Call an object as a function in JavaScript - javascript

I'm trying to execute JavaScript functions that are called when a event (for example onClick event) is performed on a web page with JavaScript code. I'm getting the function from the event like this :
var attributval = document.getElementsByTagName("a").getAttribute('onClick');
and I'm trying to execute this object (which a JavaScript function in fact) as a function (suppose we have <a onClick = alert('whatever');> on this example, I tried:
var attributval = document.getElementsByTagName("a").getAttribute('onClick');
attributval() = function(){attributval};
attributval();
but it didn't work.

A DOM attribute is not the same as a JavaScript property (even though they can have the same name onclick). You should use
var attributval = document.getElementsByTagName("a")[0].onclick;
to retrieve a function (or null) from the JS object (as opposed to getAttribute(), which will most likely return a toString() for the property).
Now, attributval() = is illegal syntax, as attributval() is not an l-value (you cannot assign to it).
attributval(); will work but without the second line (which is illegal JavaScript) it will invoke the original A element onclick handler (if one is defined) or throw an exception (if the onclick handler is null).

Skip trying to create a function around the function. Just call it:
var attributval = document.getElementsByTagName("a")[0].onclick;
attributval();

try
var attributval = document.getElementsByTagName("a")[0].getAttribute('onClick');

By using get attribute you are returning a string so your only way is to use eval(onclickString) or var fn = new Function(onClickString); fn();

attributval is simply a string, correct? If you trust this code, execute it with eval(attributval) -- however any reference to this won't work.
What you probably want is to manually trigger an event. jQuery makes that easy.

If you want to do more than a click, then Chris McDonald's answer at Is it possible to trigger a link's (or any element's) click event through JavaScript? seems to fit the bill, although you might need to heed the third comment.

I thought I'd add a short answer on how to work with events using jQuery, since it seems relevant.
// Select the link using it's ID field (assuming it has one)
var myLink = $('a#myLink')
// Add a click event to the link
myLink.on('click', function(e) {
console.log("I've been clicked!");
});
// Trigger the click event manually. This would result in the above
// function being run. Interestingly, this will not cause the browser
// to follow the link like a real click would
myLink.trigger('click');
// Remove the click event (this removes ALL click events)
myLink.off('click');
// Add a click event to the link that only runs once, then removes itself
myLink.one('click', function() {
alert("I'll only bother you once!");
});
// Add a click event that you can identify from other click events.
// This means that you can trigger it or remove it without bothering other
// click events
myLink.on('click.myClick', function() {
alert("This click event has been identified as 'myClick'");
});
// Now you can trigger it without triggering other click events
myLink.trigger('click.myClick');
// And remove it, also with no harm coming to other click events
myLink.off('click.myClick');
Hope this helps

Related

What does e/event property means in JS

<script>
var ohnoesEl = document.getElementById("ohnoes");
var onOhNoesClick = function(e) {
e.preventDefault();
var audioEl = document.createElement("audio");
audioEl.src = "https://www.kasandbox.org/programming-sounds/rpg/giant-no.mp3";
audioEl.autoplay = "true";
document.body.appendChild(audioEl);
};
ohnoesEl.addEventListener("click", onOhNoesClick);
</script>
In this code, I didn't understand one thing. I checked internet and StackOverflow but couldn't find anything.
I have a problem to understand event property.
Why do we put e as an argument before we use properties such as preventDefault?
How will I realize whether I should use it or not?
I have a problem to understand event property.
Well, it's not a property. All event handling functions are automatically passed a reference to the event object that represents the event currently being handled. This object can tell you quite a bit about the circumstances at the time of the event (i.e. which mouse button was clicked, what key was pressed, where on the screen the mouse was when the click happened, what object triggered the event, etc.).
Why do we put e as an argument before we use properties such as
preventDefault?
The syntax of e.preventDefault() is simply common Object-Oriented Programming syntax of: Object.method(). We are accessing the Event object that was passed into the function with the e identifier and then invoking the preventDefault method stored within that object.
It's how you get at some object-specific behavior. .preventDefault() is not a global function, you can't just call it on its own. It's only something that an event object can do, so you have to reference the object before calling the method.
As with all function arguments, you may call the argument any valid name you like, but since the object will be an event object, e, evt, and event are quite common.
How will I realize whether I should use it or not?
In your code: e.preventDefault(), indicates that the event that was triggered should not perform its built-in action, effectively cancelling the event.
You would use this technique in situations where the user has initiated some event, but your code determines that the process should not continue. The best example is with a form's submit event. If the user hasn't filled out all the required fields and then hits the submit button, we don't want the form to be submitted, so we check to see if the required fields were filled in and, if not, we cancel the submit event.
Here's an example:
// Get a reference to the link:
var link = document.getElementById("nasaLink");
// Set up a click event callback function that will automatically
// be passed a reference to the click event when it occurs. In this
// example, the event will be received as "evt".
link.addEventListener("click", function(evt){
console.clear(); // Cancel previous log entries
// Get the type of event that was received and the object that triggered it
console.log("You triggered a " + evt.type + " on :", evt.target)
// Cancelling an event is generally based on some condition
// Here, we'll make it simple and say that if you click on the
// link when the second is an even second, the navigation will be cancelled
if(new Date().getSeconds() % 2 === 0){
// Normally, clicking a valid hyperlink will navigate you away from the current page
// But, we'll cancel that native behavior by cancelling the event:
evt.preventDefault();
console.log(evt.type + " cancelled! No navigation will occur.");
}
console.log("The mouse was postioned at: " + evt.screenX + " x " + evt.screenY);
console.log("The SHIFT key was pressed at the time? " + evt.shiftKey);
console.log("\tTry clicking again, but with SHIFT held down this time.");
});
Click for NASA
The event property is an object that is passed to every event handler.
This event object then has many properties and methods you can call to manipulate the event process and action in the handler.
For instance, in the event object you have this method called preventDefault() . What does preventDefault() do? Each event is triggered by a particular html dom element in the page. Sometimes this html elements have behaviour attached to them. For instance, and <a> element has the potential of changing the browser url for a particular window. If the element that triggered the event is then an <a>, with preventDefault() you just cut the default behaviour for that <a> anchor and that will avoid an url load/change.
I recommend you find a reference for this event object and pay a read to it. So you'll become more familiar to what it is available within it.

jQuery persist all event listeners on element for future setting

Using jQuery I need to:
persists list of all event handlers that are added to element,
remove them all for few seconds and
return things to initial state (reassign the same event handlers)
I found that get list of current listeners with (some jQuery inner mechanisms):
var eventsSubmitBtn = $._data(submitButton[0], "events");
Then I can remove all event listeners with
submitButton.off();
But last stem seems not to be working
setTimeout(function () {
$._data(submitButton[0], "events", eventsSubmitBtn);
}, 5000);
eventsSubmitBtn is an empty array.
Is this the way this should be done with initial setting and I'm need something like deep cloning for those objects or this can't be done with $._data?
N.B. I have possibility to add my cistom code after all other system js code, thus I can't place the code assigning to $.fn.on before anything. Code that I write will run the last on startup and other event listeners are attached before my scripts will run.
As you get a reference to the object returned by $._data(), any change to that object will not go unnoticed, i.e. after you invoke .off(), that object will have changed to reflect that there are no handlers attached any more.
You could solve this by taking a shallow copy of the object, (e.g. with Object.assign).
But this is not really a recommended way to proceed. According to a jQuery blog, "jQuery._data(element, "events") ... is an internal data structure that is undocumented and should not be modified.". As you are modifying it when restoring the handlers, this cannot be regarded best practice. But even only reading it should only be used for debugging, not production code.
It would be more prudent to put a condition in your event handling code:
var ignoreEventsFor = $(); // empty list
$("#button").on('click', function () {
if (ignoreEventsFor.is(this)) return;
// ...
});
Then, at the time it is needed, set ignoreEventsFor to the element(s) you want to ignore events for. And when you want to revert back to normal, set it to $() again.
Now adding this to all your event handlers may become a burden. If you stick to using on() for attaching event handlers, then you could instead extend $.fn.on so it will add this logic to the handlers you pass to it.
The following demo has a button which will respond to a click by changing the background color. With a checkbox you can disable this from happening:
/* Place this part immediately after jQuery is loaded, but before any
other library is included
*/
var ignoreEventsFor = $(), // empty list
originalOn = $.fn.on;
$.fn.on = function (...args) {
var f = args[args.length-1];
if (typeof f === 'function') {
args[args.length-1] = function (...args2) {
if (ignoreEventsFor.is(this)) return;
f.call(this, ...args2);
};
}
originalOn.call(this, ...args);
}
/* This next part belongs to the demo, and can be placed anywhere */
$(function () {
$("#colorButton").on('click', function () {
// Just some handler that changes the background
var random = ('00' + (Math.random() * 16*16*16).toString(16)).substr(-3);
$('body').css({ backgroundColor: "#" + random });
});
$("#toggler").on('change', function () {
// Toggle the further handling of events for the color button:
ignoreEventsFor = $(this).is(':checked') ? $("#colorButton") : $();
});
});
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<button id="colorButton">Change color</button><br>
<input type="checkbox" id="toggler">Disable events
Notice: the above code uses ES6 spread/rest syntax: if you need support for IE then that would have to be written using the arguments variable, apply, ...etc.

How can I use onclick method?

I made object of window by javascript. And I want to add some functions to the windows object. But .onclick, .setattribute, and .addEventListener do not work with my object. Even mouse cursor also can not be changed when I give value of css or javascript. It is on 499 line of my source. Could you tell me why these do not work? Should I change language? Is it bug?
function seSizeValue(theNumOfWins,outlayerId){
var seSizeValue;
var seSizeValueId;
seSizeValueId = "seSizeValueId" + theNumOfWins;
seSizeValue = document.createElement("div");
document.getElementById(outlayerId).appendChild(seSizeValue);
seSizeValue.setAttribute("id",seSizeValueId);
document.getElementById(seSizeValueId).setAttribute("class","seSizeValueCSS");
document.getElementById(seSizeValueId).onclick = function(event) {seSizeChange();};
return seSizeValueId;
}
"""document.getElementById(seSizeValueId).onclick = function(event) {seSizeChange();};"""
This line does not work. I already tried to change '.setattribute', and '.addEventListener'.
There was no error. But it does not work.
https://drive.google.com/file/d/0B4p8lZSEMXcqN0FzQTkyVW8wRGc/view
this is my full source.
It does not work on my source.
Should I use jQuery?
I think so Save the window reference in a variable with a sentence like that
var new_window = window.open
is posible ser manually events in DOM subelements using jquery with On() method o directly with javascript.
Im not sure about that but I suggest a posible solution.
There is no function defined as "seSizeChange" so when the onclick event fires, it's going to run a function that doesn't exist.
Also, when you call this "seSizeValue" function, are you calling it after the specified element has been loaded on to the document?
If not, the code won't be able to find the specified element.
document.getElementById(seSizeValueId).onclick = function(event) {seSizeChange();};
is definitely wrong.
See http://jsfiddle.net/2txugfq5/
Everything is working except fact that 'seSizeChange' is not defined.
John makes good points.
See here http://jsfiddle.net/hqw7ocwj/2/ , works fine. No changes to anything of importance from your code, apart from:
$(document).ready(function() {});
and:
function seSizeChange() {
alert("seSizeChange method fired.");
}
Main points:
1. Ensure your JavaScript is called only after the DOM is ready: $(document).ready(function{});
2. Make sure that the method in the function that the onclick refers to has been defined.

inline javascript onclick event

This is my html code
Hit
This is my javascript file
function clickHandler(evt) {
var thisLink = (evt)?evt.target:Window.event.srcElement;
alert(thisLink.innerHTML);
return false;
}
But when i click the Hit Link, it redirects.
you need to pass in the event if you wish to preventDefault.
html:
Hit
script:
function runFunction (evt) {
evt.preventDefault();
evt.stopPropagation();
}
To tie both of the very-correct answers together, what's happened is you've inlined a function where you've written onclick="return runFunction();"
If you look at that, what it's really doing is going like this:
var link = document.getElementById("myLink");
link.onclick = function () { runFunction(); };
See the problem?
My runFunction is being called without any event object passed in, at all.
...which means that var thisLink = (evt) ? is going to return false, which means that it's going to try to run in oldIE-mode.
By writing onclick="runFunction", that's the same as saying:
link.onclick = runFunction;
Which means that when the onclick event happens, runFunction will be called, and in W3C-compliant browsers, it will be sent an event object.
Which is why that solution works.
The best way to avoid a lot of this confusion is to deal with JavaScript from inside of JavaScript, and to deal with HTML inside of HTML, so that you don't have to worry about how strings translate into code.
Now, to get all of this to work, AND prevent redirection, you want to do this:
for W3C browsers (the ones that pass the event parameter):
function runFunction (evt) {
// stops the default-action from happening
// means you need to find another way to fire it, if you want to later
evt.preventDefault();
// stops higher-up elements from hearing about the event
// like if you stop a submit button from "clicking", that doesn't stop the form
// from submitting
evt.stopPropagation();
//the oldIE versions of both of these are
event.cancelBubble = true;
event.returnValue = false;
}
When I plugged your code into chrome, I got this as the error in the console:
Uncaught TypeError: Cannot read property 'srcElement' of undefined
IF the javascript bombs out while processing, it never gets a chance to return at all so the browser tends to disregard what is in the onclick handler after the exception.
Since it bombed out... default behavior of anchor tags, which is to send you off to wherever the href says to go.
Try wrapping the contents of the function in a try/catch block and see what turns up if this kind of thing plagues you.

Listening and firing events with Javascript and maybe jQuery

In my JavaScript and Flex applications, users often perform actions that I want other JavaScript code on the page to listen for. For example, if someone adds a friend. I want my JavaScript app to then call something like triggerEvent("addedFriend", name);. Then any other code that was listening for the "addedFriend" event will get called along with the name.
Is there a built-in JavaScript mechanism for handling events? I'm ok with using jQuery for this too and I know jQuery makes extensive use of events. But with jQuery, it seems that its event mechanism is all based around elements. As I understand, you have to tie a custom event to an element. I guess I can do that to a dummy element, but my need has nothing to do with DOM elements on a webpage.
Should I just implement this event mechanism myself?
You have a few options:
jQuery does allow you to do this with objects not associated with the document. An example is provided below.
If you're not already using jQuery on your page, then adding it is probably overkill. There are other libraries designed for this. The pattern you are referring to is called PubSub or Publish/Subscribe.
Implement it yourself, as you've suggested, since this is not difficult if you're looking only for basic functionality.
jQuery example:
var a = {};
jQuery(a).bind("change", function () {
alert("I changed!");
});
jQuery(a).trigger("change");
I would implement such using MVVM pattern with knockjs library.
Just create an element, and use jquery events on it.
It can be just a global variable, doesn't even have to be connected to the DOM.
That way you accomplish your task easily and without any extra libs.
Isn't it possible to bind onchange events in addition to click events? For instance, if addFriend is called and modifies a list on the page, you could bind the change event to then invoke additional functionality.
$('#addFriendButton').click( function() {
// modify the #friendList list
});
$('#friendList').change( function() {
myOtherAction();
});
This is total Host independent, no need for jQuery or dom in this case!
function CustomEvents(){
//object holding eventhandlers
this.handlers_ = {};
}
//check if the event type does not exist, create it.
//then push new callback in array.
CustomEvents.prototype.addEventListner = function (type, callBack){
if (!this.handlers_[type]) this.handlers_[type] = [];
this.handlers_[type].push(callBack);
}
CustomEvents.prototype.triggerEvent = function (type){
//trigger all handlers attached to events
if (!this.handlers_[type]) return;
for (var i=0, handler; handler = this.handlers_[type][i]; i++)
{
//call handler function and supply all the original arguments of this function
//minus the first argument which is the type of the event itself
if (typeof handler === "function") handler.apply(this,arguments.slice(1));
}
}
//delete all handlers to an event
CustomEvents.prototype.purgeEventType = function(type){
return delete this.handlers_[type];
}
test:
var customEvents = new CustomEvents();
customEvents.addEventListner("event A", function(arg){alert('Event A with arguments' + arg);));
customEvents.triggerEvent("event A", "the args");
EDIT added arguments passing

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