How to add event listener to element created dynamically within prototype?

Question

HTML

<div id="main"></div>

JS

var TestClass = function() {};
TestClass.prototype = {
    addElement: function(parentId) {
    var parent = document.querySelector('#' + parentId);
    var a = document.createElement('a');
    a.textContent = '+';
    a.href = '#';
    a.addEventListener('click', this.onElementClick, false); //nothing happens on click
    parent.innerHTML = a.outerHTML;
  },

  onElementClick: function() {
    console.log('dynamic element clicked');
  }
};

new TestClass().addElement('main'); // on DOM ready

Output

<div id="main"><a href="#">+</a></div>

Answer 1

Like this:

var TestClass = function() {};
TestClass.prototype = {
    addElement: function(parentId) {
    var parent = document.getElementById(parentId); //Changed to getElementById
    var a = document.createElement('a');
    a.textContent = '+';
    a.href = '#';
    a.addEventListener('click', this.onElementClick, false);
    parent.append(a); //Changed
  },

  onElementClick: function() {
    console.log('dynamic element clicked');
  }
};

new TestClass().addElement('main'); // on DOM ready

<div id="main"></div>

You did not add your a to parent, instead you just added its HTML.