Extract the text out of HTML string using JavaScript

Create an element, store the HTML in it, and get its textContent:

function extractContent(s) {
  var span = document.createElement('span');
  span.innerHTML = s;
  return span.textContent || span.innerText;
};
    
alert(extractContent("<p>Hello</p><a href='http://w3c.org'>W3C</a>"));

Here's a version that allows you to have spaces between nodes, although you'd probably want that for block-level elements only:

function extractContent(s, space) {
  var span= document.createElement('span');
  span.innerHTML= s;
  if(space) {
    var children= span.querySelectorAll('*');
    for(var i = 0 ; i < children.length ; i++) {
      if(children[i].textContent)
        children[i].textContent+= ' ';
      else
        children[i].innerText+= ' ';
    }
  }
  return [span.textContent || span.innerText].toString().replace(/ +/g,' ');
};
    
console.log(extractContent("<p>Hello</p><a href='http://w3c.org'>W3C</a>.  Nice to <em>see</em><strong><em>you!</em></strong>"));

console.log(extractContent("<p>Hello</p><a href='http://w3c.org'>W3C</a>.  Nice to <em>see</em><strong><em>you!</em></strong>",true));

One line (more precisely, one statement) version:

function extractContent(html) {
    return new DOMParser()
        .parseFromString(html, "text/html")
        .documentElement.textContent;
}

textContext is a very good technique for achieving desired results but sometimes we don't want to load DOM. So simple workaround will be following regular expression:

let htmlString = "<p>Hello</p><a href='http://w3c.org'>W3C</a>"
let plainText = htmlString.replace(/<[^>]+>/g, '');