I dont understand javascript syntax well,my question:
How jquery define click function for get parameter inside anonymous function?
The Case:
$("a").click(function(event) {
alert(event.type);
});
in C the function should be defined:
void click(fn,event){
}
in javascript its looks to me that she defined as- (but where defined event?):
click (fn){
}
please explain to me the jquery syntax of click function code source here.
Thanks,
Yosef
If you just want to find out where the event object is passed to your handler, that would be line 2568 of the jQuery-1.5.2 redistributable source code (or line 438 of the actual, un-contatenated source file):
var ret = handleObj.handler.apply( this, args );
In the above line of code, handler is your anonymous function and args is an array whose first element is the event object. jQuery uses the apply method of the JavaScript Function object to invoke the handler and pass in the arguments
The jQuery source code is quite complex when it comes to full sequence of adding and handling events so, unless you want a line-by-line explanation of hundreds of lines of code, I suggest you rephrase your question to a smaller scope (e.g. You could create a toy demonstration of the scenario you want to understand).
Perhaps this will help?
dosomething(function(message) {
alert(message);
});
function dosomething(fn) {
fn("Hello!");
}
The first part of the jQuery is the selector $("a") which selects and returns object(s) selected from the DOM. In this case, it will return a list of all anchor tag objects on the page.
Then, you are chaining the .click() method to that, jQuery attaches an event listener to all of the anchor tags. When the event listener is attached, it is more or less the equivalent of doing
<a href='..' onclick='someFunction(event)'>some link</a>
...which passes the event object to the function.
For example, compare to this:
<a onclick='blah(event)'>click</a>
<script type='text/javascript'>
function blah(e) {
alert (e.type);
}
</script>
If I click on it, I will see "click" in the alert. In principle, jQuery is doing the same thing.
Related
If I have multiple functions passed to a click event i.e.
#click="
inputHandler();
sendToken(computedUser.email);
responseMessage();
"
The function with an event parameter:
inputHandler(e) {
// code
}
Won't run. If I pass it on it's own:
#click="inputHandler"
It works fine.
Why is this and how can I get around it?
Internally Vue uses some RegExps to decide what form of event handler you're using.
If it seems to be the name of a method it will call it and pass the event.
If it seems to be inline code it'll just run it. In this case the event object is accessible as $event.
So:
#click="inputHandler($event)"
is roughly equivalent to:
#click="inputHandler"
Strictly speaking these aren't quite the equivalent, for example with component events that emit multiple arguments you'll only get the first one using $event like this.
See https://v2.vuejs.org/v2/guide/events.html#Methods-in-Inline-Handlers
For a deeper understanding see the Vue source code:
https://github.com/vuejs/vue/blob/0baa129d4cad44cf1847b0eaf07e95d4c71ab494/src/compiler/codegen/events.js#L96
Give your eyes a few minutes to adjust and it isn't too difficult to understand.
Personally I try to avoid anything more complicated than a single method call in the inline listener. Instead I'd suggest having something like #click="onSendClick" and let the method onSendClick worry about the details.
If I recall correctly, vue creates a wrapper function, if the passed value isn't a function. So
inputHandler();
sendToken(computedUser.email);
responseMessage();
actually get's turned into
function wrapper(){
inputHandler();
sendToken(computedUser.email);
responseMessage();
}
And as you can see the arguments passed to wrapper are lost.
The easiest way to fix this is probably to create a new method that accepts the event parameter and calls all of your function and use that one in the event handler.
I am new to JavaScript/jQuery and I've been learning how to make functions. A lot of functions have cropped up with (e) in brackets. Let me show you what I mean:
$(this).click(function(e) {
// does something
});
It always appears that the function doesn't even use the value of (e), so why is it there so often?
e is the short var reference for event object which will be passed to event handlers.
The event object essentially has lot of interesting methods and properties that can be used in the event handlers.
In the example you have posted is a click handler which is a MouseEvent
$(<element selector>).click(function(e) {
// does something
alert(e.type); //will return you click
}
DEMO - Mouse Events DEMO uses e.which and e.type
Some useful references:
http://api.jquery.com/category/events/
http://www.quirksmode.org/js/events_properties.html
http://www.javascriptkit.com/jsref/event.shtml
http://www.quirksmode.org/dom/events/index.html
http://www.w3.org/TR/DOM-Level-3-Events/#event-types-list
DISCLAIMER: This is a very late response to this particular post but as I've been reading through various responses to this question, it struck me that most of the answers use terminology that can only be understood by experienced coders. This answer is an attempt to address the original question with a novice audience in mind.
Intro
The little '(e)' thing is actually part of broader scope of something in Javascript called an event handling function. Every event handling function receives an event object. For the purpose of this discussion, think of an object as a "thing" that holds a bunch of properties (variables) and methods (functions), much like objects in other languages. The handle, the 'e' inside the little (e) thing, is like a variable that allows you to interact with the object (and I use the term variable VERY loosely).
Consider the following jQuery examples:
$("#someLink").on("click", function(e){ // My preferred method
e.preventDefault();
});
$("#someLink").click(function(e){ // Some use this method too
e.preventDefault();
});
Explanation
"#someLink" is your element selector (which HTML tag will trigger this).
"click" is an event (when the selected element is clicked).
"function(e)" is the event handling function (on event, object is created).
"e" is the object handler (object is made accessible).
"preventDefault()" is a method (function) provided by the object.
What's happening?
When a user clicks on the element with the id "#someLink" (probably an anchor tag), call an anonymous function, "function(e)", and assign the resulting object to a handler, "e". Now take that handler and call one of its methods, "e.preventDefault()", which should prevent the browser from performing the default action for that element.
Note: The handle can pretty much be named anything you want (i.e. 'function(billybob)'). The 'e' stands for 'event', which seems to be pretty standard for this type of function.
Although 'e.preventDefault()' is probably the most common use of the event handler, the object itself contains many properties and methods that can be accessed via the event handler.
Some really good information on this topic can be found at jQuery's learning site, http://learn.jquery.com. Pay special attention to the Using jQuery Core and Events sections.
e doesn't have any special meaning. It's just a convention to use e as function parameter name when the parameter is event.
It can be
$(this).click(function(loremipsumdolorsitamet) {
// does something
}
as well.
In that example, e is just a parameter for that function, but it's the event object that gets passed in through it.
The e argument is short for the event object. For example, you might want to create code for anchors that cancels the default action. To do this you would write something like:
$('a').click(function(e) {
e.preventDefault();
}
This means when an <a> tag is clicked, prevent the default action of the click event.
While you may see it often, it's not something you have to use within the function even though you have specified it as an argument.
In jQuery e short for event, the current event object. It's usually passed as a parameter for the event function to be fired.
Demo: jQuery Events
In the demo I used e
$("img").on("click dblclick mouseover mouseout",function(e){
$("h1").html("Event: " + e.type);
});
I may as well have used event
$("img").on("click dblclick mouseover mouseout",function(event){
$("h1").html("Event: " + event.type);
});
Same thing!
Programmers are lazy we use a lot of shorthand, partly it decreases our work, partly is helps with readability. Understanding that will help you understand the mentality of writing code.
Today I just wrote a post about "Why do we use the letters like “e” in e.preventDefault()?" and I think my answer will make some sense...
At first,let us see the syntax of addEventListener
Normally it will be:
target.addEventListener(type, listener[, useCapture]);
And the definition of the parameters of addEventlistener are:
type ：A string representing the event type to listen out for.
listener ：The object which receives a notification (an object that implements the Event interface) when an event of the specified type
occurs. This must be an object implementing the EventListener
interface, or a JavaScript function.
(From MDN)
But I think there is one thing should be remarked:
When you use Javascript function as the listener, the object that implements the Event interface(object event) will be automatically assigned to the "first parameter" of the function.So,if you use function(e) ,the object will be assigned to "e" because "e" is the only parameter of the function(definitly the first one !),then you can use e.preventDefault to prevent something....
let us try the example as below:
<p>Please click on the checkbox control.</p>
<form>
<label for="id-checkbox">Checkbox</label>
<input type="checkbox" id="id-checkbox"/>
</div>
</form>
<script>
document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
//var e=3;
var v=5;
var t=e+v;
console.log(t);
e.preventDefault();
}, false);
</script>
the result will be : [object MouseEvent]5 and you will prevent the click event.
but if you remove the comment sign like :
<script>
document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
var e=3;
var v=5;
var t=e+v;
console.log(t);
e.preventDefault();
}, false);
</script>
you will get : 8 and an error:"Uncaught TypeError: e.preventDefault is not a function
at HTMLInputElement. (VM409:69)".
Certainly,the click event will not be prevented this time.Because the "e" was defined again in the function.
However,if you change the code to:
<script>
document.querySelector("#id-checkbox").addEventListener("click", function(e,v){
var e=3;
var v=5;
var t=e+v;
console.log(t);
event.preventDefault();
}, false);
</script>
every thing will work propertly again...you will get 8 and the click event be prevented...
Therefore, "e" is just a parameter of your function and you need an "e" in you function() to receive the "event object" then perform e.preventDefault(). This is also the reason why you can change the "e" to any words that is not reserved by js.
It's a reference to the current event object
this will be my first stackoverflow help but I am confident that my answer will help anyone reading this.
Basically, e is just an object containing information about the EVENT which has just occured.
if it is 'click', then the object will contain about the click,
if it is 'submit', then the object will contain about the submit,
and they are typically found in addEventListener.
clickMe.addEventListener('click', e => {
console.log(e)
}
meaning, whenever I 'click' the button, it will console.log the INFOMRATION about the event that happened which is I did is to 'click' it, this will print information about the click event.
e is very useful because you can access and use the event to your very own project such as printing the location of x value... like
clickMe.addEventListener('click', e => {
console.log(e.clientX)
}
then it will print the location where you 'click' that event.. mine it returns 32
if you prefer video, please watch this
https://www.youtube.com/watch?v=_BVkOvpyRI0
video is not mine
upvoting me will truly help me since I am a student and looking for opportunity to help here. Love lots!
$(this).click(function(e) {
// does something
});
In reference to the above code
$(this) is the element which as some variable.
click is the event that needs to be performed.
the parameter e is automatically passed from js to your function which holds the value of $(this) value and can be used further in your code to do some operation.
I am a beginner in javascript. and have no experience in programming, at all.
So I'd like you to be generous to beginner.
And here is my question.
I'm trying to code javascript unobtrusively.
So I put in all of my js codes into external js file. for example : test.js
and deleted these codes. to do unobtrusive js coding. for example :
and I tried to use these 2 methods :
variable.onclick=test(arg1, arg2);
variable.addEventListener('click',test(arg1, arg2),true);
but these triggers didn't work.
to put it delicately, function test(arg1, arg2) worked right after dom loding finished. regardless of activating 'click' trigger.
So I spent several hours solving this problem, and finally got a solution. this is it.
variable.onclick = function(){
variable.addEventListener('click',test('arg1','arg2'),true);
}
I wanna know why first two methods didn't work, and why that solution works well.
I solved the problem, but don't know why, and how...
In JavaScript, when you reference a function by name and follow that reference by a parenthesized list of arguments, that means that you want to call the function, right then and there. Thus a statement like
variable.onclick=test(arg1, arg2);
will assign to the "onclick" property the value obtained by calling the "test" function. In other words that statement means
Please call the function "test" passing it "arg1" and "arg2", and assign whatever it returns to the "onclick" property of the object referenced by "variable".
An event handler must be a function, however, and your "test" handler probably returns either nothing, or something that's not a function. So it didn't work.
Your solution, however, is also incorrect. You're successfully assigning a function to the handler property, but your function is itself installing another event handler. There's no reason to do that here, and in general setting up event handlers from within other event handlers is a suspicious practice. All you need is:
variable.onclick = function() { test(arg1, arg2); };
variable.onclick requires a function declaration by design. In your case you could have just done
variable.onclick = function(){
test(arg1,arg2);
};
The way you did it won't work because you're not giving the click handler any instructions. The corrections I have made say that when the variable (the one with the click handler attached) is clicked trigger this function that will in turn trigger the test function.
Same thing goes for the second one
variable.addEventListener('click', function(){
test(arg1,arg2);
});
This works again because you are saying when this variable is clicked run the function that will trigger the test function.
Basically you are trying to assign the result of running a function, the test function as a task for the click handler to run. This won't work except maybe your test function returns a function that contains code that you want to run when the click event is triggered. Hope this helps.
I have following code that works -
$(function(){
$('#testbutton').click(function(){
var checkedValue = $('[name="someRadioGroup"]:radio:checked').val();
$('#result').html('The radio element with value <tt>' + checkedValue + '</tt> is checked');
});
});
I understand that $('#testbutton') fetches the HTML element with id testbutton and assigns an event handler to it.
But I don't understand why I need to put that code inside a function literal?
If I remove the $(function(){ (and the corresponding }); ) it does not work.
However other code can work without being wrapped in a function literal. Example -
alert('Hi there')
What is the difference? Why does alert work but the event assignment statement does not?
$(function(){...}); is shorthand for $(document).ready(function(){...});. The purpose of it is to not run your code until the elements that you intend to work with exist in the DOM (which generally is after the document is ready.)
It is not a requirement.
You are putting your code in an event handler that is invoked when the DOM is ready.
If you don't put your code in a DOM ready handler, and if your code performs DOM selection, you need to find some other way to to make sure the DOM is ready before it runs.
If your code doesn't perform DOM selection, then it's not needed. That's why your alert() works, because it doesn't need to fetch any elements.
An alternative is this.
<body>
<button id="testbutton">click</button>
<div id="result"></div>
<script type="text/javascript" src="/your/script.js"></script>
</body>
This places your code below all the elements on the page, so they are certain to be available when your code runs.
Another alternative is to use a window.onload handler.
window.onload = function() {
// your code
};
or using jQuery.
$(window).on("load", function() {
// your code
});
This is similar to the DOM ready handler, except that it waits for all resource, like images, to be loaded.
At the most basic level, something of the form (function(){...})() is a function literal that is executed immediately. What this means is that you have defined a function and you are calling it immediately.
This form is useful for information hiding and encapsulation since anything you define inside that function remains local to that function and inaccessible from the outside world (unless you specifically expose it - usually via a returned object literal).
A variation of this basic form is what you see in jQuery plugins (or in this module pattern in general). Since you are defining a function literal, it doesn't have access to anything from the outside world unless you explicitly pass in the information. Hence:
(function($) {
...
})(jQuery);
Which means you're passing in a reference to the actual jQuery object, but it's known as $ within the scope of the function literal.
I have something like the following..
$(document).ready(function() {
$('#doReport').click(doReport);
});
function doReport(type) {
if (type === undefined) {
type = 'blah';
}
alert (type);
}
If I run doReport() from the console or standalone in the javascript with nothing in it, it will return 'blah' (as expected), and obviously if I call doReport('wibble'); it returns 'wibble' as you would expect.
But if I run it by clicking the element with ID doReport (utilising the bind I set up in .ready) it returns [object Object]
I don't understand why that would be the case.
The jQuery library passes your event handlers an "event" object. It will always be there. It's a "wrapped" or "fixed" version of the native browser object, making it somewhat easier to deal with.
Here is the documentation for such objects.
Also of note is the fact that jQuery will invoke your handler functions such that this refers to the DOM element for which the handler is being invoked.
Also also, as #Ericson578 points out in a good comment, jQuery allows additional parameters to be set up, which means that your handler may be passed additional parameters. That might be useful if you've got a single event handler function to be bound to different elements, but you'd like to qualify its behavior with some different flags or whatever based on the particulars of an element.
Event handlers receive an event object as a parameter.
This is because event handlers are triggered with an object (specifically, the event object) passed as the first argument.
This is the reason you see such syntax as
$('#doReport').click(function(e) {
If you want to call your function without any parameters, you'll need to create a wrapping function to do so:
$(document).ready(function() {
$('#doReport').click(function() {
doReport();
});
});
When jQuery calls the function passed as parameter to click, it passed event object as the argument hence you are getting the alert as [object Object].
Check this:
http://api.jquery.com/click/
From JQuery - .click()
.click( handler(eventObject) )
handler(eventObject)A function to execute each time the event is triggered.
Your doReport() function is getting an event object.
wrap it with another function if you need to pass an argument to your function.
$(document).ready(function() {
$('#doReport').click(function(event){
doReport('blah');
});
});