I want to make an eventHandler that passes the event and some parameters. The problem is that the function doesn't get the element. Here is an example:
doClick = function(func){
var elem = .. // the element where it is all about
elem.onclick = function(e){
func(e, elem);
}
}
doClick(function(e, element){
// do stuff with element and the event
});
The 'elem' must be defined outside of anonymous function. How can i get the passed element to use within the anonymous function? Is there a way to do this?
And what about addEventListener? I don't seem to be able to pass the event through an addEventListener at all do I ?
Update
I seemed to fix the problem with 'this'
doClick = function(func){
var that = this;
this.element.onclick = function(e){
func(e, that);
}
}
Where this contains this.element that i can access in the function.
The addEventListener
But i'm wondering about the addEventListener:
function doClick(elem, func){
element.addEventListener('click', func(event, elem), false);
}
I don't understand exactly what your code is trying to do, but you can make variables available in any event handler using the advantages of function closures:
function addClickHandler(elem, arg1, arg2) {
elem.addEventListener('click', function(e) {
// in the event handler function here, you can directly refer
// to arg1 and arg2 from the parent function arguments
}, false);
}
Depending upon your exact coding situation, you can pretty much always make some sort of closure preserve access to the variables for you.
From your comments, if what you're trying to accomplish is this:
element.addEventListener('click', func(event, this.elements[i]))
Then, you could do this with a self executing function (IIFE) that captures the arguments you want in a closure as it executes and returns the actual event handler function:
element.addEventListener('click', (function(passedInElement) {
return function(e) {func(e, passedInElement); };
}) (this.elements[i]), false);
For more info on how an IIFE works, see these other references:
Javascript wrapping code inside anonymous function
Immediately-Invoked Function Expression (IIFE) In JavaScript - Passing jQuery
What are good use cases for JavaScript self executing anonymous functions?
This last version is perhaps easier to see what it's doing like this:
// return our event handler while capturing an argument in the closure
function handleEvent(passedInElement) {
return function(e) {
func(e, passedInElement);
};
}
element.addEventListener('click', handleEvent(this.elements[i]));
It is also possible to use .bind()
to add arguments to a callback. Any arguments you pass to .bind()
will be prepended to the arguments that the callback itself will have. So, you could do this:
elem.addEventListener('click', function(a1, a2, e) {
// inside the event handler, you have access to both your arguments
// and the event object that the event handler passes
}.bind(elem, arg1, arg2));
element.addEventListener('click', func(event, this.elements[i]))
. Javascript just doesn't work that way. There are other ways around that, using closures, but you can't do it the way you wrote. addEventListener calls it's callback with exactly one argument (the event) and you can't change that in any way - jfriend00 2012-04-03 20:39
function eventFunction (arg, evt) {
console.log(arg[0],arg[1],arg[2])
console.log(evt)
}
var el = document.getElementById('elementID');
el.addEventListener('click', eventFunction.bind(el,[1,2,3]))
Note that the event argument is always last in the arguments for the function, and is not explicitly declared in the bind method - Knight Yoshi 2015-01-05 13:27
Something you can try is using the bind method, I think this achieves what you were asking for. If nothing else, it's still very useful.
function doClick(elem, func) {
var diffElem = document.getElementById('some_element'); //could be the same or different element than the element in the doClick argument
diffElem.addEventListener('click', func.bind(diffElem, elem))
}
function clickEvent(elem, evt) {
console.log(this);
console.log(elem);
// 'this' and elem can be the same thing if the first parameter
// of the bind method is the element the event is being attached to from the argument passed to doClick
console.log(evt);
}
var elem = document.getElementById('elem_to_do_stuff_with');
doClick(elem, clickEvent);
It is an old question but a common one. So let me add this one here.
With ES2015 syntax you can achieve it more succinct way:
function event_handler(event, arg) {
console.log(event, arg);
}
element.addEventListener('click', (event) => event_handler(event, 'Here comes the argument'));
var e = (event) => event_handler(event, 'Here comes the argument');
element.addEventListener('click', e);
element.removeEventListener('click', e);
SnnSnn 2018-04-27 10:48
Given the update to the original question, it seems like there is trouble with the context ("this") while passing event handlers. The basics are explained e.g. here http://www.w3schools.com/js/js_function_invocation.asp
A simple working version of your example could read
var doClick = function(event, additionalParameter){
// do stuff with event and this being the triggering event and caller
}
element.addEventListener('click', function(event)
{
var additionalParameter = ...;
doClick.call(this, event, additionalParameter );
}, false);
this
inside of doThings
is the window object. Try this instead:
var doThings = function (element) {
var eventHandler = function(ev, func){
if (element[ev] == undefined) {
return;
}
element[ev] = function(e){
func(e, element);
}
};
return {
eventHandler: eventHandler
};
};
Short answer:
x.addEventListener("click", function(e){myfunction(e, param1, param2)});
...
function myfunction(e, param1, param1) {
...
}
let obj = MyObject();
elem.someEvent( function(){ obj.func(param) } );
//calls the MyObject.func, passing the param.