Current index of a button array

Question

How do I return the index of only the button that is clicked?

Here's the code that got me worried:

window.onload = function(){

var    description = document.getElementsByClassName('description'),
        buttons = document.getElementsByClassName('button');

var currD = 0; // this var stands for the current description that should be shown

var show = function(){

    for( var i = 0; i < description.length; i++ ){

        if( i !== currD ){
            description[i].style.display='none';
        }

        else if( i === currD ){
            description[i].style.display='block';   
        }

    }

};

  for (var i = 0; i < buttons.length; i++){
    buttons[i].addEventListener('click', function(){
        currD = i;
        console.log(i);
    });
}

    window.setInterval(show,300);
};

Every time I click the button the for loop return the last element.

Since I didn't have many buttons on this page I went for the unproficient old way which is:

window.onload = function(){

var description = document.getElementsByClassName('description'),
    buttons = document.getElementsByClassName('button');

    var currD = 0; // this var stands for the current description that should be shown

    var show = function(){

        for( var i = 0; i < description.length; i++ ){

            if( i !== currD ){
                description[i].style.display='none';
            }

            else if( i === currD ){
                description[i].style.display='block';   
            }

        }

    };

buttons[0].addEventListener('click', function(){
    currD = 0;
});

buttons[1].addEventListener('click', function(){
    currD = 1;
});

buttons[2].addEventListener('click', function(){
    currD = 2;
});

        window.setInterval(show,300);
    };

This works but if I want to add more buttons it'd be a loss of time setting all the event listeners.

Possible duplicate of JavaScript closure inside loops – simple practical example — Teemu
– Teemu, Commented Jan 19, 2016 at 10:29

lucian.nicolaescu · Accepted Answer · 2016-01-19 10:54:26Z

1

This is a common CLOSURE issue. This should work:

for (var i = 0; i < buttons.length; i++){

    (function (_i) {
        buttons[_i].addEventListener('click', function(){
          currD = _i;
          console.log(_i);
        });
    })(i);

}

Another solution:

for (var i = 0; i < buttons.length; i++){

    buttons[i].addEventListener('click', function(){
        return function() {
            currD = i;
            console.log(i);
        }
    });
}

You can read more about closure even here: http://www.w3schools.com/js/js_function_closures.asp but there are a lot of other interesting articles. Just google javascript closure

For your specific example, maybe even something like this can work:

for (var i = 0; i < buttons.length; i++){
    buttons[i].addEventListener('click', function(e){
        currD = buttons.indexOf( e.currentTarget );
        console.log(currD);
    });
}

edited Jan 19, 2016 at 10:54

answered Jan 19, 2016 at 10:34

lucian.nicolaescu

6473 silver badges9 bronze badges

Sign up to request clarification or add additional context in comments.

9 Comments

Teemu Over a year ago

The first solution is awful, the second solution doesn't work : (.

lucian.nicolaescu Over a year ago

Lol. Those are the solutions for closure. Why a function wrapper is awful? Anyway, can you create a jsFiddle so we can show it working?

Teemu Over a year ago

In the first solution you're creating a new function on every round, that works though, but is considered a bad practice. In the second snippet i inside of the handler function is the event object, not the looping variable.

lucian.nicolaescu Over a year ago

Also, the second solution does the same thing as the first one. Yes, that's the idea of the closure, you need to create a function for each element. Otherwise you will not have the i variable!. You need to set different functions for each button with the corresponding "i".

Teemu Over a year ago

Actually the second solution doesn't work at all.

|

Collectives™ on Stack Overflow

Current index of a button array

1 Answer 1

9 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

9 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related