javascript - react link vs a tag and arrow function

Question

I just started on react router.

I have two questions. What is the difference between using <Link to="/page"> and <a href="page">? Both make the exact same get request to /page but I get an error when I use <a href="page"> but it works when I use <Link to="/page"> when I am nesting routes. I don't understand, how there could be any difference, when I know for fact that both render to exact same url?

Second is the weird arrow function in react router v4 documentation

const About = () => (
  <div>
    <h2>About</h2>
  </div>
)

I know () => {} these are new in ES6 but I cannot find anything on normal brackets instead of parentheses. What are they?

Edit

My index.js class (I have all the imports)

render((
    <Router>
        <div>
            <Route component={App}/>
        </div>
    </Router>
), document.getElementById('root')
);

My App.js class

class App extends Component {
render() {
    return (
        <div className="container">
            <header>
                <span className="icn-logo"><i className="material-icons">code</i></span>
                <ul className="main-nav">
                    <li><Link to="/">Home</Link></li>
                    <li><Link to="/about">About</Link></li>
                    <li><Link to="/teachers">Teachers</Link></li>
                    <li><Link to="/courses">Courses</Link></li>
                </ul>
            </header>
            <Route exact path="/" component={Home}/>
            <Route path="/about" component={About}/>
            <Route path="/teachers" component={Teachers}/>
            <Route path="/courses" component={Course}/>
        </div>
    );
}
}

export default App;

The error I'm getting. Cannot GET /about on the browser when I try to move to localhost:8080/about. However, when I click the about button, it goes to exactly the same url /about and renders perfectly

Answer 1

This may be a bit late to address your issue and you may well have figured it out. But here's my take:

First:

What is the difference between using <Link to="/page"> and <a href="page">

• On the surface, you seem to be comparing apples and oranges here. The path in your anchor tag is a relative path while that one in the Link is absolute (rightly so, I don't think react-router supports relative paths yet). The problem this creates is say you are on /blah, while clicking on your Link will go to /page, clicking on the <a href='page' /> will take you to /blah/page. This may not be an issue though since you confirmed the correctness of the url, but thought to note.
• A bit deeper difference, which is just an addon to @Dennis answer (and the docs he pointed to), is when you are already in a route that matches what the Link points to. Say we are currently on /page and the Link points to /page or even /page/:id, this won't trigger a full page refresh while an <a /> tag naturally will. See issue on Github.

A fix I used to solve my little need around this was to pass in a state property into link like so <Link to={{pathname: "/page", state: "desiredState"}}>Page</Link>. Then I can check for this in the target component's (say <Page />) componentWillReceiveProps like so:

componentWillReceiveProps(nextProps){
  if (nextProps.location.state === 'desiredState') {
    // do stuffs
  }
}

Second question:

the weird arrow function in react router v4 documentation... I cannot find anything on normal brackets instead of parentheses. What are they?

Arrow functions; again @Dennis and @Jaromanda X have kind of addressed it. However, I've got three bits to add:

• When you have () => blah without the curly braces {}, you are implicitly returning whatever follows the => in this case blah. But when you have curly braces immediately after the arrow, then it's now your responsibility to return something if you so desire. So () => blah (which by the way is synonymous to () => (blah)) will be more similar to () => { return blah } and not () => { blah }.
• So what happens if you want to return an object: { blah: blah }; this is what @Jaromanda X was pointing at. You will then need to do () => ({ blah: blah }) or simply () => ({ blah }) for implicit return or you could return explicitly like so () => { return { blah: blah } }.
• My third bit is to point you to MDN

Hope it helps.