Why do I have to .bind(this) for methods defined in React component class, but not in regular ES6 class

Question

Something that is puzzling me is why when I define a react component class, values contained in the this object are undefined in methods defined (this

Answer 1

Functions in ES6 Classes - the case is explained very well by @Felix Kling. Every time you call a function on an object, this points to the object.

Lifecycle methods in React.Component - whenever React instantiates your component like myComponent = new MyComponent() it knows which object to call the lifecycle methods on, namely myComponent. So a simple call myComponent.componentDidUpdate() makes this available in the componentDidUpdate lifecycle method. Same for the other lifecycle methods.

Handlers & Bound in React.Component - this.state is undefined, because this is actually window - log it and see. The reason is that React invokes handlers on the global context, unless you have the handler bound to another context which overrides window (see @Phi Nguyen's answer also). I think they have done that to allow you more flexibility, because in complex applications your handler may come from another component passed through props and then you would like to have the possibility to say: "Hey, React - this is not my component, but it's parent."

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React's documentation is a bid misleading when it says

Methods follow the same semantics as regular ES6 classes, meaning that they don't automatically bind this to the instance.

What they mean is that

var dog = new Dog('Mitzie');
speak = d.speak;

dog.speak() // this will be dog, because the function is called on dog
speak() // this will be window, and not dog, because the function is not bound