Here is an example of how you can create a variable that contains a DOM element using HTML and JavaScript querySelector.

<p class="demo">Hello World</p>

<script>
    var foo = document.querySelector('.demo');
    console.log(foo)
</script>

Navigating to the web page containing this markup should return something like this.

textContent can be used to return the content of the DOM element.

<p class="demo">Hello World</p>

<script>
    var foo = document.querySelector('.demo');

    console.log(foo)

    var textContent = foo.textContent
    console.log(textContent)
</script>

Which should return something like this.

And innerHTML replace can be used to replace values in the DOM element.

<p class="demo">Hello World</p>

<script>
    var foo = document.querySelector('.demo');
    foo.innerHTML = foo.textContent.replace("Hello", "Goodbye")
    console.log(foo)
</script>

And now "Hello World" is "Goodbye World".