routing in web apps is essentially a set of rules to decide...

  • given this request
  • what code do we run?

the "code we run" is also called an endpoint or a route or a script or a handler or...

Routing is simple...

Many web app server frameworks have complicated systems for routing, but that complexity is not essential.

Routing can be a simple series of if..else statements, or a switch statement

and most of the fancy framework code is simply to build up a list of matching rules which the server then walks through in first-to-last order.

...but don't reinvent the wheel

Frameworks like Express give you more than the implementation of features like routing and parameter passing

they also give you an interface that will make your calling code easier to read

as well as a shared context of documentation and tutorials so other coders don't have as much to learn before understanding your code

Express Routing

  • supports handling all HTTP methods with the pattern app.method(path, handler)

  • whenever an incoming request of the given method matches the path parameter, Express will invoke the handler callback function

Express Routing Example

In the Hello, Express lesson we saw the following route:

app.get('/', (request, response) => response.send('Hello World!'))

This means,

code explanation
app my application,
.get when the client does a GET request
(request, response) => will call this handler function with a request object and a response object
response.send send a response
('Hello, World') with this string as its body

Express Route Matching Rules

  • paths can include special characters that are like regular expressions

    • ? "zero or one of these"
    • + "one of more of these"
    • * "zero or more" (but see below)
    • () "these go together"
  • ...but are not regular expressions

    • . and - are interpreted literally
    • : means "this is a parameter" (see next slide)
    • * means "zero or more characters" (which is .* in real regexes)
  • ...or you can use actual regular expressions

    // This route path will match butterfly 
    // and dragonfly, but not butterflyman
    app.get(/.*fly$/, function (request, response) {
        response.send('I am a fly')
  • for more info, see the full Express Routing Guide on their web site

Path Parameters in Express

The special character : means "this is a path parameter"


Path: /hello/Gandalf
Route: /hello/:name
Params: {name: 'Gandalf'}

Express will grab the value from the path itself, and put it into the request.params object for you to use later.

LAB: Hello, Path Friend!

Change your "Hello, Express" server to have the following route:

app.get('/hello/:friend', (request, response)=> {
    response.send('Hello, ' + request.params.friend + '!')

Prove that it works by visiting http://localhost:5000/hello/Gandalf (or use your own name)

LAB: Hello, You!

Now add a new route

app.get('/hello/:you/from/:me', (request, response)=> {
    response.send(`${} says, "Hello, ${}!')

Does http://localhost:5000/hello/Gandalf/from/Sauron work? If not, why not?

(Answer on next slide.)

Route Matching is Top-Down

Remember, Express routes are a list of matching rules which the server then walks through in first-to-last order.

So if an early route matches, it wins... even if there's a more specific rule that also matches later in the list.


Put more specific rules above more general rules.

app.get('/hello/:you/from/:me', (request, response)=> { ... 

app.get('/hello/:friend', (request, response)=> { ...