The Handler

Handler is a function that takes in a request that can return a response to fulfill the request or return nothing to continue to next handler if available or send a default 404 response like "cannot get /"

Example

/// App init logic

app.use((request) => {
    const { pathname } = new URL(request.url)
    if (request.method === "GET" && pathname === "/") {
        return new Response("Home Page")
    }
})

app.use((request) => {
    const { pathname } = new URL(request.url)
    if (request.method === "POST" && pathname === "/contact") {
        return new Response("Contact form submitted")
    }
})

/// ...rest

In the above example the first handler checks if the request is a get method to the index ("/") route and responds if it is and returns nothing otherwise

Now the control goes to the next handler that checks for a post request to the contact route and responds to that request

If the above handlers return nothing there is no other handler left so the default 404 response is sent

Context

The createApp method optionally takes in function that takes the current request and returns an object called the context that can be accessed by all other Handlers registered. Click here to learn more about context