Nested EventListner looking for content loaded? - javascript

I am trying to detect when a div within the entire document is loaded.
but for some reason is this not being detected?
https://codepen.io/noflaco/pen/eoeexM?editors=1111
document.addEventListener("DOMContentLoaded", function() {
const test = document.getElementById("test");
function consent() {
alert("test");
}
test.addEventListener("DOMContentLoaded", Consent);
});
why not?

why not?
Arbitrary DOM elements to not trigger a DOMContentLoaded event, so the event handler is never executed. This event is only triggered on the document element when whole DOM has been parsed and loaded.
You can learn more about this event on MDN.
Ironically, the fact that you can load the div via document.getElementById("test") means that it already exists, so there is no point in adding a handler to wait for it to load.

Related

Why isn't document.onload called?

By looking at the output of console.log(document), I find document has these properties:onload=null;onloadstart=null;onloadend=null;onreadystatechange=null; I write the following code:
<html>
<head><title>test</title></head>
<body>
<script>
document.onload=function(){alert("document.onload");};
document.onloadstart=function(){alert("document.onloadstart");};
document.onloadend=function(){alert("document.onloadend");};
document.onreadystatechange=function(){alert("document.onreadystatechange");};
</script>
<div>hello</div>
</body>
</html>
Interestingly, document.onload,document.onloadstart,document.onloadend are never called, while document.onreadystatechange is called twice, why?
Why isn't document.onload called?
First of all the load events (created by the browser) do not bubble (because that's how it is specified):
document.body.onload = function(e) {
console.dir(e.bubbles)
}
So you can only listen for these load events, on the element for which they occur.
And for Document there is no load event listed that will be triggered based on the standard.
readystatechange on the other hand is listed as an event that can happen for document.
You can however for sure listen for load events on the document and the event callback will be called. But that will only happen if you e.g. manually trigger one on document:
const event = new Event('load');
document.onload = function (e) {
console.log('load document')
}
document.dispatchEvent(event);
Or if you emit a load event that bubbles on a descendant:
const event = new Event('load', {bubbles: true});
document.onload = function (e) {
console.log('load document')
}
document.body.dispatchEvent(event);
So why does onload then appear as a property? That's due to the GlobalEventHandlers and Document includes GlobalEventHandlers), and due to event handler IDL attribute, exposing those as on… event properties.
I'm unsure, but try this:
window.addEventListener('load', () => {
alert("document.onload")
})
I only use window onload event, but if I never saw document onload event, just change window to document.
Like I said´, I'm unsure what to do, because the amount of your code, which you showed is to less.
.onload is part of the window object, not for document.
GlobalEventHandlers.onload
The onload property of the GlobalEventHandlers mixin is an event handler that processes load events on a Window, XMLHttpRequest, <iframe> and <img> elements, etc.
And from what i've seen on MDN .onloadstart and .onloadend are used for resources like images, don't know if they are availiable for document/window. GlobalEventHandlers.onloadstart
But I think with onreadystatechange you should be able to emulate the behavior of onloadstart and onloadend.
Document.readyState

When to use $(function() {}) when registering click handlers with jQuery?

What is the difference between
$(function()
{
$(".some").click(function()
{
...
});
});
and
$(".some").click(function()
{
...
});
I know from here that $(function() is shorthand for $(document).ready(function(). But why are we waiting for the document to be ready? Will the function not be only called when some is clicked anyway?
Note: #2 does not work in my case.
The difference is that #1 waits for the DOM to fully load before running the JavaScript.
The second code runs the JavaScript when it receives it which means it looks for .class elements before they have finished loading. This is why it doesn't work.
You need the document to be ready, i.e. all elements of the document to be available, before you can add an event listener to an element.
The reason is: consider a button, and you want an event listener (listening for the click event, for example.
When your sript runs but the button is not yet present, the attempt to attach the listener will fail. As a result, the associated function cannot be called once the button is actually clicked.
Does that answer your question?
You use the $(function()) simply because you need the DOM to fully load.
For example you have a button and you want to add some action on click. You click the button, but nothing happened, because the button was handled prior to the DOM loading.
If you won't check that the DOM is fully loaded, some unexpected behavior might occur.
Please do not confuse between onload() to ready(), as on load executes once the page is loaded and ready() executes only when the DOCUMENT is fully ready.
$(function(){...}) triggers the function when the DOM is load, it's similar to window.onload but part of jquery lib.
you can also use $(NAMEOFFUNCTION);
It's there to be sure the event has a element to listen to.

How to listen for specific elements to load?

Is it possible to listen for a specific DOM element to be loaded? I have a window.onload event that is called when the whole page loads, but I'm looking for a way for it to be called for each DOM element that gets loaded. (Does onload not bubble up?)
Example:
function init() {
alert(this.id); // Should be in context of DOM element that bubbled up when loaded
}
window.onload = init;
<div id="one">One</div>
<div id="two">One</div>
<div id="three">One</div>
I want this code to alert one, two, three.
onload events don't bubble like this, you have 2 options here:
poll for the element, running a check for it on an interval.
use DOM mutation events (which vary across browsers - mainly IE implementation headaches) to listen, though this will be expensive, since you'll have to filter everything other than your element.

window.onload vs $(document).ready()

What are the differences between JavaScript's window.onload and jQuery's $(document).ready() method?
The ready event occurs after the HTML document has been loaded, while the onload event occurs later, when all content (e.g. images) also has been loaded.
The onload event is a standard event in the DOM, while the ready event is specific to jQuery. The purpose of the ready event is that it should occur as early as possible after the document has loaded, so that code that adds functionality to the elements in the page doesn't have to wait for all content to load.
window.onload is the built-in JavaScript event, but as its implementation had subtle quirks across browsers (Firefox, Internet Explorer 6, Internet Explorer 8, and Opera), jQuery provides document.ready, which abstracts those away, and fires as soon as the page's DOM is ready (doesn't wait for images, etc.).
$(document).ready (note that it's not document.ready, which is undefined) is a jQuery function, wrapping and providing consistency to the following events:
DOMContentLoaded - a newish event which fires when the document's DOM is loaded (which may be some time before the images, etc. are loaded); again, slightly different in Internet Explorer and in rest of the world
and window.onload (which is implemented even in old browsers), which fires when the entire page loads (images, styles, etc.)
$(document).ready() is a jQuery event. JQuery’s $(document).ready() method gets called as soon as the DOM is ready (which means that the browser has parsed the HTML and built the DOM tree). This allows you to run code as soon as the document is ready to be manipulated.
For example, if a browser supports the DOMContentLoaded event (as many non-IE browsers do), then it will fire on that event. (Note that the DOMContentLoaded event was only added to IE in IE9+.)
Two syntaxes can be used for this:
$( document ).ready(function() {
console.log( "ready!" );
});
Or the shorthand version:
$(function() {
console.log( "ready!" );
});
Main points for $(document).ready():
It will not wait for the images to be loaded.
Used to execute JavaScript when the DOM is completely loaded. Put event handlers here.
Can be used multiple times.
Replace $ with jQuery when you receive "$ is not defined."
Not used if you want to manipulate images. Use $(window).load() instead.
window.onload() is a native JavaScript function. The window.onload() event fires when all the content on your page has loaded, including the DOM (document object model), banner ads and images. Another difference between the two is that, while we can have more than one $(document).ready() function, we can only have one onload function.
A little tip:
Always use the window.addEventListener to add an event to window. Because that way you can execute the code in different event handlers .
Correct code:
window.addEventListener('load', function () {
alert('Hello!')
})
window.addEventListener('load', function () {
alert('Bye!')
})
Invalid code:
window.onload = function () {
alert('Hello!') // it will not work!!!
}
window.onload = function () {
alert('Bye!')
}
This is because onload is just property of the object, which is overwritten.
By analogy with addEventListener, it is better to use $(document).ready() rather than onload.
A Windows load event fires when all the content on your page is fully loaded including the DOM (document object model) content and asynchronous JavaScript, frames and images. You can also use body onload=. Both are the same; window.onload = function(){} and <body onload="func();"> are different ways of using the same event.
jQuery $document.ready function event executes a bit earlier than window.onload and is called once the DOM(Document object model) is loaded on your page. It will not wait for the images, frames to get fully load.
Taken from the following article:
how $document.ready() is different from window.onload()
$(document).ready(function() {
// Executes when the HTML document is loaded and the DOM is ready
alert("Document is ready");
});
// .load() method deprecated from jQuery 1.8 onward
$(window).on("load", function() {
// Executes when complete page is fully loaded, including
// all frames, objects and images
alert("Window is loaded");
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js"></script>
The $(document).ready() is a jQuery event which occurs when the HTML document has been fully loaded, while the window.onload event occurs later, when everything including images on the page loaded.
Also window.onload is a pure javascript event in the DOM, while the $(document).ready() event is a method in jQuery.
$(document).ready() is usually the wrapper for jQuery to make sure the elements all loaded in to be used in jQuery...
Look at to jQuery source code to understand how it's working:
jQuery.ready.promise = function( obj ) {
if ( !readyList ) {
readyList = jQuery.Deferred();
// Catch cases where $(document).ready() is called after the browser event has already occurred.
// we once tried to use readyState "interactive" here, but it caused issues like the one
// discovered by ChrisS here: http://bugs.jquery.com/ticket/12282#comment:15
if ( document.readyState === "complete" ) {
// Handle it asynchronously to allow scripts the opportunity to delay ready
setTimeout( jQuery.ready );
// Standards-based browsers support DOMContentLoaded
} else if ( document.addEventListener ) {
// Use the handy event callback
document.addEventListener( "DOMContentLoaded", completed, false );
// A fallback to window.onload, that will always work
window.addEventListener( "load", completed, false );
// If IE event model is used
} else {
// Ensure firing before onload, maybe late but safe also for iframes
document.attachEvent( "onreadystatechange", completed );
// A fallback to window.onload, that will always work
window.attachEvent( "onload", completed );
// If IE and not a frame
// continually check to see if the document is ready
var top = false;
try {
top = window.frameElement == null && document.documentElement;
} catch(e) {}
if ( top && top.doScroll ) {
(function doScrollCheck() {
if ( !jQuery.isReady ) {
try {
// Use the trick by Diego Perini
// http://javascript.nwbox.com/IEContentLoaded/
top.doScroll("left");
} catch(e) {
return setTimeout( doScrollCheck, 50 );
}
// detach all dom ready events
detach();
// and execute any waiting functions
jQuery.ready();
}
})();
}
}
}
return readyList.promise( obj );
};
jQuery.fn.ready = function( fn ) {
// Add the callback
jQuery.ready.promise().done( fn );
return this;
};
Also I have created the image below as a quick references for both:
A word of caution on using $(document).ready() with Internet Explorer. If an HTTP request is interrupted before the entire document is loaded (for example, while a page is streaming to the browser, another link is clicked) IE will trigger the $(document).ready event.
If any code within the $(document).ready() event references DOM objects, the potential exists for those objects to be not found, and Javascript errors can occur. Either guard your references to those objects, or defer code which references those objects to the window.load event.
I have not been able to reproduce this problem in other browsers (specifically, Chrome and Firefox)
Events
$(document).on('ready', handler) binds to the ready event from jQuery. The handler is called when the DOM is loaded. Assets like images maybe still are missing. It will never be called if the document is ready at the time of binding. jQuery uses the DOMContentLoaded-Event for that, emulating it if not available.
$(document).on('load', handler) is an event that will be fired once all resources are loaded from the server. Images are loaded now. While onload is a raw HTML event, ready is built by jQuery.
Functions
$(document).ready(handler) actually is a promise. The handler will be called immediately if document is ready at the time of calling. Otherwise it binds to the ready-Event.
Before jQuery 1.8, $(document).load(handler) existed as an alias to $(document).on('load',handler).
Further Reading
The timing
On the function ready
An example
Promises
The removed event alias
window.onload: A normal JavaScript event.
document.ready: A specific jQuery event when the entire HTML has been loaded.
One thing to remember (or should I say recall) is that you cannot stack onloads like you can with ready. In other words, jQuery magic allows multiple readys on the same page, but you can't do that with onload.
The last onload will overrule any previous onloads.
A nice way to deal with that is with a function apparently written by one Simon Willison and described in Using Multiple JavaScript Onload Functions.
function addLoadEvent(func) {
var oldonload = window.onload;
if (typeof window.onload != 'function') {
window.onload = func;
}
else {
window.onload = function() {
if (oldonload) {
oldonload();
}
func();
}
}
}
// Example use:
addLoadEvent(nameOfSomeFunctionToRunOnPageLoad);
addLoadEvent(function() {
/* More code to run on page load */
});
Document.ready (a jQuery event) will fire when all the elements are in place, and they can be referenced in the JavaScript code, but the content is not necessarily loaded. Document.ready executes when the HTML document is loaded.
$(document).ready(function() {
// Code to be executed
alert("Document is ready");
});
The window.load however will wait for the page to be fully loaded. This includes inner frames, images, etc.
$(window).load(function() {
//Fires when the page is loaded completely
alert("window is loaded");
});
The document.ready event occurs when the HTML document has been loaded, and the window.onload event occurs always later, when all content (images, etc) has been loaded.
You can use the document.ready event if you want to intervene "early" in the rendering process, without waiting for the images to load.
If you need the images (or any other "content") ready before your script "does something", you need to wait until window.onload.
For instance, if you are implementing a "Slide Show" pattern, and you need to perform calculations based on image sizes, you may want to wait until window.onload. Otherwise, you might experience some random problems, depending on how fast the images will get loaded. Your script would be running concurrently with the thread that loads images. If your script is long enough, or the server is fast enough, you may not notice a problem, if images happen to arrive in time. But the safest practice would be allowing for images to get loaded.
document.ready could be a nice event for you to show some "loading..." sign to users, and upon window.onload, you can complete any scripting that needed resources loaded, and then finally remove the "Loading..." sign.
Examples :-
// document ready events
$(document).ready(function(){
alert("document is ready..");
})
// using JQuery
$(function(){
alert("document is ready..");
})
// window on load event
function myFunction(){
alert("window is loaded..");
}
window.onload = myFunction;
Time flies, it's ECMAScript 2021 now and IE11 is used by people less and less. The most two events in contrast are load and DOMContentLoaded.
DOMContentLoaded fires after the initial HTML document has been completely loaded and parsed.
load fires after DOMContentLoaded and the whole page has loaded,
waiting for all dependent resources to finish loading. Example of resources: scripts, stylesheets, images and iframes etc.
IMPORTANT: Synchronous scripts will pause parsing of the DOM.
Both two events can be used to determine the DOM is able to use or not. For examples:
<script>
// DOM hasn't been completely parsed
document.body; // null
window.addEventListener('DOMContentLoaded', () => {
// Now safely manipulate DOM
document.querySelector('#id');
document.body.appendChild();
});
/**
* Should be used only to detect a fully-loaded page.
*
* If you just want to manipulate DOM safely, `DOMContentLoaded` is better.
*/
window.addEventListener('load', () => {
// Safely manipulate DOM too
document.links;
});
</script>
window.onload is a JavaScript inbuilt function. window.onload trigger when the HTML page loaded. window.onload can be written only once.
document.ready is a function of the jQuery library. document.ready triggers when HTML and all JavaScript code, CSS, and images which are included in the HTML file are completely loaded.
document.ready can be written multiple times according to requirements.
When you say $(document).ready(f), you tell script engine to do the following:
get the object document and push it, since it's not in local scope, it must do a hash table lookup to find where document is, fortunately document is globally bound so it is a single lookup.
find the object $ and select it, since it's not in local scope, it must do a hash table lookup, which may or may not have collisions.
find the object f in global scope, which is another hash table lookup, or push function object and initialize it.
call ready of selected object, which involves another hash table lookup into the selected object to find the method and invoke it.
done.
In the best case, this is 2 hash table lookups, but that's ignoring the heavy work done by jQuery, where $ is the kitchen sink of all possible inputs to jQuery, so another map is likely there to dispatch the query to correct handler.
Alternatively, you could do this:
window.onload = function() {...}
which will
find the object window in global scope, if the JavaScript is optimized, it will know that since window isn't changed, it has already the selected object, so nothing needs to be done.
function object is pushed on the operand stack.
check if onload is a property or not by doing a hash table lookup, since it is, it is called like a function.
In the best case, this costs a single hash table lookup, which is necessary because onload must be fetched.
Ideally, jQuery would compile their queries to strings that can be pasted to do what you wanted jQuery to do but without the runtime dispatching of jQuery. This way you have an option of either
do dynamic dispatch of jquery like we do today.
have jQuery compile your query to pure JavaScript string that can be passed to eval to do what you want.
copy the result of 2 directly into your code, and skip the cost of eval.
window.onload is provided by DOM api and it says " the load event fires when a given resource has loaded".
"The load event fires at the end of the document loading process. At this point, all of the objects in the document are in the DOM, and all the images, scripts, links and sub-frames have finished loading."
DOM onload
But in jQuery $(document).ready() will only run once the page Document Object Model (DOM) is ready for JavaScript code to execute. This does not include images, scripts, iframes etc. jquery ready event
So the jquery ready method will run earlier than the dom onload event.

Bind ready event on javascript dom element creation

i would like to know if we can bind a loaded or ready event on an item created by a script when the dom is loaded. I heard from live() but it's not something clickable, it's just an item which has to load.
Thanks for your help!
I guess your best shot is the load event there.
$('element').load(function(){
alert('loaded');
});
native
var elem = document.getElementById('element_id');
elem.onload = function(){
alert('loaded');
};
Another example for dynamic creation:
$('<img/>', {
src: '/images/myimage.png',
load: function(){
alert('image loaded');
}
}).appendTo(document.body);
If you want to be able to separate the pieces of code for creating the item and the load event handling you could try having your dynamically created element trigger a custom event on the window:
var myElement = $('<img/>', {
src: '/images/myimage.png'
}).appendTo(document.body);
$(window).trigger( {type: "myElementInit", myObject : myElement} );
With a pointer back to itself in the extra parameters, you could then have a separate handler set-up within a jQuery(document).ready to look for the "myElementInit" window event and grab the reference to the element out of the extra parameters:
jQuery.('window').bind( "myElementInit", function(event){
var theElement = event.myObject;
...
} );
You can use the delegated form of the .on() event, as documented here:
Delegated events have the advantage that they can process events from
descendant elements that are added to the document at a later time. By picking an element that is guaranteed to be present at the time the
delegated event handler is attached, you can use delegated events to
avoid the need to frequently attach and remove event handlers. This
element could be the container element of a view in a
Model-View-Controller design, for example, or document if the event
handler wants to monitor all bubbling events in the document. The
document element is available in the head of the document before
loading any other HTML, so it is safe to attach events there without
waiting for the document to be ready.

Categories

Resources