class App extends Component { render() { return ( <div className="App"> <header className="App-header"> <img src={logo} className="App-logo" alt="logo" /> <p> Edit <code>src/App.js</code> and save to reload. </p> <a className="App-link" href="https://reactjs.org" target="_blank" rel="noopener noreferrer" > Learn React </a> </header> </div> ); } }
function Welcome(props) { return <h1>Hello, {props.name}</h1>; } class Welcome extends React.Component { render() { return <h1>Hello, {this.props.name}</h1>; } }
render进行return时,对于单行的内容,没必要加括号,但对于多行的内容要加括号
原因在于,JSX转为js后,js会在每行自动加';'
,如果return后换行了,那么就会变成return;
下面通过babel来做一下编译:
class Welcome extends React.Component { render() { return <h1>Hello, {this.props.name}</h1>; } } //******************** var Welcome = /*#__PURE__*/function (_React$Component) { _inherits(Welcome, _React$Component); var _super = _createSuper(Welcome); function Welcome() { _classCallCheck(this, Welcome); return _super.apply(this, arguments); } _createClass(Welcome, [{ key: "render", value: function render() { return /*#__PURE__*/React.createElement("h1", null, "Hello, ", this.props.name); } }]); return Welcome; }(React.Component);
做下对比
class Welcome extends React.Component { render() { return <h1>Hello, {this.props.name}</h1>; } } //****************** var Welcome = /*#__PURE__*/function (_React$Component) { _inherits(Welcome, _React$Component); var _super = _createSuper(Welcome); function Welcome() { _classCallCheck(this, Welcome); return _super.apply(this, arguments); } _createClass(Welcome, [{ key: "render", value: function render() { return; /*#__PURE__*/ React.createElement("h1", null, "Hello, ", this.props.name); } }]); return Welcome; }(React.Component);
所以针对文章开头举的app的例子,如果不加括号的话,是需要右内容和return在同一行的,就像下面这样
class App extends Component { render() { return <div className="App"> <header className="App-header"> <img src={logo} className="App-logo" alt="logo" /> <p> Edit <code>src/App.js</code> and save to reload. </p> <a className="App-link" href="https://reactjs.org" target="_blank" rel="noopener noreferrer" > Learn React </a> </header> </div> } }
这样写也是可以编译通过的,但如果不加括号,还在return后换行了,那就会报错
class App extends Component { render() { return <div className="App"> <header className="App-header"> <img src={logo} className="App-logo" alt="logo" /> <p> Edit <code>src/App.js</code> and save to reload. </p> <a className="App-link" href="https://reactjs.org" target="_blank" rel="noopener noreferrer" > Learn React </a> </header> </div> } }
另外还要注意的一点是,对于return的内容,只能有一个根节点,所以返回的内容必须有一个tag进行包裹,否则,不管加不加括号都是不行的,编译报错的描述也很直观了