I'm trying to make a programme which flashes an image when the "a" key is pressed

<div class="planeta" id="planeta">
  <img src="planeta.jpg" style="width: 150px; height: 150px;">
  <div class="text-blocka">
    <h4>Kepler-186f</h4>
    <p>550 light years away</p>
  </div>
</div>

The first part of the code works and the image begins to flash, but i want it to cancel the flash when the key is released and this part is not working for me. Below is the javascript i have written so far. Thanks in advance!

function checkKeydowna(akey) {
    if (akey.keyCode == "65") {
      var img = document.getElementById('planeta');

var interval = window.setInterval(function(){
    if(img.style.visibility == 'hidden'){
        img.style.visibility = 'visible';
    } else {
      img.style.visibility == 'hidden'
      }
}, 50);
    }
}

window.addEventListener("keyup", checkKeyupa, false);

function checkKeyupa(abkey) {
    if (abkey.keyCode == "65") {
    delete window.setInterval();
    }
  }

Solution

You forgot addEventListener for checkKeydowna.
delete window.setInterval(); is meaningless. that doesn't clear interval. you should clear interval with window.clearInterval(intervalInstance). So if you want to clear an interval you should store that in a global varible when you set it. That is why I declare var interval = false; which also means interval didn't set.
Keydown event will be triggered multiple times, so you should check if interval is set or not to avoid reinitialize it.

var interval = false;
function checkKeydowna(akey) {
  if (akey.keyCode == "65") {
    var img = document.getElementById('planeta');

    if (!interval) {
      interval = window.setInterval(function() {
        if (img.style.visibility == 'hidden') {
          img.style.visibility = 'visible';
        } else {
          img.style.visibility = 'hidden'
        }
      }, 50);
    }
  }
}

document.body.addEventListener("keyup", checkKeyupa, false);
document.body.addEventListener("keydown", checkKeydowna, false);

function checkKeyupa(abkey) {
  if (abkey.keyCode == "65") {
    window.clearInterval(interval);
    interval = false;
    var img = document.getElementById('planeta');
    img.style.visibility = 'visible';
  }
}

<img style='visibility:visible' src="https://www.google.com.tr/images/branding/googlelogo/2x/googlelogo_color_120x44dp.png" id="planeta" />