document.getElementById('myButton').onclick = function() {

    this.classList.toggle('active');

}

/* var or const followed by its name */.getElementById(/* HTML element ID */);

/* var or const's name */.classList.toggle(/* Name of CSS class */);

// I hope this helps! 

   element.classList.toggle('class');

#include "SFML/System/Vector2.hpp"

​

// also use "using namespace std;" to save a lot of time and headaches.

document.getElementById('myButton').onclick = function() {

​

    var className = ' ' + myButton.className + ' ';

​

    if ( ~className.indexOf(' active ') ) {

        this.className = className.replace(' active ', ' ');

    } else {

        this.className += ' active';

    }              

}