Dimensions

useWindowDimensions is the preffered API for React components. Unlike Dimensions, it updates as the window's dimensions update. This works nicely with the React paradigm.
import { Dimensions } from 'react-native';
You can get the application window's width and height using the following code:
const windowWidth = Dimensions.get('window').width;
const windowHeight = Dimensions.get('window').height;
Although dimensions are available immediately, they may change (e.g due to device rotation, foldable devices etc) so any rendering logic or styles that depend on these constants should try to call this function on every render, rather than caching the value (for example, using inline styles rather than setting a value in a StyleSheet).
If you are targeting foldable devices or devices which can change the screen size or app window size, you can use the event listener available in the Dimensions module as shown in the below example.

import React, { useState, useEffect } from "react";
import { View, StyleSheet, Text, Dimensions } from "react-native";

const window = Dimensions.get("window");
const screen = Dimensions.get("screen");

export default function App() {
  const [dimensions, setDimensions] = useState({ window, screen });

  const onChange = ({ window, screen }) => {
    setDimensions({ window, screen });
  };

  useEffect(() => {
    Dimensions.addEventListener("change", onChange);
    return () => {
      Dimensions.removeEventListener("change", onChange);
    };
  });

  return (
    <View style={styles.container}>
      <Text>{`Window Dimensions: height - ${dimensions.window.height}, width - ${dimensions.window.width}`}</Text>
      <Text>{`Screen Dimensions: height - ${dimensions.screen.height}, width - ${dimensions.screen.width}`}</Text>
    </View>
  );
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: "center",
    alignItems: "center"
  }
});

Reference

static addEventListener(type, handler)
Add an event handler. Supported events:
  • change: Fires when a property within the Dimensions object changes. The argument to the event handler is an object with window and screen properties whose values are the same as the return values of Dimensions.get('window') and Dimensions.get('screen'), respectively.
    • window - Size of the visible Application window
    • screen - Size of the device's screen

static get(dim)
Initial dimensions are set before runApplication is called so they should be available before any other require's are run, but may be updated later.
Example: const {height, width} = Dimensions.get('window');
Parameters:
NameTypeRequiredDescription
dimstringYesName of dimension as defined when calling set. @returns {Object?} Value for the dimension.
For Android the window dimension will exclude the size used by the status bar (if not translucent) and bottom navigation bar

static removeEventListener(type, handler)

Remove an event handler.

static set(dims)

This should only be called from native code by sending the didUpdateDimensions event.
Parameters:
NameTypeRequiredDescription
dimsobjectYesstring-keyed object of dimensions to set