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React router path: making optional value inside path

Consider the following two react route paths:
/site/my/photos/:id
/site/photos/:id
I would like the 'my' inside first path to be optional.
How can I make it optional (probably something to do with ? symbol), and how can I then retrieve if the my is present or not in the URL?

You can pass an array of paths to the Route from react-router-dom, for example:
<Route path={["/site/my/photos/:id", "/site/photos/:id"]} component={MyComponent} />

i think you should define two routes with same component like below :
<Route path="/site/my/photos/:id" component={MyComponent} />
<Route path="/site/photos/:id" component={MyComponent} />

having issues in understand path passed to react router

How does the two path differ . I tried going through the docs of react router but it still creates confusion .
First one is
<Route
path="/cricket/:description/:id"
component={xyz}
/>
and another is
<Route
exact
path="/cricket"
component={xyz1}
/>
I see that the first Route is mentioned in App.js .It leads to confusion as which one got executed and how ?

Router will render both componenets if path is something like /cricket/foo/123 because it matches both pattern.
If you want only one to render you have to use exact prop like
<Route exact path="/cricket"
component={xyz1}
/>

React router v4 match path if not `/user`

I have a case in react router v4:
<Route path="/:time?/:zone?" component={TimeComponent} />
However I don't want to render that TimeComponent if route is:
/user/userId123
As you can see above, the path matches user as :time and userId123 as :zone.
My goal: I don't want path to match if route contains /user.
I know I can use regex, but how to specify that I dont want to match the path if contains /user?
Looking for help

You can use switch from react router dom like this:
<Switch>
<Route path="/user/:userId?" component={UserComponent} />
<Route path="/:time?/:zone?" component={TimeComponent} />
</Switch>
With this it will match only one route at a time and you 2 param route is at the end. So using this approach you can catch /user route before time component route.
Please try this.

React router v4 prevent matching child routes

I encountered a problem with React Router v4 Switch component. I'm quite surprised that i couldn't find a relevant thread for this problem. A common Switch will look like this:
<Switch>
<Route path='/path1' component={Path1Component}/>
<Route path='/path2' component={Path2Component}/>
<Route exact path='/' component={Home}/>
<Route component={NotFound}/>
</Switch>
This means that when i'm on a path: '/' i get a Home component, on '/path1' i get a Path1Component and on path '/foobar' i get a NotFound component. And that is perfectly fine
However when i'm on '/path1/foobar' route i also get the Path1Component. This behaviour is not correct in every case - this time i do not want any nested routes for '/path1' route. '/path1/foobar' should get a NotFound component, any string, with '/' or without after '/path1' should return NotFound component.
What would be the preferred resolution to this problem? I could just add exact to every path, but wouldn't that be overbloating the code? I feel like that should be the default, but it's not the case.
Even on React Router v4 docs, like here. I see this problem - here '/will-match/foo' will also match. What are your thoughts?

There is a discussion here, but to make it short : it would break existing code. If this were to change, you'd have to do exact={false} if you want to match child routes without doing 'path1/child1'.

React Router, nesting routes after the params

So, I currently have the following as part of my url map for react.
<Route path="dog" component={DogWrapper}>
<Route path=":id" component={DogDetails}/>
<Route path=":id/genealogy" component={Genealogy}/>
</Route>
But I'm never hitting the Genealogy component. Any ideas as to why?

Your issue is with ordering. React-router works on precedence so it will always take the :id route first. Move the geneaolgy route up and you should be all set.
https://github.com/reactjs/react-router/blob/master/docs/guides/RouteMatching.md#precedence