Question

To make a simple request, we create a request object with the XMLHttpRequest constructor and call its open and send methods.

var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", false);
req.send(null);
console.log(req.responseText);
// → This is the content of data.txt

The open method configures the request. In this case, we choose to make a GET request for the example/data.txt file. URLs that don’t start with a protocol name (such as http:) are relative, which means that they are interpreted relative to the current document. When they start with a slash (/), they replace the current path, which is the part after the server name. When they do not, the part of the current path up to and including its last slash character is put in front of the relative URL.

After opening the request, we can send it with the send method. The argument to send is the request body. For GET requests, we can pass null . If the third argument to open was false , send will return only after the response to our request was received. We can read the request object’s responseText property to get the response body.

The other information included in the response can also be extracted from this object. The status code is accessible through the status property, and the human-readable status text is accessible through statusText . Headers can be read with getResponseHeader .

var req = new XMLHttpRequest();
req.open("GET", "example/data.txt", false);
req.send(null);
console.log(req.status, req.statusText);
// → 200 OK
console.log(req.getResponseHeader("content-type"));
// → text/plain

Header names are case-insensitive. They are usually written with a capital letter at the start of each word, such as “Content-Type”, but “content-type” and “cOnTeNt-TyPe” refer to the same header.

The browser will automatically add some request headers, such as “Host” and those needed for the server to figure out the size of the body. But you can add your own headers with the setRequestHeader method. This is needed only for advanced uses and requires the cooperation of the server you are talking to—a server is free to ignore headers it does not know how to handle.

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.