Pass this to addEventListener() as parameter

12,332

Solution 1

Inside the forEach callback, this does not refer to a DOM element. It doesn't refer to any useful value.

Secondly, you are immediately calling cbChange and pass its return value to addEventListener, which is undefined. But addEventListener expects to be passed a function, so you either have to pass cbChange or a function that calls cbChange.

Lastly, while you could define the event handler to accept the element as first argument, it's much simpler if it accepts the event object, because that is the default API.

Having said all that, the simplest solution would be:

var checkboxes = document.getElementsByClassName('cb');
Array.from(checkboxes).forEach(function(element){
//                                      ^^^^^^^
  element.addEventListener("change", cbChange, false);
//^^^^^^^  
});

function cbChange(){
  console.log(this.value);
//            ^^^^
}

Since inside an event handler, this refers to the element the handler is bound to, using this inside cbChange just works.


And here are some alternatives:

// Use the event object
var checkboxes = document.getElementsByClassName('cb');
Array.from(checkboxes).forEach(function(element){
  element.addEventListener("change", cbChange, false);
});

function cbChange(event){
  console.log(event.target.value);
}


// Using a wrapper that calls `cbChange`
var checkboxes = document.getElementsByClassName('cb');
Array.from(checkboxes).forEach(function(element){
  element.addEventListener("change", function() { cbChange(this); }, false);
});

function cbChange(ele){
  console.log(ele.value);
}

Solution 2

you don't need to pass this to event handler. you can access event.target inside event handler. you can do something like this:

var checkboxes = document.getElementsByClassName('cb');
Array.from(checkboxes).forEach(function(){
  this.addEventListener("change", cbChange, false);
})

function cbChange(event){
  var ele = event.target;
  console.log(ele.value);
}
<input class="cb" type="checkbox" name="candidate" value="1"/>
<input class="cb" type="checkbox" name="candidate" value="2"/>
<input class="cb" type="checkbox" name="candidate" value="3"/>
<input class="cb" type="checkbox" name="candidate" value="4"/>

Solution 3

Remove (this) from cbChange(this). This will immediately execute the function.

To get the value you need the target.target is the element on which event is executed.On consoling ele you will see all available options

var checkboxes = document.getElementsByClassName('cb');
Array.from(checkboxes).forEach(function() {
  this.addEventListener("change", cbChange, false);
});

function cbChange(ele) {
  console.log(ele.target.value);
}
<input class="cb" type="checkbox" name="candidate" value="1" />
<input class="cb" type="checkbox" name="candidate" value="2" />
<input class="cb" type="checkbox" name="candidate" value="3" />
<input class="cb" type="checkbox" name="candidate" value="4" />

Solution 4

From my opinion, i thinks you can use for..of loop to get the value of each checkbox.

var checkboxes = document.getElementsByClassName('cb');
for (let checkboxe of checkboxes ) {
  checkboxe.addEventListener('change', () => {
      console.log(checkboxe.value)
  })
}
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Hassan Imam
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Hassan Imam

Full Stack Developer with 6+ years of experience. Proficient in NodeJs, ReactJs, Javascript, MySQL, MongoDB. Graduated from GGSIPU in 2014 in Computer Science and Engineering. Linkedin - Hassan Imam

Updated on June 09, 2022

Comments

  • Hassan Imam
    Hassan Imam over 1 year

    I want to add change event to a group of checkboxes, how can I access this in my event function, so that when I do the event I can access value of the checkbox.

    This is my current code:

    var checkboxes = document.getElementsByClassName('cb');
    Array.from(checkboxes).forEach(function(){
      this.addEventListener("change", cbChange(this), false);
    });
    
    function cbChange(ele){
      console.log(ele.value);
    }
    <input class="cb" type="checkbox" name="candidate" value="1"/>
    <input class="cb" type="checkbox" name="candidate" value="2"/>
    <input class="cb" type="checkbox" name="candidate" value="3"/>
    <input class="cb" type="checkbox" name="candidate" value="4"/>