React this.state is undefined?

When you call {this.addUser} , it gets called, here this is an instance of your class(component), and thus it gives no error to you because addUser method does exist in your class scope, but when you are under addUser method you are using this to update the state which exist in the scope of class(component), but currently you are within the scope of addUser method and so it gives you an error as under addUser Scope you got nothing like state, user etc. So to deal with this problem you need to bind this while you are calling addUser method.So that your method always knows the instance of this.

So the final change in your code will look like this:-

<Form addUser={this.addUser.bind(this)}/>

OR

---

You can bind this in the constructor,because it is the place when you should intialize things because constructor methods are called first when the components render to the DOM.

So you can do it in this way:-

  constructor(props) {
    super(props);
    this.state = {
        users: null
    }
    this.addUser=this.addUser.bind(this);
}

And now you can call it in normal way as you did before:-

<Form addUser={this.addUser}/>

I hope this will work,And I made it clear to You.

@Vinit Raj's approaches works perfectly - tho i prefer to use the arrow function syntax like so.

<Form addUser={ () => this.addUser() }/>

Using an anonymous function like this, you don't need to bind it anywhere.

If you prefer using arrow function do this. The arrow function syntax as like below

addUser = event => { 
    const { value }  = event.target;
    console.log("value", value);
}

To prevent this function being called on every render or re-render you need to

Change

<Form addUser={this.addUser}/>

To

<Form addUser={() => this.addUser()}/>

So that addUser gets called only when event is happened/triggered

I had the same problem but my issue was trying to access this.state before the this.state = { ... } call finished. Was doing something like this this.state = { ...this.function1() } and function1 = () => { a: this.state.b }. Hope this helps someone