Why react setstate is not updating immediately?

The setState function in React does not immediately update the component's state. This is because React employs a performance optimization strategy called batched updates. When you call setState, React actually queues these state changes rather than executing them immediately. This approach aims to minimize unnecessary DOM operations and re-renders, thereby improving application performance.

Here are several key points explaining why setState does not update immediately:

1. Asynchronous Updates: setState is an asynchronous operation. React collects multiple state changes and applies them in a single batch, typically before the browser renders each frame.

2. Component Lifecycle: React's design philosophy involves unifying state updates and rendering at specific points in the component lifecycle. If setState triggered re-renders immediately, it would cause performance issues with complex components.

3. Avoiding Unnecessary Renders: Suppose you call setState multiple times within an event handler. If each call immediately updated, the browser might perform redundant render operations, which is inefficient. By batching updates, React merges these changes and performs only one render.

4. Concurrent Mode: In future React versions, such as React 18's concurrent mode, React schedules updates more intelligently to leverage browser rendering capabilities and deliver a smooth user experience.

For example, suppose you call setState three times consecutively within a component's event handler, each time modifying a value in the component's state:

javascript
this.setState({ value: this.state.value + 1 });
this.setState({ value: this.state.value + 1 });
this.setState({ value: this.state.value + 1 });

In the above code, you might expect the value to increase three times. However, due to React's batching and asynchronous updates, these three calls may be merged into a single update, causing value to increase only once.

Understanding that setState is asynchronous is crucial for writing correct React code. If you need to execute certain operations immediately after a state update, use the callback function of setState or lifecycle methods like componentDidUpdate.

javascript
this.setState({ value: this.state.value + 1 }, () => {
  console.log('State update complete, new value is:', this.state.value);
});

In this example, the operation to log the state executes after the state update and component re-render.

This is because it occurs asynchronously, meaning the update might not have been applied yet.

According to the React v.16 documentation, you should use the second form of setState() that accepts a function instead of an object:

State updates may be asynchronous

React can batch multiple setState() calls into a single update to improve performance.

Since this.props and this.state may be updated asynchronously, you should not rely on their values to compute the next state.

For example, the following code may fail to update the counter:

javascript
// Wrong
this.setState({
  counter: this.state.counter + this.props.increment,
});

To fix this issue, use the second form of setState() that accepts a function instead of an object. This function will receive the previous state as the first argument and the props at the time of the update as the second argument:

javascript
// Correct
this.setState((prevState, props) => ({
  counter: prevState.counter + props.increment
}));

You should use the second function as the callback for setState because setState is asynchronous. For example:

javascript
this.setState({pencil: !this.state.pencil}, myFunction)

However, in your case, since you want to pass parameters to the function, you should create a custom function to call the props function:

javascript
myFunction = () => {
  this.props.updateItem(this.state)
}

Combining them together should work.