A serverless function is a program written to handle one request per program execution. In other words, the program is started, handed a request, and run only until that request is handled. Then it is shut down.

Earlier in the guide, we saw an example of a simple serverless function written in JavaScript:

const encoder = new TextEncoder()

// Declare a function that handles a request (in this case, an HTTP request)
export async function handleRequest(request) {

    // Send back an object that describes a response (in this case, an HTTP response)
    return {
        status: 200,
        body: encoder.encode("I'm a Serverless Function").buffer
    }
}

The entry point of a serverless function is (act surprised when I say this) a function. More specifically, it is a function that gives the request input and returns a response as output optionally.

We call this kind of program a serverless function. It does not start a server that listens for requests. It does not run for long periods.

The serverless function is the building block of what we will look at next: the serverless app. There we will talk about how some platforms, like AWS Lambda, use the terms “serverless function” and “serverless app” interchangeably. And in most cases, that is fine.