Question

In this chapter, we saw that HTTP is a protocol for accessing resources over the Internet. A client sends a request, which contains a method (usually GET ) and a path that identifies a resource. The server then decides what to do with the request and responds with a status code and a response body. Both requests and responses may contain headers that provide additional information.

Browsers make GET requests to fetch the resources needed to display a web page. A web page may also contain forms, which allow information entered by the user to be sent along in the request made when the form is submitted. You will learn more about that in the next chapter .

The interface through which browser JavaScript can make HTTP requests is called XMLHttpRequest . You can usually ignore the “XML” part of that name (but you still have to type it). There are two ways in which it can be used—synchronous, which blocks everything until the request finishes, and asynchronous, which requires an event handler to notice that the response came in. In almost all cases, asynchronous is preferable. Making a request looks like this:

var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", true);
req.addEventListener("load", function() {
  console.log(req.status);
});
req.send(null);

Asynchronous programming is tricky. Promises are an interface that makes it slightly easier by helping route error conditions and exceptions to the right handler and by abstracting away some of the more repetitive and error-prone elements in this style of programming.

This is a book about getting computers to do what you want them to do. Computers are about as common as screwdrivers today, but they contain a lot more hidden complexity and thus are harder to operate and understand. To many, they remain alien, slightly threatening things.