Why Doesn't `setState` Immediately Update React Component State?

Why Does Calling the setState Method Not Immediately Mutate the State?

In React, the setState method is an asynchronous function used to set the component's state. This means that the state is not updated immediately after calling setState. This is because setState triggers a re-render of the component, which can be an expensive operation. To avoid making the browser unresponsive, setState calls are batched and executed later.

Asynchronous Nature of setState

According to the React documentation: "setState() does not immediately mutate this.state, but creates a pending state transition." Thus, accessing this.state after calling setState may still return the existing value.

Batched setState Calls

For performance optimization, multiple setState calls may be batched together and executed in a single re-render. This prevents unnecessary re-renders and improves UI responsiveness.

Using a Callback

To access the updated state value after setState, you can use a callback function as the second argument. This callback will be executed after the state has been updated:

setState({ key: value }, () => {
    console.log('updated state value', this.state.key)
});

Example

Consider the following example:

class NightlifeTypes extends React.Component {
   handleOnChange = (event) => {
      let value = event.target.checked;

      if(event.target.className == "barClubLounge") {
         this.setState({ barClubLounge: value}, () => { 
             console.log(value);
             console.log(this.state.barClubLounge);
             //both will print same value
         });        
      }
   }
}

In this example, the handleOnChange function sets the barClubLounge state property to the value argument. The callback function in the setState call ensures that the console logs will display the updated state value.

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