How does React.js useEffect hook work behind the scene?

The useEffect hook in React is a powerful tool that allows you to perform side effects in function components. Under the hood, React manages the lifecycle of function components using a mechanism called the "Fiber" architecture. When a function component is rendered or updated, React schedules these components for execution using a reconciliation process.

Here's how useEffect works under the hood:

Initialization: When a function component containing a useEffect hook is first rendered, React schedules the execution of the effect after the browser has painted the screen. This ensures that the UI is updated first before any side effects occur.
Dependencies Tracking: If the useEffect hook has a second argument - an array of dependencies, React compares the current values of these dependencies with their previous values (from the last render). React keeps track of these dependencies.
Effect Execution: If the dependencies have changed, React considers the effect to be "dirty" and executes it. If there are no dependencies (or no second argument is provided), the effect is executed after every render.
Cleanup: If the component is unmounted or the dependencies change, React performs cleanup before re-running the effect. This cleanup is especially useful for removing event listeners, subscriptions, or any other resources that might lead to memory leaks or stale data.
Optimization: React optimizes the execution of effects by batching multiple updates and only re-rendering components when necessary. It also ensures that effects are only executed when necessary, minimizing unnecessary side effects.
Effect Dependencies Update: If the dependencies array changes between renders, React re-evaluates the effect and performs the cleanup of the previous effect before running the new one. This ensures that the effect reflects the latest state of its dependencies.

Overall, useEffect provides a declarative way to manage side effects in React function components, allowing developers to write cleaner and more maintainable code. Its underlying implementation takes advantage of React's efficient rendering and reconciliation processes to ensure optimal performance.