How to get the pure text without HTML element using JavaScript?

I have the 1 button and some text in my HTML like the following:

function get_content(){
   // I don't know how to do in here!!!

<input type="button" onclick="get_content()" value="Get Content"/>
<p id='txt'>
<span class="A">I am</span>
<span class="B">working in </span>
<span class="C">ABC company.</span>

When the user clicks the button, the content in the <p id='txt'> will become the follow expected result:

<p id='txt'>
// All the HTML element within the <p> will be disappear
I am working in ABC company.

Can anyone help me how to write the JavaScript function?

Thank you.