Javascript XMLHttpRequest Tutorial

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1- XMLHttpRequest

Interface XMLHttpRequest in  Javascript is designed to read data source from an  URL. Its name can cause confusion that it can read only text/xml data sources. In fact, it can read everything from an URL.
XMLHttpRequest is designed to read the data source from  URL synchronously or asynchronously. Reading data asynchronously helps users are still able to manipulate with the browser during the  XMLHttpRequest is reading data source remotely.
If you want to read files on user's computer, the  FileReader is the thing you need. It is designed as similarly as  XMLHttpRequest.

Constructor

// Create a XMLHttpRequest object:
 
var xhr = new XMLHttpRequest();
 

Properties

Property Mô tả
readyState Returns a number, the state of the request.
timeout Specify the maximum time to receive a reply, if it is not received within that time, it is considered a failure and automatically terminated.
withCredentials Its value is true or false (Default is false). If true means that this request can use cookies, authorization headers, but it must still comply with the same origin policy.
 
responseType Specify the type of data you want to receive. The default value is "text". (See possible values of this property below)
response Returns an ArrayBuffer, Blob, Document object, or a DOM String, depending on the value of XMLHttpRequest.responseType, that contains the response entity body.
responseText Returns a DOM String that contains the response to the request as text, or null if the request was unsuccessful or has not yet been sent.
responseURL Returns the serialized URL of the response or the empty string if the URL is null.
responseXML Returns a Document containing the response to the request, or null if the request was unsuccessful, has not yet been sent, or cannot be parsed as XML or HTML.
 
status Returns a number with the status of the response of the request.
statusText Returns a DOM String containing the response string returned by the HTTP Server. For example "200 OK".
readyState
Possible values of  readyState:
State Value Description
UNSENT 0 XMLHttpRequest has been created. open() not called yet.
OPENED 1 open() has been called.
HEADERS_RECEIVED 2 send() has been called, and headers & status are available.
LOADING 3 Downloading; responseText holds partial data.
DONE 4 The operation is complete.
responseType
Possible values of  responseType:
Value Description
"" If no value is specified for the responseType, or empty value, it is treated as "text".
"arraybuffer" The response is a ArrayBuffer containing binary data.
"blob" The response is a Blob object containing the binary data.
"document" The response is an HTML Document or XMLDocument, as appropriate based on the MIME type of the received data.
"json" The response is a JavaScript object created by parsing the contents of received data as JSON.
"text" The response is text in a DOMString object.

Events

During sending a request for reading a data source from an  URL, XMLHttpRequest will fire events described in interface ProgressEvent.
Note: The events marked by (?) are testing standard, which are not supported by most of browsers.
  Event Description
? loadstart Indicates that the process of loading data has begun. This event always fires first.
  progress Event fires multiple times as data is being loaded, giving access to intermediate data.
? error Event fires when loading has failed.
? abort Event fires when data loading has been canceled by calling abort() method (Method available on both XMLHttpRequest & FileReader).
  load Event fires only when all data has been successfully read.
? loadend Event fires when the object has finished transferring data. Always fires after error, abort, or load.
  • TODO Link!

Handlers

Note: The handlers marked by (?) are a testing standard, which is not supported by most browsers.
  Handler Mô tả
  onreadystatechange(event) A handler, which is called when the readyState property changes.
     
? onloadstart(progressEvt) A handler for the loadstart event.
  onprogress(progressEvt) A handler for the progress event.
? onerror(progressEvt) A handler for the error event.
? onabort(progressEvt) A handler for the abort event.
  onload(progressEvt) A handler for the load event.
? onloadend(progressEvt) A handler for the loadend event.

Steps to work with XMLHttpRequest

2- Examples with XMLHttpRequest

Example of using  XMLHttpResponse to read  XML data source given by an  URL.
xhr-example.html
<!DOCTYPE html>
<html>

<head>
    <title>XMLHttpRequest Example</title>
    <meta charset="UTF-8">

    <script src="xhr-example.js"></script>

</head>

<body>

    <h3>XMLHttpRequest example</h3>
    <a href="">Reset</a> <br><br>

    <button onclick = "clickHandler(event)">Click Me</button>
    <br><br>

    <textarea id="textarea-log" cols="50" rows="15"></textarea>

</body>

</html>
 
xhr-example.js
function clickHandler(evt) {


    var URL= "https://rawgit.com/o7planning/webexamples/master/_testdatas_/simple-xml-data.xml";
    // var URL= "https://rawgit.com/o7planning/webexamples/master/_testdatas_/triceratops.png";

    resetLog();

    // Create XMLHttpRequest.
    let xhr = new XMLHttpRequest();


    appendLog("URL: " + URL);
    appendLog("\n\n");

    xhr.onprogress = function(progressEvent) {
        appendLog("onprogress! " + progressEvent);
    }

    // readyState (State of request):
    // 0 - XMLHttpRequest.UNSENT
    // 1 - XMLHttpRequest.OPENED
    // 2 - XMLHttpRequest.HEADERS_RECEIVED
    // 3 - XMLHttpRequest.LOADING
    // 4 - XMLHttpRequest.DONE
    xhr.onreadystatechange = function(event)  {
        appendLog("onreadystatechange! readyState = " + xhr.readyState);
        appendLog(" status = " + xhr.status);
        appendLog(" statusText = " + xhr.statusText);
    }

    xhr.onload = function(progressEvent) {
        appendLog("onload!");
        appendLog(" status = " + xhr.status);
        appendLog(" statusText = " + xhr.statusText);
        
        appendLog(" ------ xhr.responseText ------: ");
        appendLog(xhr.responseText);

        appendLog(" ------ xhr.responseXML -------: ");
        appendLog(xhr.responseXML); // [object XMLDocument]

        // Convert XMLDocument to String.
        var xmlString = (new XMLSerializer()).serializeToString(xhr.responseXML);
        appendLog(xmlString);

    }

    xhr.onerror = function(progressEvent) {
        appendLog("onerror!");
        appendLog("Has Error!");
    }

    let async = true;
    // Initialize It.
    xhr.open("GET", URL, async);

    // Send it (Without body data)
    xhr.send();
}

function resetLog() {
    document.getElementById('textarea-log').value = "";
}

function appendLog(msg) {
    document.getElementById('textarea-log').value += "\n" + msg;
}

 
In the above example I do not mention the way to analyze XML (XMLDocument) content. If you are interested, please refer to the other posts below: 
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