Lets look at some code

Probably because you are using it wrong.

If you browse Stackoverflow for React questions, this is one of the most common questions that you will find. Well, it is this or other versions of the same question. All this after Stackoverflow marks questions as duplicates.

function handleChange(value) {
  this.setState({
    value: newValue,
  });
  console.log(this.state.value); //Why is my state not updated?
}

Why is your state not getting updated?

Straight out of React docs, setState may be asynchronous. What does that mean?

Let’s find out.

setTimeout(() => console.log('foo'), 0);
console.log('bar');

What is the output?

First the second console.log fires and then the first one inside the setTimeout function. This happens because setTimeout is asynchronous and is moved to the browser thread. So the code that runs after setTimeout gets executed however small the timeout is.

Same is the case with setState Because of it’s asynchronous state, the statement after is being called before the actual setState function.

How do we fix this?

function handleChange(value) {
  this.setState(
    {
      value: newValue,
    },
    () => {
      console.log(this.state.value);
    }
  );
}

setState actually comes with a callback version. All you have to do is provide the function to be run after the setState is executed. Here, you can give whatever action you wanted to perform once the setState is done.

Since you might already have the result you are going to setState with, it might be better to utilise that result for regular operations rather than using the callback.

PS: You could also just use console.log(this.state.value) in render() function or componentDidUpdate() but I’m guessing you already knew that.

Why is it asynchronous?

Now that you know how to fix it, you can leave.

Or you can stay and figure out why is it made asynchronous. Doesn’t it make React slower?

From the docs:

React may batch multiple setState() calls into a single update for performance.

Because this.props and this.state may be updated asynchronously, you should not rely on their values for calculating the next state.

Yes, that is just it. React it doing this for performance. You might not feel the need for it in a small application. But in a larger application where a lot of state updates may be taking place simultaneously, batching state updates comes as a boon.

The setState comes with several other neat tricks as well, with prevState which you should definitely check out if you are new to React or may be just haven’t heard of it.