Routing is the mechanism by which an application responds to request to a particular API endpoint.
For example, a client can make a GET, POST, PUT or DELETE http request for various URLs (e.g. GET http://localhost:3000/books or DELETE http://localhost:3000/students/42).
Based on the URL which is accessed, a different function on the web server will be called. For example, GET http://localhost:3000/books will get a list of books, and DELETE http://localhost:3000/students/42 will remove the student with id of 42. This is the concept of routing.
Each route can have one or more handler functions, which are executed when the route is matched.
A simple route example can be found in express_basic_example_1.js
node express_basic_example_1.js
After the application is started, access the two URLs and you should see different response on the browser page.
http://localhost:3000/students
Actually, you may define more than one route handlers
You can call app.METHOD multiple times, like
app.get(path, route_handler1);
app.get(path, route_handler2);or you can declare the handler in one line, like
app.get(path, route_handler1, route_handler2);Note that the handlers are executed in the same order as declaration. In the example above, route_handler1 is called before route_handler2.
This is illustrated in express_basic_example_2.js
node express_basic_example_2.js
If you send a request to `http://localhost:3000/students", you should see the output of both route handlers, printed in the right sequence.
There are something to take note when there are multiple handlers are used for the same route:
- How an route handler passes the request to the next route handler via next() call
- When we are not ready to send back the response yet, we use
response.write()method to update the response, instead of usingresponse.send()method - If a route handler needs to send the response back to client, it should call
response.end()method.