Creating a custom useContext-like hook in vanilla JavaScript involves using closures and creating a simple context provider. Below is a basic example demonstrating how you might achieve this:

// CustomContext.js
function createContext(defaultValue) {
  let contextValue = defaultValue;
  const listeners = new Set();

  function useContext() {
    return contextValue;
  }

  function updateContext(newValue) {
    if (contextValue !== newValue) {
      contextValue = newValue;
      listeners.forEach((listener) => listener());
    }
  }

  function subscribe(listener) {
    listeners.add(listener);
    return () => {
      listeners.delete(listener);
    };
  }

  return { useContext, updateContext, subscribe };
}

// Usage
const ExampleContext = createContext('Hello from Context!');

// ExampleComponent.js
function ExampleComponent() {
  const contextValue = ExampleContext.useContext();

  // Subscribe to context updates
  const unsubscribe = ExampleContext.subscribe(() => {
    const updatedValue = ExampleContext.useContext();
    console.log('Context updated:', updatedValue);
  });

  // Unsubscribe when the component is unmounted (optional)
  // useEffect(() => {
  //   return () => unsubscribe();
  // }, []);

  return contextValue;
}

// ExampleProvider.js
function ExampleProvider({ children }) {
  // Wrap your component tree with the provider
  return children;
}

// Usage in the application
const appRoot = document.getElementById('app');

const App = () => {
  return (
    <ExampleProvider>
      <ExampleComponent />
    </ExampleProvider>
  );
};

ReactDOM.render(<App />, appRoot);

This example demonstrates a basic context implementation using closures. The createContext function creates a context object with useContext, updateContext, and subscribe functions. Components can use these functions to access and update the context value.

Keep in mind that this is a simplified version, and real-world scenarios might involve additional considerations, such as handling multiple providers, context hierarchies, or optimizing updates. In a practical application, using a library like React for state management is often more convenient and efficient.

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How to create your own useContext hook in vanilla js?