Executing JavaScript's addEventListener() function without specifying the event that triggers it

Question

Executing JavaScript's addEventListener() function without specifying the event that triggers it

I am currently working on creating a grid of boxes that can be clicked, sending notifications with their respective IDs to the server using SocketIO.

<body>
<div class="wrapper">
    <div id="1" class="box"></div>
    <div id="2" class="box"></div>
</div>
</body>

let boxes = document.querySelectorAll('.box');

Array.from(boxes, function(box) {
    box.addEventListener('click', function()  {
        id = box.id

        //Send socketio message to server
        socket.emit('box_event_client', {data: id});
    });
});

Although this method works and allows me to click on each box and send events to the server, I encountered an issue where the event listeners could not be removed. I found out from https://www.w3schools.com/jsref/met_element_removeeventlistener.asp

Note: To remove event handlers, the function specified with the addEventListener() method must be an external function, Anonymous functions, like "element.removeEventListener("event", function(){ myScript });" will not work.

To address this problem, I made some changes:

Array.from(boxes, function(box) {
    box.addEventListener('click', addEL(box.id));
});

function addEL(boxID) {
    console.log("Box clicked: " + boxID)

    //Send socketio message to server
    socket.emit('box', {data: boxID});
}

However, after loading the page into the browser, all boxes in the grid are automatically 'clicked' and events are sent to the server. Can someone provide insight into why this is happening?

javascript addeventlistener

Answer 1

Answer №1

Could someone please assist me in understanding why this issue is occurring?

Array.from(boxes, function(box) {
    box.addEventListener('click', addEL(box.id));
});

addEL(box.id) is a function call, which means it takes the id and executes it on each box's click event listener.

Solution:

AddEventListener requires the event name and the callback function name or the function itself (inline function).

Simply provide the function name as the parameter.

Array.from(boxes, function(box) {
    box.addEventListener('click', addEL);
});

You can access the id of the box from the event object like event.currentTarget.id

function addEL(event) {
    let boxId = event.currentTarget.id;
    console.log("Box clicked: " + boxId);
    //Send socketio message to server
    socket.emit('box', {data: boxId});
}

Answer 2

Could someone please assist me in understanding why this issue is occurring?

Array.from(boxes, function(box) {
    box.addEventListener('click', addEL(box.id));
});

addEL(box.id) is a function call, which means it takes the id and executes it on each box's click event listener.

Solution:

AddEventListener requires the event name and the callback function name or the function itself (inline function).

Simply provide the function name as the parameter.

Array.from(boxes, function(box) {
    box.addEventListener('click', addEL);
});

You can access the id of the box from the event object like event.currentTarget.id

function addEL(event) {
    let boxId = event.currentTarget.id;
    console.log("Box clicked: " + boxId);
    //Send socketio message to server
    socket.emit('box', {data: boxId});
}

Answer 3

Answer №2

When the code

box.addEventListener('click', addEL(box.id));

is executed, it invokes addEL(box.id) and passes the result of that invocation as the second argument to addEventListener. This is why addEL is called directly without waiting for any event.

If you wish to remove the listener later on, here is how you should write the code:

Array.from(boxes, function(box) {
    let clickCallback = function()  {
        id = box.id

        // Send a socket.io message to the server
        socket.emit('box_event_client', {data: id});
    }

    box.addEventListener('click', clickCallback);

    // You can remove the listener within the same scope where clickCallback is defined
    // box.removeEventListener('click', clickCallback);
});

Answer 4

When the code

box.addEventListener('click', addEL(box.id));

is executed, it invokes addEL(box.id) and passes the result of that invocation as the second argument to addEventListener. This is why addEL is called directly without waiting for any event.

If you wish to remove the listener later on, here is how you should write the code:

Array.from(boxes, function(box) {
    let clickCallback = function()  {
        id = box.id

        // Send a socket.io message to the server
        socket.emit('box_event_client', {data: id});
    }

    box.addEventListener('click', clickCallback);

    // You can remove the listener within the same scope where clickCallback is defined
    // box.removeEventListener('click', clickCallback);
});

Answer 5

Answer №3

modify

box.addEventListener('click', useEventListener(box.id));

to

box.addEventListener('click', function() { useEventListener(box.id) });

or make it simpler (utilize ES6)

box.addEventListener('click', () => useEventListener(box.id));

or

box.addEventListener('click', useEventListener.bind(null, box.id));

this change is necessary because you are using the result of the function, not the function itself

UPD: to be able to remove the handler, declare the function as

var eventHandler = useEventListener.bind(null, box.id)

and use

box.addEventListener('click', eventHandler);

you can also have an array of such functions like

eventHandlers.push(useEventListener.bind(null, box.id))

,

box.addEventListener('click', eventHandlers[i]);

Answer 6

modify

box.addEventListener('click', useEventListener(box.id));

to

box.addEventListener('click', function() { useEventListener(box.id) });

or make it simpler (utilize ES6)

box.addEventListener('click', () => useEventListener(box.id));

or

box.addEventListener('click', useEventListener.bind(null, box.id));

this change is necessary because you are using the result of the function, not the function itself

UPD: to be able to remove the handler, declare the function as

var eventHandler = useEventListener.bind(null, box.id)

and use

box.addEventListener('click', eventHandler);

you can also have an array of such functions like

eventHandlers.push(useEventListener.bind(null, box.id))

,

box.addEventListener('click', eventHandlers[i]);

Executing JavaScript's addEventListener() function without specifying the event that triggers it

Answer №1

Answer №2

Answer №3

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