Callback Functions are functions that are passed as arguments to other function.
- Example (see the comments in code):
<p>When the following button is clicked, it is added its tag-name in #resp</p>
<button id='btn'>Click</button>
<blockquote id='resp'>#resp</blockquote>
<script>
// defines a callback function, receives an object with the event
function callback(ev){
//adds in #resp the name of the html element that emitted the event
document.getElementById('resp').innerHTML = ev.target.tagName;
}
// registers the click event on #btn to call the callback() function
document.getElementById('btn').addEventListener('click', callback);
</script>
setTimeout() and setInterval().
<ht>Example callback function with setInterval()</h4>
<blockquote id='resp'>#resp</blockquote>
<script>
var seconds = 0;
var resp = document.getElementById('resp');
// used as callback function in setInterval()
function addSeconds(x) {
// increments the seconds and adds the value in #resp
seconds++;
resp.innerHTML = seconds;
}
// calls the addSeconds() to every second (1000 milliseconds)
setInterval('addSeconds()', 1000);
</script>
<ht>Example anonymous function with setTimeout()</h4>
<blockquote id='resp'>#resp</blockquote>
<script>
var resp = document.getElementById('resp');
//adds a text in #resp after 2 seconds
setTimeout(()=>{
resp.innerHTML ='https://CoursesWeb.net/';
}, 2000);
</script>
<script>
// gets data from external file
// receives the URL address of the file, and a callback function that will be called when the response is received
function ajaxF(url, callback) {
var request = (window.XMLHttpRequest) ? new XMLHttpRequest() : new ActiveXObject('Microsoft.XMLHTTP'); // sets the XMLHttpRequest instance
request.open('GET', url); // define the request
request.send(null); // sends data
// Check request status, when the response is completely received pass it to callback function
request.onreadystatechange = function() {
if (request.readyState == 4) {
callback(request.responseText);
}
}
}
// the callback function
function addResp(resp) {
alert(resp);
}
// calls the ajaxF() function, passing the file address, and the callback function
ajaxF('test.txt', addResp);
// continue the script while the Ajax and callback functions are executed
alert('Message 1');
</script>
<embed src="flash_game.swf" width="450" height="350" />
#id:first-line {
font-weight: bold;
color: blue;
}var url = window.location; alert(url);
$homepage = file_get_contents("http://coursesweb.net/");
echo $homepage;