Add click handler to H1 elements

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Added on 2019/09/16

AI Summary

This assignment is about creating a photo album application using JavaScript, HTML, and PHP. The task involves adding click event handlers to delete links in the photo album, which will call a function that sends a POST request to the server to delete the photo. The server-side script (json-photoalbum-delete.php) checks if the deletion ID is set, calls the deletePhotograph function, and returns a JSON response indicating success or failure.

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Digital Business Application – Lab Exercises
JavaScript and Ajax
EXERCISE 3: PHOTO ALBUM CLIENT-RENDERED VERSION C
Photo Album Client-Rendered Version C
The next version of the photoalbum adds the ability to delete photographs from the album. The first
thing this requires is a link displayed next to each photograph to allow it to be deleted. This link
needs to be added to the createPhotoHTML function. Like the PHP photoalbum, this link needs to
embed the id number of the photograph. Unlike the PHP version, it does not need to have the href
attribute pointing back to the PHP script, in fact, it does not even need to be a link using the <a> tag
at all! This is because when we click on delete it is going to run a JavaScript function that will do
everything else.
Make a copy of photoalbum-client-rendered-version-b.html and call it photoalbum-client-rendered-
version-c.html. Then add the highlighted lines to the existing createPhotoHTML function. Note that
this function is given a JavaScript object called photoInfo which has named fields for the image,
name, description and photoId values.
function createPhotoHTML( photoInfo) {
return " <div class='w3-container w3-row w3-col l4'>" +
" <div class='w3-container w3-card-8 w3-margin-8'>" +
" <div class='w3-container w3-half w3-image'>" +
" <a class='example-image-link' data-lightbox='example-1'
href='images/" + photoInfo.image + "'><img src='images/" + photoInfo.image
+ "' class='example-image w3-circle'></a>" +
" <div class='w3-title w3-text-purple'>" + photoInfo.name +
"</div>" +
" </div>" +
" <div class='w3-container w3-half'>" + photoInfo.description
+
// next line should be conditional on whether logged in or not
" <span class='link deletelink' id='" + photoInfo.photoid +
"'>(Delete)</span>" +
" </div>" +
" </div>" +
" </div>";
}
The link that will be added beside each picture will look something like this (for the photo with id
12):
<span class='link deletelink' id='12'>(Delete)</span>"
Note that this is just a <span> element, not an <a> element. This does not matter, because we can
attach a JavaScript onClick event handler to any element in the DOM, so a span works just as well.
To add an onClick handler to an element we need to get a reference to the element and assign a
function to its onclick property. Fortunately, JQuery provides us an easy way to do this. Every one of
the delete link spans will have the CSS class deletelink. JQuery lets us select DOM elements using
CSS selector rules, so $(“.deletelink”) will return a collection of all DOM elements with that class,
and in our case that means the delete <span>s for every photograph on the page. JQuery also allows
us to add a single handler function to ALL the elements it finds in one line. Try the following example
in a new file:
Page 1 of 5

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Digital Business Application – Lab Exercises
<html>
<head>
<script src='http://code.jquery.com/jquery-1.11.3.min.js'></script>
<script>
$(document).ready(
function() {
$("h1").click( function( event) {
alert( "You clicked: " + event.target.innerHTML);
})
}
);
</script>
</head>
<body>
<h1>Click Me</h1>
<h1>Click me too!</h1>
</body>
</html>
$("h1") selects all the h1 elements in the document.
$("h1").click( FUNCTION ) adds the given FUNCTION as a click handler to each of the h1 elements in
the document.
In this case the function is :
function( event) {
alert( "You clicked: " + event.target.innerHTML);
}
Note that the function takes a parameter called event. This is an object provided by the JavaScript
engine describing a user interface event. In this case it will be a mouse click, but this object also has
a field called target which is a reference to the DOM element that was clicked on. This function uses
the built in function alert to display a dialog box which displays the content of the target of the
event – that is, the text in the h1 tag you clicked on.
So from this it follows that $(“.deletelink”).click( FUNCTION) could be used to add click handlers to
all the photographs on the page, as they all have the class deletelink assigned to them.
It is important to realise that this can only be done once they exist as elements in the DOM, and the
HTML for the photographs is dynamically added by JavaScript in the loadPhotos() function.
Therefore, we must add the click handlers after the photos have been added to the DOM, at the end
of the code in the loadPhotos() function.
Modify loadPhotos() as shown below to add a click handler to all delete links. This adds a function
similar to the example above, which simply displays a dialog box and tells you the id number of the
photograph we want to delete.
Page 2 of 5

Digital Business Application – Lab Exercises
function loadPhotos() {
$.getJSON( "json.txt",
function( photos) {
$('#album').empty();
for (i = 0; i < photos.length; i += 3) {
var rowDiv = $( "<div class='w3-row'>");
$('#album').append( rowDiv);
for( j = i; j < i + 3; j++ ) {
if ( j < photos.length) {
rowDiv.append( createPhotoHTML( photos[j]));
}
}
}
// add listener to all deletelinks
$('.deletelink').click(
function( event) {
alert('Photo id: ' + event.target.id);
}
);
}
);
}
We now have links which run a function when clicked on, and that function knows the id number of
the photo in question. What we need now is a function that will actually delete the photo.
Remember that the photographs are actually stored on the server, so we will have to ask the server
to do the deletion. JQuery helps here by allowing us to send a POST request to the server easily, and
handle its response to make sure it succeeded. Here is the JavaScript we need. Add it to your file.
function deletePhoto( event) {
$.post( "json-photoalbum-delete.php",
"deletionid=" + event.target.id,
function( data) {
if ( data.result != "success") {
alert( "ERROR: " + data.error);
} else {
loadPhotos();
}
});
}
This function needs to be called with the mouse click event object generated by clicking on one of
the delete links. It then uses the JQuery $.post function to create an HTTP request using the POST
method. This request is sent to the URL given as the first parameter to $.post, in this case json-
photoalbum-delete.php. The second parameter is the name value pairs to send in the body of the
message, in this case deletionid with the value taken from the delete link that was clicked on. The
third parameter is a function to be executed when the response from the server is received. This will
be given an object constructed from the body of the response message. The PHP script this is calling
will return a JSON encoded object which JQuery will automatically turn into a corresponding
JavaScript object. It will contain a field called result with the value success or failure, depending on
whether the server was able to delete the requested photograph or not. The function given will
display an error in an alert if one happened, but otherwise it will just call the loadPhotos() function
again. This will discard the existing photographs and reload them from the server so that the deleted
picture will no longer appear.
Page 3 of 5

Digital Business Application – Lab Exercises
The next thing you need to do is change the code that adds a click handler to the delete links so that
instead of doing alert( ‘Photo id: ‘ + event.target.id); it does deletePhoto( event); instead.
The JavaScript is now complete, but we need a PHP script on the server for it to invoke. Save the
following as json-photoalbum-delete.php.
<?php
header("content-type: application/json");
require_once("lib/dbutils.php");
require_once("photoalbum-common.php");
$pdo = connect();
if ( isset( $_POST['deletionid'])) {
$errorMessage = deletePhotograph( $pdo, $_POST['deletionid']);
} else {
$errorMessage = "No id provided in post";
}
if ( $errorMessage == "") {
print "{\"result\": \"success\"}";
} else {
print "{";
print "\"result\": \"failure\"";
print ",";
print "\"error\": \"$errorMessage\"";
print "}";
}
?>
Note that this script begins by emitting a content-type header declaring that it will return JSON data
as its result. If you view it directly in a web browser you will see the JSON data encoding an error
message.
Next note that it imports the file photoalbum-common.php. This file contains the deletePhotograph
function as used by the PHP photoalbum. We use the same function here.
The script then checks to see if the request contains a deletionid value. If it does, it calls the
deletePhotograph function, giving it the database connection and the id number to delete. This
function returns an empty string on success, or an error message if something went wrong. If there
is no deletionid in the request (as there would not be when you viewed the file directly in a web
browser) the error message is set appropriately.
The last part of the script prints out the JSON data for the response message. This consists of the
field result with the value success or failure, and in the case of failure, another field called error
which contains the errormessage..
With this script on the server and all the JavaScript changes in place, your photoalbum should allow
the deletion of photographs.
Page 4 of 5

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Digital Business Application – Lab Exercises
EXERCISE 3: PHOTO ALBUM CLIENT-RENDERED VERSION C
Required in your report (basic exercise).
 A clickable link to photoalbum-client-rendered-c.html.
 Reflection on the tasks and all activities involved.
Extended Tasks
 No extended tasks are defined for this exercise.
Original additional work:
Investigate and experiment with any related subject matter that interests you.
Page 5 of 5

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Add click handler to H1 elements

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