Is there a synchronous alternative of setState() in Reactjs

You could wrap setState in a function returning a promise, and then use this function with the await keyword to cause your code to wait until the state has been applied.

Personally, I would never do this in real code, instead I would just put the code I wish to execute after the state update in the setState callback.

Nerveless, here is an example.

class MyComponent extends React.Component {

    function setStateSynchronous(stateUpdate) {
        return new Promise(resolve => {
            this.setState(stateUpdate, () => resolve());
        });
    }

    async function foo() {
        // state.count has value of 0
        await setStateSynchronous(state => ({count: state.count+1}));
        // execution will only resume here once state has been applied
        console.log(this.state.count);  // output will be 1
    }
} 

In the foo function, the await keyword causes the code execution to pause until the promise returned by setStateSynchronous has been resolved, which only happens once the callback passed to setState is called, which only happens when the state has been applied. So execution only reaches the console.log call once the state update has been applied.

docs for async/await:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/await

If this is required I would suggest using a callback in your setState function (and I also suggest using a functional setState).

The callback will be called after the state has been updated.

For example, your example would be

//initial value of cnt:0
this.setState(
    (state) => ({cnt: state.cnt+1}),
    () => { alert(this.state.cnt)}
)

as per documentation here : https://facebook.github.io/react/docs/react-component.html#setstate

Note: Official docs say, "Generally we recommend using componentDidUpdate() for such logic instead."

As you have read from the documentation, there is NO sync alternative, reason as described is performance gains.

However I presume you want to perform an action after you have changed your state, you can achieve this via:

class MyComponent extends React.Component {
  constructor(props) {
    super(props);
    this.state = {
     x: 1
    };
    
    console.log('initial state', this.state);
  }
  
  updateState = () => {
   console.log('changing state');
    this.setState({
      x: 2
    },() => { console.log('new state', this.state); })
  }
  
  render() {
    return (
      <div>
      <button onClick={this.updateState}>Change state</button>
    </div>
    );
   
  }
}

ReactDOM.render(
  <MyComponent />,
  document.getElementById("react")
);

<div id="react"></div>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/15.1.0/react-dom.min.js"></script>

No, there is not. React will update the state when it sees fit, doing things such as batching setState calls together for efficiency. It may interest you that you are able to pass a function into setState instead, which takes the previous state, so you may choose your new state with good knowledge of the previous one.