Adding event listeners with Javascript DOM - javascript

I'm having trouble adding eventListener to through javascript. Ok, I have a function that creates 4 anchor elements. I want to add an event to them onmouseover that calls a function to change their backkground color. Here's the code (look at the next to last line of createAnchor() to find the relevant code line.
function createAanchor(index) {
var a = document.createElement("a");
var text = getText(index);
var a = document.createElement("a");
var t = document.createTextNode(text);
a.href = getHref(index);
a.appendChild(t);
a.style.textAlign = "center";
a.style.fontSize = "1.2em";
a.style.color = "white";
a.style.fontFamily = "arial";
a.style.fontWeight = "bold";
a.style.textDecoration = "none";
a.style.lineHeight = "238px";
a.style.width = "238px";
a.style.margin = "5px";
a.style.background = eightColors(index);
a.style.position = "absolute";
a.addEventListener("onmouseover", changeColor());
return a;
}
function changeColor() {
alert("EVENT WORKING");
}
Ok here's the problem. When the function gets to a.addEventListener("onmouseover", changeColor()); the function changeColors() executes, but it does not execute later on onmouseover Why is this?

There is no such event onmouseover, the event is called mouseover.
You have to pass a function reference to addEventlistener. () calls the function, as you already noticed, so... don't call it.
This is how it should be:
a.addEventListener("mouseover", changeColor);
I recommend to read the excellent articles about event handling on quirksmode.org.

It's because you wrote changeColors() instead of just changeColors. The () tell JavaScript to call the function.
In other words, changeColors by itself is a reference to the function, while changeColors() refers to the function and then calls it. The result of the function call (the return value from the function) is what's ultimately passed to addEventListener().

Ok I think we need to understand when to use the prefix "on" with the event type. In IE 8 or less then IE8 we use attachEvent and detachEvent which are equivalent to addEventListener and removeEventListener. There are some differences which are not required for this question.
While using attachEvent the event type is prefixed with "on" but in addEventListener no prefix is used.
hence,
elem.attachEvent("onclick",listener); // <= IE8
elem.addEventListener("click",listener,[,useCapture]); // other browsers

I've needed to do something like that too. I have an infoWindow in my map and I need to handle a click event on paragraph in that infoWindow. So I did it like this:
google.maps.event.addListener(infoWindow, 'domready', function()
{
paragraph = document.getElementById("idOfThatParagraph");
paragraph.addEventListener("click", function()
{
//actions that I need to do
});
});
It works for me. So I hope it will help someone :)

Related

Why isn't the class added/removed? - Javascript [duplicate]

This question already has answers here:
javascript onclick happening on page load
(5 answers)
Closed 5 months ago.
I am trying to create a link, which looks and feels like an <a> tag item, but runs a function instead of using the href.
When I try to apply the onclick function to the link it immediately calls the function regardless of the fact that the link was never clicked. Any attempt to click the link thereafter fails.
What am I doing wrong?
HTML
<div id="parent">
Send
</div>
Javascript
startFunction();
function secondFunction(){
window.alert("Already called!?");
}
function startFunction() {
var sentNode = document.createElement('a');
sentNode.setAttribute('href', "#");
sentNode.setAttribute('onclick', secondFunction());
//sentNode.onclick = secondFunction();
sentNode.innerHTML = "Sent Items";
//Add new element to parent
var parentNode = document.getElementById('parent');
var childNode = document.getElementById('sendNode');
parentNode.insertBefore(sentNode, childNode);
}
JsFiddle
As you can see I tried two different ways of adding this onclick function, both of which have the same effect.
You want .onclick = secondFunction
NOT .onclick = secondFunction()
The latter calls (executes) secondFunction whereas the former passes a reference to the secondFunction to be called upon the onclick event
function start() {
var a = document.createElement("a");
a.setAttribute("href", "#");
a.onclick = secondFunction;
a.appendChild(document.createTextNode("click me"));
document.body.appendChild(a);
}
function secondFunction() {
window.alert("hello!");
}
start();
You could also use elem#addEventListener
a.addEventListener("click", secondFunction);
// OR
a.addEventListener("click", function(event) {
secondFunction();
event.preventDefault();
});

How is the parameter involved in order to let function one "know" that it shall react to what happens in function two?

The following code actually works, but I don't understand why. How come that when I pass the "event"-parameter to the function zaehle(), the function actually "knows" that it is supposed to react on what happens in the setup function?
I just can't see what connnects the zaehle() and the setup() function or how the parameter that I pass to zaehle() would be involved.
I hope I could make the question clear. If not I'll gladly try to explain it somehow else. It really bugs me and I feel like I can't go on studying until I get it.
<body>
<div id="eins">0</div>
<div id="zwei">0</div>
<div id="drei">0</div>
<div id="vier">0</div>
<div id="funf">0</div>
</body>
JS
var mouseoverZaehler = 0;
function zaehle(event) {
mouseoverZaehler++;
event.target.innerHTML = mouseoverZaehler;
}
function setup() {
document.getElementById("eins").addEventListener("mouseover", zaehle);
document.getElementById("zwei").addEventListener("mouseover", zaehle);
document.getElementById("drei").addEventListener("mouseover", zaehle);
document.getElementById("vier").addEventListener("mouseover", zaehle);
document.getElementById("funf").addEventListener("mouseover", zaehle);
}
window.addEventListener("load", setup);
Here is what happens step by step:
Page loads
setup function is called (because of window.addEventListener("load", setup))
Each element in setup function gets a mouseover event listener attached to it and when it fires zaehle function is called (because of document.getElementById("number").addEventListener("mouseover", zaehle))
You move your mouse over any of the elements
zaehle function gets called - mouseoverZaehler is incremented and innerHTML of the targeted element is set to the updated value of mouseoverZaehler
Check out addEventListener docs for further details.
The addEventListener calls in your setup function tell the browser that when a mouseover event occurs on the relevant element, it should call the function you're giving it (zaehle, in your case). It's the browser that passes the argument to zaehle, later, when calling it.
You could imagine addEventListener, conceptually, as putting that handler function on a list for the event on the element:
// VERY conceptual, leaves out a lot of details
function addEventListener(eventName, handler) {
this.events[eventName].handlers.push(handler);
}
...and then later, when the event occurs, the browser creates an event object and calls those handlers:
// Again, VERY conceptual, leaves out a lot of details
var event = /*...*/;
element.events[eventName].handlers.forEach(function(handler) {
handler.call(element, event);
});
Here's a working analogue of what's going on:
function FakeElement () {
this.events = Object.create(null);
}
FakeElement.prototype.addEventListener = function(eventName, handler) {
var eventEntry = this.events[eventName];
if (!eventEntry) {
eventEntry = this.events[eventName] = {
handlers: []
};
}
eventEntry.handlers.push(handler);
};
FakeElement.prototype.trigger = function(eventName) {
var event = {type: eventName}; // "Browser" creates the event
var eventEntry = this.events[eventName];
var handlers = eventEntry && eventEntry.handlers;
if (handlers) {
handlers.forEach(function(handler) {
handler.call(this, event); // "Browser" calls handler, passing
}); // the event into it
}
};
// Using it:
function zaehle(event) {
console.log("zaehle got event: " + event.type);
}
var e = new FakeElement();
e.addEventListener("mouseover", zaehle);
console.log("added handler for mouseover to element");
// Simulate the event occurring
var timer = setInterval(function() {
e.trigger("mouseover");
}, 500);
setTimeout(function() {
clearInterval(timer);
}, 3000);
You have registered your callback/function zaehle() for mouseover event. So when that event occurs for a specific div, browser calls the callback with event object which contains information about the event and the target i.e event occurred on which element.

Define the order of execution of functions after event in javascript

Just, before reading, I have read about this thread: Order of execution of functions bound to an event in Javascript but its not helping. Actually,
I have an anonymous function, define like that:
<input type="button" name="blablabla" value="Send" onclick="javascript:blablabla">
So, this function is on a button, use to validate forms. As you can see, It's an anonymous function, and I don't have any access on this code. This function start when I click on it. Okay, I have understood that
But, this function is not totally full, and I want to add my own, with her own logic of check. So I want my checks first, and then call the anonymous function. Here is my code:
function check() {
console.log("debut de check");
var participant = document.getElementById("new_participant_name");
var participant1 = document.getElementById("new_participant2_name");
var participant2 = document.getElementById("new_participant3_name");
participant = participant.value;
participant1 = participant1.value;
participant2 = participant2.value;
var trois_participants = (participant2) ? true : false;
if (!participant1 || !participant)
{
console.log("pas de participant1 ou participant, sert à rien de gérer la suite");
//if the script come here, I want to stop processing, and don't want to call the anonymous function.
return ;
}
}
window.onload = function()
{
document.getElementById("InsertButton").addEventListener('click', function () {
check();
})};
So, I want to call my function (check) before the anonymous function, but, with the same event. I don't know if I am totally understable... thanks per avance
EDIT: Sorry guys, My code have a bug before, yes the code is inlined, I will try all of your solutions tomorrow, thanks guys
If (and only if) the existing handler is attached using an inline onclick="..." handler, you can obtain its value, and then overwrite it:
window.onload = function() {
var el = document.getElementById('InsertButton');
var old_click = el.onclick;
el.onclick = undefined;
el.addEventListener('click', function() {
check();
old_click(this);
});
}
Why not create your own handler??
Element.prototype.myEventListener=function(name,func){
this.addEventListener(name,function(){
if(!check()){return;}
func();
});
};
Now you can do:
document.body.myEventListener("click",function(){
alert("t");
});
Check will always be called before the registered handler.
Note, to block the call, check must return false:
function check(){
return false;//no custom eventlistener fires
return true;//all will fire
}
Use the useCapture flag so you can intercept the event while it's travelling down to the button.
At that point you can perform your check, and if it fails you can call stopPropagation on the event to prevent it from reaching the handlers that are attached to its bubbling phase.
Also, by nature, events are quite bad at managing the order of execution. In general they depend on the order of registration of the listeners.
// code over which you have no control and can't change
var btn = document.getElementById("greeter");
btn.addEventListener("click", function() {
console.log("hello");
})
// code you can add later
function check() {
return Math.random() > 0.5;
}
window.addEventListener("click", function(e) {
var greeter = document.getElementById("greeter");
if (e.target === greeter && !check()) {
e.stopPropagation();
}
}, true)
<button id="greeter">hello world</button>

Calling JavaScript functions from HTML

How do I execute a JS object's function property from an HTML link?
I have the following JS:
function Tester(elem) {
this.elem = document.getElementById(elem);
}
Tester.prototype.show = function() {
this.elem.innerHTML = 'test';
};
Tester.prototype.test = function() {
alert("a");
};
​
Here is the HTML:
<script type="text/javascript">
var test = new Tester("test");
test.show();
</script>
When I click on the link that gets rendered, it cannot identify the test() function. How would I get it so when a user clicks on the link, the test() function is executed?
The proper way would be to create a DOM element and attach the event handler with JavaScript:
Tester.prototype.show = function() {
var a = document.createElement('a'),
self = this; // assign this to a variable we can access in the
// event handler
a.href = '#';
a.innerHTML = 'test';
a.onclick = function() {
self.test();
return false; // to prevent the browser following the link
};
this.elem.appendChild(a);
};
Since the event handler forms a closure, it has access to the variables defined in the outer function (Tester.prototype.show). Note that inside the event handler, this does not refer to your instance, but to the element the handler is bound to (in this case a). MDN has a good description of this.
quirksmode.org has some great articles about event handling, the various ways you can bind event handlers, their advantages and disadvantages, differences in browsers and how this behaves in event handlers.
It's also certainly helpful to make yourself familiar with the DOM interface.

Problem using loop values in function

Beginners Javascript question here.
I am trying to create a function that finds all the links in a given div and sets up an onclick event for each one. I can get the link hrefs correctly, but when I try using them in the onclick function, Javascript seems to only use the last value found:
I.E
I have these links
#purpose
#future
#faq
When I use the onclick function, every link is reported as the #faq link.
Here's the code:
function prepareLinks () {
var nav = document.getElementById('navigation');
var links = nav.getElementsByTagName ('a');
for (var i = 0; i<links.length; i++) {
var linkRef = links[i].getAttribute('href').split("#")[1];
links[i].onclick = function () {
var popUp = "You clicked the " +linkRef +" link";
alert (popUp);
}
}
}
Here you have a closure creation. External variable linkRef becomes saved in inner onclick function. Try this way:
clickFunction() {
var popUp = "You clicked the " + this.href.split("#")[1] +" link";
// this should mean current clicked element
alert (popUp);
}
for (var i = 0; i<links.length; i++) {
links[i].onclick = clickFunction;
}
This is a scoping problem. The expression "You clicked the " +linkRef +" link" is evaluated when the onclick event fires, but what is the value of linkRef at this point?
You're trying to attach a separate onClick handler to each link, but you're accidentally attaching the same one.
You could generate a new function each time by using the new Function() constructor as
links[i].onclick = new Function("","alert('You clicked the '"+linkRef+"' link');");
See http://tadhg.com/wp/2006/12/12/using-new-function-in-javascript/ for more details.
But it's generally better to see if you can have a single handler. It's interesting that when you get to an event handler, the "this" keyword refers to the generator of the event. So you could have your original code refer to this.getAttribute("href"). Too many handlers will make your javascript event processing slow.

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