react link vs a tag and arrow function

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.

There is no better then looking at the code source.

https://github.com/ReactTraining/react-router/blob/master/packages/react-router-dom/modules/Link.js

You can see that Link is a component, that internally use history. Which is the module|library behind the history and navigation for react-router. And come with different modes (in memory history, browserHistory, hashHistory. And even custom).

Yea as a similarity it render an anchor tag but the default behavior is overridden (preventDefault()). They could have used just a div. But not completely right. As for the reason bellow.

So basically it work like that:

Observe the condition bellow

  if (
          !event.defaultPrevented && // onClick prevented default
          event.button === 0 && // ignore everything but left clicks
          (!this.props.target || this.props.target === "_self") && // let browser handle "target=_blank" etc.
          !isModifiedEvent(event) // ignore clicks with modifier keys
    ) {

}

if the condition above is met. It will use history (push or replace). Otherwise it will leave the browser normal behavior. And so in that case it will be just a normal anchor tag <a />. Example letting the browser handle target='blank'. The condition are well explained. Then depending on the type of history object. The behavior change. Not the behavior of ` itself. But just the result of the history object type.

In resume:

<Link /> is a component, that render a <a /> anchor tag. However in the main conditions the default behavior is prevented (preventDefault()). That allow it to apply the change to the history object (onClick event). Which react-router navigation is based on. And on the some conditions as mentioned above. It just fall back to the browser behavior. And just be exactly a <a /> anchor tag (no preventDefault()).

For the use. If you are using React-router. Then you just need to use Link.

The component allows you to do more than the normal link element. For instance, because it's a React component you have the benefits of having a state and what not (if you want that). You can see more documentation on here. Without the error I'm not sure what happens, but I suspect the routing library wants you to use the component, over a normal html element.

With regards to () => {} this is a construct which is called an anonymous function, or a lambda expression. It's basically the same as saving a function in a variable: var x = function(){ return (<div>...) }; if you have anything in the first parenthesis, it's a parameter which you have access to: const x = (y) => return y*2; The reason it's done in React is to expose the function scope to the component it lies in.

The href attribute would trigger a page refresh which would reset the application states. However the link and navlink of react-router doesn't trigger a page refresh. Since React is used to create single page applications most of the time make sure you choose Link or Navlink when working with routing