You need to pass the click event then get the target element which in this case is the clicked element.

function myFunction(event) {
  if (event.target.classList.contains("my-class")) {
    alert("I Do things becuase i have (my-class)")
  }
}

<button class="my-class" onclick="myFunction(event)">Click me</button>
<br/>
<br/>
<br/>
<br/>
<button onclick="myFunction(event)">I Do nothing</button>

As @collapsar mentioned in comment, element is't set. I recommand you to use addEventListener and event.target.

document.getElementById("your-element").addEventListener("click", () =>{
  if (event.target.classList.contains("my-class")) {
    console.log("your-element has \"my-class\" class")
  }
})

<div id="your-element" class="my-class">Click</div>

When the HTML element rendered statically you should consider two things:

1. Wait for the DOM ready event in order to make modifications on the element.
2. Attach the event dynamically, making sure that you bind the event handler to new elements after adding them to the DOM.

HTML

<div class="my-class" />

Javascript

function myFunction(event) {
  var element = event.target;
  if (element.classList.contains("my-class")) {
    //do something
  }
}

document.addEventListener("DOMContentLoaded", function(event) { 
  // DOM is ready
  const elements = document.getElementsByClassName("my-class");
  for (let i = 0; i < elements.length; i++) {
    elements[i].addEventListener('click', myFunction);
  }
});