You can call useState to declare a state variable within a functional component. This initializes the state variable with the provided initial value.
2. Accessing State
   After declaring a state variable using useState, you can access its current value directly by referencing the variable returned by useState.
3. Updating State
   To update the state variable, you call the setStateFunction provided by useState, passing the new value as an argument. React will then re-render the component with the updated state.
   
   

Example

1import React, { useState } from 'react';
2import { Button, Text, View } from 'react-native';
3const Counter = () => {
4  // Declare a state variable 'count' with initial value 0
5  const [count, setCount] = useState(0);
6  return (
7    <View>
8      <Text>Count: {count}</Text>
9      <Button
10        title="Increment"
11        onPress={() => setCount(count + 1)} // Update 'count' by incrementing it by 1
12      />
13      <Button
14        title="Decrement"
15        onPress={() => setCount(count - 1)} // Update 'count' by decrementing it by 1
16      />
17    </View>
18  );
19};
20export default Counter;

In this example, useState is used to declare a state variable named count with an initial value of 0. Two buttons are rendered—one for incrementing the count and the other for decrementing it. Pressing these buttons triggers the setCount function with the updated value, causing the component to re-render with the new count value.

Syntax

1useEffect(() => {
2  // Side effect code here
3  return () => {
4    // Cleanup code here (optional)
5  };
6}, [dependencies]);
7

Parameters

• Effect Function: The first parameter is a function that contains the code for the side effect you want to perform. This function will run after every render, including the initial render.
• Dependencies Array: The second parameter is an optional array of dependencies. It allows you to specify which values the effect depends on. If any of the values in the dependencies array change between renders, the effect function will re-run.
  

Usage

1 . Performing Data Fetching
You can use useEffect to fetch data from an API when the component mounts or when certain props change.

2. Subscribing to Events
   useEffect can be used to subscribe to events such as keyboard events or WebSocket messages.
   
3. Managing Document Titles
   It's possible to use useEffect to update the document title dynamically based on the state of your application.
   
4. Cleaning Up Resources
   The cleanup function returned by useEffect allows you to perform cleanup tasks when the component unmounts or when the dependencies change. This can include unsubscribing from event listeners or canceling network requests.

Example

1import React, { useState, useEffect } from 'react';
2import { Text, View, Button } from 'react-native';
3const ExampleComponent = () => {
4  const [count, setCount] = useState(0);
5  useEffect(() => {
6    // Update the document title with the current count
7    document.title = `Count: ${count}`;
8    // Cleanup function
9    return () => {
10      // Reset the document title when the component unmounts
11      document.title = 'React App';
12    };
13  }, [count]); // Only re-run the effect if count changes
14  return (
15    <View>
16      <Text>Count: {count}</Text>
17      <Button onPress={() => setCount(count + 1)} title="Increment" />
18    </View>
19  );
20};
21export default ExampleComponent;

In this example, useEffect is used to update the document title dynamically based on the count state variable. The effect function runs after every render, but it only re-runs if the count value changes. The cleanup function resets the document title when the component unmounts.

Syntax

1const value = useContext(MyContext);

Parameters

• MyContext: This parameter is a reference to the context object created by React.createContext(). It represents the context that you want to access within the component.
  

Usage

1. Consuming Context
   Use useContext inside a functional component to access the value provided by the nearest Context.Provider component in the component tree.
   

Example

Let's consider an example where a theme context is created and used in a functional component:

1import React, { useContext } from 'react';
2import { View, Text, StyleSheet } from 'react-native';
3// Create a context for the theme
4const ThemeContext = React.createContext('light');
5// A component that provides the theme context
6const ThemeProvider = ({ children }) => {
7  return (
8    <ThemeContext.Provider value="dark">
9      {children}
10    </ThemeContext.Provider>
11  );
12};
13// A component that consumes the theme context
14const ThemedComponent = () => {
15  const theme = useContext(ThemeContext);
16  return (
17    <View style={styles[theme]}>
18      <Text style={styles.text}>Themed Component</Text>
19    </View>
20  );
21};
22// Styles for different themes
23const styles = StyleSheet.create({
24  light: {
25    backgroundColor: '#ffffff',
26    color: '#000000',
27    padding: 10,
28  },
29  dark: {
30    backgroundColor: '#000000',
31    color: '#ffffff',
32    padding: 10,
33  },
34  text: {
35    fontSize: 20,
36  },
37});
38// Wrap the ThemedComponent with the ThemeProvider
39const App = () => {
40  return (
41    <ThemeProvider>
42      <ThemedComponent />
43    </ThemeProvider>
44  );
45};
46export default App;

In this example

ThemeContext is created using React.createContext('light'), with a default value of 'light'.

ThemeProvider provides the theme context with a value of 'dark'.

ThemedComponent consumes the theme context using useContext(ThemeContext).

Based on the current theme value, the component renders with different styles.

Using useContext, the ThemedComponent can access the theme value provided by the nearest ThemeProvider ancestor without the need for prop drilling. This simplifies the code and makes it more maintainable.

Syntax

1const [state, dispatch] = useReducer(reducer, initialState);

Parameters

• reducer: A function that determines how the state should be updated based on the dispatched action.
• initialState: The initial state value.

Usage

1. Setting Up Reducer
   You define a reducer function that specifies how the state should change in response to dispatched actions.
2. Initializing State
   When using useReducer, you provide an initial state value. This state can be a simple value, an object, or an array.
3. Dispatching Actions
   To update the state, you dispatch actions to the reducer function. The reducer then determines how to modify the state based on the action type and payload.

Example

1import React, { useReducer } from 'react';
2import { Button, Text, View } from 'react-native';
3const initialState = { count: 0 };
4const reducer = (state, action) => {
5  switch (action.type) {
6    case 'increment':
7      return { count: state.count + 1 };
8    case 'decrement':
9      return { count: state.count - 1 };
10    default:
11      throw new Error();
12  }
13};
14const Counter = () => {
15  const [state, dispatch] = useReducer(reducer, initialState);
16  return (
17    <View>
18      <Text>Count: {state.count}</Text>
19      <Button
20        title="Increment"
21        onPress={() => dispatch({ type: 'increment' })}
22      />
23      <Button
24        title="Decrement"
25        onPress={() => dispatch({ type: 'decrement' })}
26      />
27    </View>
28  );
29};
30export default Counter;

In this example, useReducer is utilized to manage the state of the count. A reducer function is defined to handle actions of type 'increment' and 'decrement', modifying the count value accordingly. The initial state is provided as an object with a count property set to 0. Pressing the increment or decrement button dispatches the corresponding action to the reducer, which updates the state accordingly.

Syntax

1const memoizedCallback = useCallback(
2  () => {
3    // Function logic
4  },
5  [dependencies]
6);

Parameters

• Function: The function that you want to memoize.
• Dependencies: An array of values. useCallback will only re-create the memoized function if any of these values change.

Usage

1. Memoizing Functions
   useCallback takes a function as its first argument and returns a memoized version of that function.
2. Specifying Dependency
   You can provide an array of dependencies as the second argument to useCallback. The memoized function will only be re-created if any of these dependencies change.
3. Optimizing Performance
   useCallback is useful when passing callbacks to child components to prevent unnecessary re-renders caused by the creation of new function references on each render.

Example

1import React, { useState, useCallback } from 'react';
2import { Button, View } from 'react-native';
3const MemoizedButton = React.memo(({ onPress }) => {
4  console.log('Rendering MemoizedButton');
5  return <Button title="Click Me" onPress={onPress} />;
6});
7const Counter = () => {
8  const [count, setCount] = useState(0);
9  const increment = useCallback(() => {
10    setCount((prevCount) => prevCount + 1);
11  }, []);
12  const decrement = useCallback(() => {
13    setCount((prevCount) => prevCount - 1);
14  }, []);
15  return (
16    <View>
17      <MemoizedButton onPress={increment} />
18      <MemoizedButton onPress={decrement} />
19    </View>
20  );
21};
22export default Counter;

In this example, useCallback is used to memoize the increment and decrement functions. These memoized functions are then passed as props to the MemoizedButton component. By memoizing these functions, we ensure that they remain the same reference across re-renders as long as their dependencies (in this case, none) remain unchanged, thus optimizing performance.

Syntax

1const memoizedValue = useMemo(() => computeExpensiveValue(a, b), [a, b]);

Parameters

• Function: The function that computes the value you want to memoize.
• Dependencies: An array of values. useMemo will recompute the memoized value only if any of these dependencies have changed.

Usage

1. Memoizing Values
   useMemo takes a function as its first argument, which computes the value you want to memoize.
2. Specifying Dependencies
   You can provide an array of dependencies as the second argument to useMemo. The memoized value will only be recomputed if any of these dependencies change.
3. Optimizing Performance
   useMemo is beneficial when you have expensive computations that depend on certain values and you want to avoid recalculating them unnecessarily on each render.
   
   

Example

1import React, { useState, useMemo } from 'react';
2import { Text, View } from 'react-native';
3const computeFactorial = (number) => {
4  console.log('Computing factorial of', number);
5  let factorial = 1;
6  for (let i = 1; i <= number; i++) {
7    factorial *= i;
8  }
9  return factorial;
10};
11const FactorialDisplay = ({ number }) => {
12  const factorial = useMemo(() => computeFactorial(number), [number]);
13  return (
14    <View>
15      <Text>Factorial of {number} is: {factorial}</Text>
16    </View>
17  );
18};
19const App = () => {
20  const [number, setNumber] = useState(5);
21  return (
22    <View>
23      <FactorialDisplay number={number} />
24      <Text>Change number:</Text>
25      <Text>{number}</Text>
26      <Text onPress={() => setNumber(number + 1)}>Increment</Text>
27      <Text onPress={() => setNumber(number - 1)}>Decrement</Text>
28    </View>
29  );
30};
31export default App;

]In this example, useMemo is used to memoize the factorial of a number. The compute factorial function is memoized using useMemo, and it is recalculated only when the number changes. This optimization prevents unnecessary recalculations of the factorial on each render, improving the performance of the application.

Syntax

1const refContainer = useRef(initialValue);

Usage

1. Creating a Reference
   You call useRef with an optional initial value to create a mutable reference. The returned ref object has a current property that holds the current value of the reference.
2. Accessing the Reference
   You can access the current value of the reference using the current property of the ref object.
3. Updating the Reference
   Changing the value of the ref object's current property does not trigger a re-render of the component.

Example

1import React, { useRef, useEffect } from 'react';
2import { Button, Text, View, TextInput } from 'react-native';
3const FocusableTextInput = () => {
4  const inputRef = useRef(null);
5  useEffect(() => {
6    // Focus on the input element when the component mounts
7    inputRef.current.focus();
8  }, []);
9  return (
10    <View>
11      <Text>Enter your name:</Text>
12      <TextInput ref={inputRef} />
13    </View>
14  );
15};
16const App = () => {
17  const renderCount = useRef(0);
18  useEffect(() => {
19    renderCount.current += 1;
20  });
21  return (
22    <View>
23      <Text>Render count: {renderCount.current}</Text>
24      <FocusableTextInput />
25    </View>
26  );
27};
28export default App;

In this example, useRef is used to create two different kinds of references:

inputRef is used to reference the TextInput component, allowing us to focus on it when the component mounts.

renderCount is used to keep track of the number of renders of the App component without causing re-renders. It is updated inside a useEffect hook, which runs after every render. Since changing the current property of a ref object does not trigger a re-render, the render count is updated without causing an infinite loop.

Syntax

1useImperativeHandle(ref, createHandle, [deps])

Parameters

• ref: A ref object created with useRef or passed from the parent component using React.forwardRef.
• createHandle: A function that returns an object with properties and methods to be exposed via the ref.
• deps (optional): An array of dependencies. The hook will re-run if any of these dependencies change.

Usage

1. Customizing Instance Value
   useImperativeHandle allows you to customize the instance value exposed by a child component when using React.forwardRef.
2. Defining Exposed Methods or Properties
   Inside the createHandle function, you define the properties and methods that you want to expose via the ref object.
3. Optimizing Performance
   You can specify dependencies to control when the hook should re-run and update the exposed instance value.

Example

1import React, { useRef, useImperativeHandle, forwardRef, useState } from 'react';
2import { Button, View, Text } from 'react-native';
3// Child component
4const ChildComponent = forwardRef((props, ref) => {
5  const [count, setCount] = useState(0);
6  // Expose increment method via ref
7  useImperativeHandle(ref, () => ({
8    increment() {
9      setCount(count + 1);
10    }
11  }));
12  return (
13    <View>
14      <Text>Count: {count}</Text>
15    </View>
16  );
17});
18// Parent component
19const ParentComponent = () => {
20  const childRef = useRef(null);
21  const handleIncrement = () => {
22    childRef.current.increment();
23  };
24  return (
25    <View>
26      <ChildComponent ref={childRef} />
27      <Button title="Increment from Parent" onPress={handleIncrement} />
28    </View>
29  );
30};
31export default ParentComponent;

In this example, useImperativeHandle is used in the ChildComponent to expose an increment method via the ref object. This method updates the count state variable in the ChildComponent when called. The ParentComponent then utilizes this exposed method to trigger an increment action from a Button component. This allows for a clean separation of concerns between the parent and child components while providing a way for them to communicate effectively.

Syntax

1useLayoutEffect(effect, [deps])

Parameters

• effect: A function that contains imperative code. This function will run synchronously after all DOM mutations.
• deps (optional): An array of dependencies. The hook will re-run if any of these dependencies change.

Usage

1. Performing Imperative DOM Operations
   useLayoutEffect is useful for performing imperative DOM operations or reading layout properties from the DOM.
2. Synchronizing State with DOM
   You can use useLayoutEffect to synchronize state with the DOM after all mutations have been applied.
3. Optimizing Performance
   useLayoutEffect should be used sparingly and only when the effect needs to run synchronously after DOM mutations.

Example

1import React, { useState, useLayoutEffect } from 'react';
2import { Text, View } from 'react-native';
3const ComponentWithLayoutEffect = () => {
4  const [width, setWidth] = useState(0);
5  // useLayoutEffect runs synchronously after all DOM mutations
6  useLayoutEffect(() => {
7    const handleResize = () => {
8      setWidth(window.innerWidth);
9    };
10    // Add event listener for window resize
11    window.addEventListener('resize', handleResize);
12    // Initial update of width
13    handleResize();
14    // Clean up the event listener
15    return () => {
16      window.removeEventListener('resize', handleResize);
17    };
18  }, []);
19  return (
20    <View>
21      <Text>Window Width: {width}</Text>
22    </View>
23  );
24};
25export default ComponentWithLayoutEffect;

In this example, useLayoutEffect is used to update the state variable width based on the window.innerWidth after all DOM mutations have been applied. The effect runs synchronously after all DOM mutations, ensuring that the state is synchronized with the current layout of the DOM. This is particularly useful for cases where you need to perform imperative operations that depend on the DOM layout

Syntax

1useDebugValue(value);

Parameters

value: The value or label to be displayed in React DevTools for the custom hook.

Usage

1. Labeling Custom Hooks
   useDebugValue is typically used inside custom hooks to provide a label or additional information about the hook's value in React DevTools.
2. Debugging
   It helps developers understand the behavior of custom hooks by displaying meaningful labels or values in the DevTools.

Example

1import { useState, useDebugValue } from 'react';
2const useCustomHook = (initialValue) => {
3  const [value, setValue] = useState(initialValue);
4  // Display label in React DevTools
5  useDebugValue(value > 10 ? 'Value is greater than 10' : 'Value is not greater than 10');
6  return [value, setValue];
7};
8// Usage of the custom hook
9const ComponentUsingCustomHook = () => {
10  const [state, setState] = useCustomHook(5);
11  return (
12    <div>
13      <p>State value: {state}</p>
14      <button onClick={() => setState(state + 1)}>Increment</button>
15    </div>
16  );
17};
18export default ComponentUsingCustomHook;

In this example, useDebugValue is used inside the custom hook useCustomHook to provide additional information about the value returned by the hook. Depending on whether the value is greater than 10 or not, a corresponding label is displayed in React DevTools. This helps developers understand the behavior of the custom hook and provides valuable debugging information.

Example

1// seCounter custom hook
2import { useState } from 'react';
3const useCounter = (initialValue = 0) => {
4  const [count, setCount] = useState(initialValue);
5  const increment = () => {
6    setCount(count + 1);
7  };
8  const decrement = () => {
9    setCount(count - 1);
10  };
11  return { count, increment, decrement };
12};
13export default useCounter;
14
15// Usage of the custom hook
16import React from 'react';
17import useCounter from './useCounter';
18const Counter = () => {
19  const { count, increment, decrement } = useCounter();
20  return (
21    <div>
22      <p>Count: {count}</p>
23      <button onClick={increment}>Increment</button>
24      <button onClick={decrement}>Decrement</button>
25    </div>
26  );
27};
28export default Counter;

In this example, a custom hook named useCounter is created to encapsulate the logic for managing a counter. The useCounter hook manages the count state variable and provides methods for incrementing and decrementing the count. This logic is then reused in the Counter component, making it more modular and easier to understand. Custom hooks enable code reuse and promote separation of concerns in your React applications.