how to give class name to an html element only when clicking one button code example

Example 1: toggle css class in javascript

/* 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!

Example 2: toggle class javascript

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