使用 ES6 反应原生文档示例

Question

The below code is from React Native Docs. As you can see, there are two function declerations with fat arrows. I understand the usage of normal parenthesis with the line containing .... previousState => ({ etc. Here use of () is needed because it returns an object literal. However, I can't understand why do we use "(" with the callback function of the setInterval . I mean this line: setInterval(() => (... . Why we do not write like setInterval(() => {.....

 class Blink extends Component { componentDidMount() { //HEART OF THE QUESTION. Why do we use "(" below, instead "{". Do we need to return for setInterval or just define a function to run? setInterval(() => ( //Here, "(" is normal because it returns object literal this.setState(previousState => ({ isShowingText: .previousState,isShowingText })) ); 1000). } //.... }

Answer 1

setInterval doesn't need a return. Both of these will work perfectly fine:

setInterval(() =>
    this.setState(previousState => ({
        counter: previousState.counter + 1 || 1
    })),
1000);

and this (which is not pretty without parenthesis, in my opinion):

setInterval(() => {
    this.setState(previousState => ({
        counter: previousState.counter + 1 || 1
    }));
}, 1000);

Usually parenthesis is needed for multiline returns, here is a good explanation: http://jamesknelson.com/javascript-return-parenthesis/