Perché JavaScript?
JavaScript è un linguaggio per computer potente, complicato, e spesso misconosciuto. Permette lo sviluppo rapido di applicazioni in cui gli utenti possono inserire i dati e vedere i risultati facilmente.
Il vantaggio primario di JavaScript, che è anche conosciuto come ECMAScript, è che è centrato intorno al Web browser, così da avere la capacità di produrre gli stessi risultati su tutte le piattaforme supportate dal browser. Gli esempi in questa pagina, proprio come Google Maps, girano su Linux, Windows, e OS X. Con la recente crescita di numerose librerie JavaScript è ora più facile traversare un documento, selezionare gli elementi del DOM, creare animazioni, trattare eventi, e sviluppare applicazioni Ajax. Diversamente dalla promozione intorno alle altre tecnologie spinta da vari interessi proprietari, JavaScript è realmente il solo linguaggio di programmazione "cross-platform" e "client-side" che è sia libero che universalmente adottato.
Cosa dovresti già conoscere
Iniziare a programmare con JavaScript è molto semplice. Tutto quello che serve per cominciare è un editor di testo e un Web browser.
Ci sono molte altre tecnologie che possono essere integrate e sviluppate insieme a Javascript che sono al di fuori di questo documento. Non aspettarti di fare un'intera applicazione come Google Maps tutta nel tuo primo giorno!
Come iniziare
Iniziare con JavaScript è molto semplice. Non devi aver precedentemente installato complicati programmi di sviluppo. Non devi conoscere come usare una shell, programmare Make, o usare un compilatore. JavaScript è interpretato dal tuo Web browser. Tutto quello che hai da fare è salvare il tuo programma come file testo e poi aprirlo nel tuo Web browser. E' tutto qui!
JavaScript è un linguaggio di programmazione ideale per linguaggi di computer introduttivi. Permette ai nuovi studenti di avere risposte immediate e insegna loro strumenti che essi troveranno di facile uso nella loro vita reale. Ciò è in assoluto contrasto con C, C++, and Java che sono veramente facili solo per sviluppatori software specializzati.
Browser compatibility issues
There are variations between what functionality is available in the different browsers. Mozilla, Chrome, Microsoft IE, Apple Safari, and Opera fluctuate in behavior. You can mitigate these issues by using the various cross-platform JavaScript APIs that are available. These APIs provide common functionality and hide these browser fluctuations from you.
How to try the examples
The examples below have some sample code. There are many ways to try these examples out. If you already have your own website, then you should be able to just save these examples as new Web pages on your website.
If you do not have your own website, you can save these examples as files on your computer and open them up with the Web browser you are using now. JavaScript is a very easy language to use for beginning programmers for this reason. You don't need a compiler or a development environment; you and your browser are all you need to get started!
You can also take use of some online programs like jsfiddle.net
Example: Catching a mouse click
The specifics of event handling (event types, handler registration, propagation, etc.) are too extensive to be fully covered in this simple example. However, this example cannot demonstrate catching a mouse click without delving a little into the JavaScript event system. Just keep in mind that this example will only graze the full details about JavaScript events and that if you wish to go beyond the basic capabilities described here, read more about the JavaScript event system.
'Mouse' events are a subset of the total events issued by a Web browser in response to user actions. The following is a list of the events emitted in response to a user's mouse action:
- Click - issued when a user clicks the mouse
- DblClick - issued when a user double-clicks the mouse
- MouseDown - issued when a user depresses a mouse button (the first half of a click)
- MouseUp - issued when a user releases a mouse button (the second half of a click)
- MouseOut - issued when the mouse pointer leaves the graphical bounds of the object
- MouseOver - issued when the mouse pointer enters the graphical bounds of the object
- MouseMove - issued when the mouse pointer moves while within the graphical bounds of the object
- ContextMenu - issued when the user clicks using the right mouse button
Note that in the latest versions of HTML, the inline event handlers, i.e. the ones added as tag attributes, are expected to be all lowercase and that event handlers in script are always all lowercase.
The simplest method for capturing these events, to register event handlers - using HTML - is to specify the individual events as attributes for your element. Example:
<span onclick="alert('Hello World!');">Click Here</span>
The JavaScript code you wish to execute can be inlined as the attribute value or you can call a function which has been defined in a <script> block within the HTML page:
<script>
function clickHandler() {
alert("Hello, World!");
}
</script>
<span onclick="clickHandler();">Click Here</span>
Additionally, the event object which is issued can be captured and referenced, providing the developer with access to specifics about the event such as which object received the event, the event's type, and which mouse button was clicked. Using the inline example again:
<script>
function clickHandler(event) {
var eType = event.type;
/* the following is for compatibility */
/* Moz populates the target property of the event object */
/* IE populates the srcElement property */
var eTarget = event.target || event.srcElement;
alert( "Captured Event (type=" + eType + ", target=" + eTarget + ")" );
}
</script>
<span onclick="clickHandler(event);">Click Here</span>
In addition to registering to receive events in your HTML, you can likewise set the same attributes of any HTMLElement objects generated by your JavaScript. The example below instantiates (creates) a span object, appends it to the page body, and registers the span object to receive mouse-over, mouse-out, mouse-down, and mouse-up events.
<body></body>
<script>
function mouseeventHandler(event) {
/* The following is for compatibility */
/* IE does NOT by default pass the event object */
/* obtain a ref to the event if one was not given */
if (!event) event = window.event;
/* obtain event type and target as earlier */
var eType = event.type;
var eTarget = event.target || event.srcElement;
alert(eType +' event on element with id: '+ eTarget.id);
}
function onloadHandler() {
/* obtain a ref to the 'body' element of the page */
var body = document.body;
/* create a span element to be clicked */
var span = document.createElement('span');
span.id = 'ExampleSpan';
span.appendChild(document.createTextNode ('Click Here!'));
/* register the span object to receive specific mouse events -
notice the lowercase of the events but the free choice in the names of the handlers you replace them with.
*/
span.onmousedown = mouseeventHandler;
span.onmouseup = mouseeventHandler;
span.onmouseover = mouseeventHandler;
span.onmouseout = mouseeventHandler;
/* display the span on the page */
body.appendChild(span);
}
window.onload = onloadHandler; // since we replace the handler, we do NOT have () after the function name
</script>
Example: Catching a keyboard event
Similar to the "Catching a mouse event" example above, catching a keyboard event relies on exploring the JavaScript event system. Keyboard events are fired whenever any key is used on the keyboard.
The list of available keyboard events emitted in response to a keyboard action is considerably smaller than those available for mouse:
- KeyPress - issued when a key is depressed and released
- KeyDown - issued when a key is depressed but hasn't yet been released
- KeyUp - issued when a key is released
- TextInput (available in Webkit browsers only at time of writing) - issued when text is input either by pasting, speaking, or keyboard. This event will not be covered in this article.
In a keypress event, the Unicode value of the key pressed is stored in either the keyCode or charCode property, never both. If the key pressed generates a character (e.g., 'a'), charCode is set to the code of that character, respecting the letter case (i.e., charCode takes into account whether the shift key is held down). Otherwise, the code of the pressed key is stored in keyCode.
The simplest method for capturing keyboard events is again to register event handlers within the HTML, specifying the individual events as attributes for your element. Example:
<input type="text" onkeypress="alert ('Hello World!');">
As with mouse events, the JavaScript code you wish to execute can be inlined as the attribute value or you can call a function which has been defined in a <script> block within the HTML page:
<script>
function keypressHandler() {
alert ("Hello, World!");
}
</script>
<input onkeypress="keypressHandler();" />
Capturing the event and referencing the target (i.e., the actual key that was pressed) is achieved in a similar way to mouse events:
<script type="text/javascript">
function keypressHandler(evt) {
var eType = evt.type; // Will return "keypress" as the event type
/* here we again need to use a cross browser method
mozilla based browsers return which and others keyCode.
The Conditional operator or ternary is a good choice */
var keyCode = evt.which?evt.which:evt.keyCode;
var eCode = 'keyCode is ' + keyCode;
var eChar = 'charCode is ' + String.fromCharCode(keyCode); // or evt.charCode
alert ("Captured Event (type=" + eType + ", key Unicode value=" + eCode + ", ASCII value=" + eChar + ")");
}
</script>
<input onkeypress="keypressHandler(event);" />
Capturing any key event from the page can be done by registering the event at the document level and handling it in a function:
<script> document.onkeypress = keypressHandler; document.onkeydown = keypressHandler; document.onkeyup = keypressHandler; </script>
Here is a complete example that shows key event handling:
<!DOCTYPE html>
<html>
<head>
<script>
var metaChar = false;
var exampleKey = 16;
function keyEvent(event) {
var key = event.keyCode || event.which; // alternative to ternary - if there is no keyCode, use which
var keychar = String.fromCharCode(key);
if (key == exampleKey) {
metaChar = true;
}
if (key != exampleKey) {
if (metaChar) {
alert("Combination of metaKey + " + keychar)
metaChar = false;
} else {
alert("Key pressed " + key);
}
}
}
function metaKeyUp(event) {
var key = event.keyCode || event.which;
if (key == exampleKey) { metaChar = false; }
}
</script>
</head>
<body onkeydown="keyEvent(event)" onkeyup="metaKeyUp(event)">
Try pressing any key!
</body>
</html>
Browser bugs and quirks
The two properties made available through the key events are keyCode and charCode. In simple terms, keyCode refers to the actual keyboard key that was pressed by the user, while charCode is intended to return that key's ASCII value. These two values may not necessarily be the same; for instance, a lower case 'a' and an upper case 'A' have the same keyCode, because the user presses the same key, but a different charCode because the resulting character is different.
The way in which browsers interpret the charCode is not a consistently-applied process. For example, Internet Explorer and Opera do not support charCode. However, they give the character information in keyCode, but only onkeypress. Onkeydown and onkeyup keyCode contain key information. Firefox uses a different word, "which", to distinguish the character.
Refer to the Mozilla Documentation on Keyboard Events for a further treatment of keyboard events.
Draft
This page is not complete.
Example: Dragging images around
The following example allows moving the image of Firefox around the page:
<!DOCTYPE html>
<html>
<head>
<style>
img { position: absolute; }
</style>
<script>
window.onload = function() {
movMeId = document.getElementById("ImgMov");
movMeId.style.top = "80px";
movMeId.style.left = "80px";
document.onmousedown = coordinates;
document.onmouseup = mouseup;
function coordinates(e) {
if (e == null) { e = window.event;}
// e.srcElement holds the target element in IE, whereas e.target holds the target element in Firefox
// Both properties return the HTML element the event took place on.
var sender = (typeof( window.event ) != "undefined" ) ? e.srcElement : e.target;
if (sender.id=="ImgMov") {
mouseover = true;
pleft = parseInt(movMeId.style.left);
ptop = parseInt(movMeId.style.top);
xcoor = e.clientX;
ycoor = e.clientY;
document.onmousemove = moveImage;
return false;
}
return false;
}
function moveImage(e) {
if (e == null) { e = window.event; }
movMeId.style.left = pleft+e.clientX-xcoor+"px";
movMeId.style.top = ptop+e.clientY-ycoor+"px";
return false;
}
function mouseup(e) {
document.onmousemove = null;
}
}
</script>
</head>
<body>
<img id="ImgMov" src="https://placehold.it/100x100&text=JS" width="64" height="64">
<p>Drag and drop around the image in this page.</p>
</body>
</html>
Example: Resizing things
<!DOCTYPE html>
<html>
<head>
<style>
#resizeImage {
margin-left: 100px;
}
</style>
<script>
window.onload = function() {
var resizeId = document.getElementById("resizeImage");
var resizeStartCoordsX,
resizeStartCoordsY,
resizeEndCoordsX,
resizeEndCoordsY;
var resizeEndCoords;
var resizing = false;
document.onmousedown = coordinatesMousedown;
document.onmouseup = coordinatesMouseup;
function coordinatesMousedown(e) {
if (e == null) {
e = window.event;
}
var element = (typeof( window.event ) != 'undefined' ) ? e.srcElement : e.target;
if (element.id == "resizeImage") {
resizing = true;
resizeStartCoordsX = e.clientX;
resizeStartCoordsY = e.clientY;
}
return false;
}
function coordinatesMouseup(e) {
if (e == null) {
e = window.event;
}
if (resizing === true) {
var currentImageWidth = parseInt(resizeId.width);
var currentImageHeight = parseInt(resizeId.height);
resizeEndCoordsX = e.clientX;
resizeEndCoordsY = e.clientY;
resizeId.style.height = currentImageHeight - (resizeStartCoordsY - resizeEndCoordsY) + 'px';
resizeId.style.width = currentImageWidth - (resizeStartCoordsX - resizeEndCoordsX) + 'px';
resizing = false;
}
return false;
}
}
</script>
</head>
<body>
<img id="resizeImage" src="https://upload.wikimedia.org/wikipedia/commons/e/e7/Mozilla_Firefox_3.5_logo_256.png"
width="64" height="64">
<p>Click on the image and drag for resizing.</p>
</body>
</html>
Example: Drawing Lines
<!DOCTYPE html>
<html>
<head>
<script>
function linedraw(ax,ay,bx,by)
{
if(ay>by)
{
bx=ax+bx;
ax=bx-ax;
bx=bx-ax;
by=ay+by;
ay=by-ay;
by=by-ay;
}
var calc=Math.atan((ay-by)/(bx-ax));
calc=calc*180/Math.PI;
var length=Math.sqrt((ax-bx)*(ax-bx)+(ay-by)*(ay-by));
document.body.innerHTML += "<div id='line' style='height:" + length + "px;width:1px;background-color:black;position:absolute;top:" + (ay) + "px;left:" + (ax) + "px;transform:rotate(" + calc + "deg);-ms-transform:rotate(" + calc + "deg);transform-origin:0% 0%;-moz-transform:rotate(" + calc + "deg);-moz-transform-origin:0% 0%;-webkit-transform:rotate(" + calc + "deg);-webkit-transform-origin:0% 0%;-o-transform:rotate(" + calc + "deg);-o-transform-origin:0% 0%;'></div>"
}
</script>
</head>
<body onload="linedraw(200,400,500,900);"> <!-- Replace with your co-ordinate -->
</body>
</html>