how to assign html code in the javascript variable?

If you only have one element with class on the page, you can do this:

var Code = document.body.querySelector(".menu");

If they are more, you should specify an index, with querySelectorAll :

var Code = document.body.querySelectorAll(".menu")[1];

The where [1] represents the second element of the page with class .menu .

The above examples will select the element as an object. If you want to get HTML, add .outerHTML :

var Code = document.body.querySelector(".menu").outerHTML;

or

var Code = document.body.querySelectorAll(".menu")[1].outerHTML;

If you want to get the element's internal HTML:

var Code = document.body.querySelector(".menu").innerHTML;

or

var Code = document.body.querySelectorAll(".menu")[1].innerHTML;

  

Remembering that the [1] index is an example that refers to the second element with the   class .menu , when there are more than 1 element with the same class.

If you want to get an html code and assign it to a variable call it by an id, class, tag name or something like this:

var code = document.getElementsByClassName('menu')[0]

console.log(code)

<div class="menu">...</div>