Cannot store arrow function in let/var in React Component

问题

I have a component in react defined by extends React.Component. Inside it, along with the render method, is:

constructor(props, context) {
    super(props, context);

    this.state = {
        open: false,
    };
}

handleClose = () => { //this works
    this.setState({open: false});
};

handleOpen = () => { //this works
    this.setState({open: true});
};

let empty = () => {}; // does not work, error  Parsing error:      Unexpected token

I seem to get an unexpected token error if I prefix any of the arrow functions with let or const. Am I missing something here?

thanks

回答1:

Read babel documentation about classes. using let or const is not part of ES6 possible prefix for classes definition

回答2:

The ECMA 2015 spec seems to only allow methods and static methods in the body of a class definition.

So, in this case, assignment statements are not allowed, hence the Unexpected token error from babel.

回答3:

You're trying to use experimental ES7 features and likely only have ES6 enabled (also you cannot prefix let or const in a class). If you're using babel6 it should be simple to switch to the correct plugin. See this answer from basically the same question: How to use ES6 arrow in class methods?

回答4:

You seem to be making use of the class properties declartion proposal. The proposed syntax is:

class ClassWithoutInits {
  myProp;
}

class ClassWithInits {
  myProp = 42;
}

As you can see, no let or const or var. let empty = () => {}; doesn't work because it is not valid syntax, just like the error message tells you.

来源：https://stackoverflow.com/questions/37838756/cannot-store-arrow-function-in-let-var-in-react-component