The `y = 10;` is a class field, which is part of a Stage 3 proposal, https://github.com/tc39/proposal-class-fields. It’s not yet finalised, though it’s getting there. No current browser supports it, and those that write their code thus depend on Babel or similar projects to transform the code into a property definition in the constructor. Chrome 72, the next release, introduces support for it. Decide for yourself whether all that counts as “perfectly legal JavaScript” or not!

Now onto bound methods: you are wrong in your code example; `instance.method()` does not require `method` to be a bound method; an unbound function is just fine. The code is equivalent to `instance.method.call(instance)`, which (for an unbound function) calls it with `instance` as its context. The problem is when, at call time, you don’t access the function as a property on another object, like this:

const fn = instance.method;
fn();

Or `(1, instance.method)()`. Then you need to worry about what the context will be (in strict mode, it’ll be `undefined`, and so any property access on it will immediately blow up, which is good).

Bound methods are created by Function.prototype.bind, or by arrow functions.

In what I will choose to call “normal code”, it’s somewhere between very rare and not particularly common to need to bind a function manually. Understand this: React is not normal code. React was designed in a distinctly unusual way (they invented an rather substantial language extension to achieve it!), and although that language and design has turned out quite well in most ways, it also has some exceptionally nasty weaknesses and rather annoying ergonomic troubles; and the need to bind functions is one of them. This is not JavaScript’s fault, but is because React has gone for a VDOM approach that (I think?) requires object identity in such cases, for reasons of efficiency. In search of one form of ergonomics, they damaged another form that others don’t have to worry about very often at all. Such is life, and the nature of tradeoffs.

Most of the trouble with needing bound functions happens with event handling. (Certainly not all, but most, in my experience.) In non-React, non-VDOM-based code, you might be doing something like this:

const foo = document.createElement('foo');
foo.addEventListener('bar', event => {
…
});

… and that’d be perfectly fine there. And a whole lot more efficient at runtime than React, I might add. * If you needed `this`, then the arrow function got it for you automatically.

Sometimes you would want a separate function to designate the event handler, and so you may want to call bind(), but it’s also just as likely that you’ll want to keep Event out of that method, and just do `foo.addEventListener('bar', event => this.handleBar(event.detail))`. Maybe, maybe not.

From your earlier example: be careful with your `x = () => …` instead of `x() { … }`, because that instantiates a new function for every instance of the object, which is very inefficient for memory consumption and performance. Binding a method from the prototype is much more efficient (though still to be avoided where unnecessary).

The most interesting approach to resolving this binding problem that React causes lies in the decorators proposal (currently at stage 2). https://github.com/tc39/proposal-decorators/blob/master/boun... is all about a @bound decorator.

* Just be careful with adding event listeners, and remember to use bubbling.