classname toggle js

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! 

var menu = document.querySelector('.menu') // Using a class instead, see note below.
menu.classList.toggle('hidden-phone');

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

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

    if ( ~className.indexOf(' active ') ) {
        this.className = className.replace(' active ', ' ');
    } else {
        this.className += ' active';
    }              
}