Answers to Exercises - Beginning JavaScript - page 627

to see whether this.async is false . If it is, synchronous code is placed within the code block to
execute.
HttpRequest.prototype.send = function() {
this.request.open("GET", this.url, this.async);
if (this.async) {
var tempRequest = this.request;
var callback = this.callBack;
function requestReadystatechange() {
if (tempRequest.readyState == 4) {
if (tempRequest.status == 200) {
callback(tempRequest.responseText);
} else {
alert("An error occurred while attempting to " +
"contact the server.");
}
}
}
this.request.onreadystatechange = requestReadystatechange;
}
this.request.send(null);
if (!this.async) {
this.callBack(this.request.responseText);
}
};
This new code finishes off the method. Starting with the first if block, a new variable called
callback is assigned the value of this.callBack . This is done for the same reasons as with the
tempRequest variable—scoping issues—because this points to the requestReadystatechange()
function instead of the HttpRequest object. Other than this change, the asynchronous code remains
the same. The requestReadystatechange() function handles the readystatechange event and
calls the callback function when the request is successful.
The second if block is much simpler. Because this code executes only if synchronous
communication is desired, all you have to do is call the callback function and pass the
XMLHttpRequest object's responseText property.
Using this newly refactored module is quite simple. The following code makes an asynchronous
request for a fictitious text file called test.txt :
function requestCallback(responseText) {
alert(responseText);
}
var http = new HttpRequest("test.txt", requestCallback);
http.send();