Reactjs : TypeError: Cannot read property 'map' of undefined

you should do a check on the nature of your data property before making a map on it.

{
 data && data.Projects.length !== 0 &&
   data.Projects.map((project) => (<li key = {project._id}>
    <a href= {`/${location}/${id}/${project._id}`}>{project.Project}</a></li> ))
}

and i recommend you to use javascrip fetch

I've encountered this very error myself many times.

First of all check if the data you are getting from the api is an array of javascript objects

Here is a simple solution.

{data ? data.Projects.map((project) => (<li key={project._id}>
                    <a href={`/${location}/${id}/${project._id}`}>{project.Project}</a></li>)) :<div>Loading..</div>}

So what I have done here is use a ternary operator, what it will do is check if data state has data yet, if it has it will show the data other wise it will show "Loading..."

Axios takes some time to fetch and set data, but the page renders before that so thats why it is showing Cannot read property 'map' of undefined

It's possible that the data returned doesn't contain what you need.

Try to print out the returned data

        axios(`http://localhost:5000/api/${location}/${id}`)
        .then((response) => {
          console.log(response.data);
          setData(response.data);
        })

it could be a valid response, but doesn't contain that variable data.Projects. If you want to handle the absence of that value, you can use something like:

(data.Projects || []).map(...)

this way, if data.Projects is falsy, it would be replaced by an empty array and it would call [].map(...).

as discussed, your setLoading(true) requirement.

function Developer(props) {
    const [data, setData] = useState();
    const [loading , setLoading] = useState(true)
    const [error, setError] = useState();

    var id = props.match.params.id
    var location = props.match.params.location


    useEffect(() => {
       // ooh look, we are in a loading state here, cause the data hasn't come back from the server at this point.
 setLoading(true);
        axios(`http://localhost:5000/api/${location}/${id}`)
           .then((response) => {
           setData(response.data);
           // awesome, data has returned from the server, we aren't loading anymore, but the finally block handles that scenario for us.

        })
        .catch((error) => {
          console.error("Error fetching data: ", error);
          setError(error);
        })
        .finally(() => {
          setLoading(false);
        });
        }, []);
        
    if (loading) return "Loading...";
    if (error) return "Error!";

    return (
        <div>
            <h1>{data.Developer}</h1>
            <ul>
                {
                data && data.Projects.length && data.Projects.map((project) => (<li key = {project._id}>
                <a href= {`/${location}/${id}/${project._id}`}>{project.Project}</a></li> ))
                }
            </ul>
        </div> 
        )}