You could keep track of the last pressed key, but I'm guessing you probably want to use keyup instead of keydown. The latter is fired continuously while keyup is only fired once. Or at least until the user presses the same key again.

The easiest way to use a flag variable, by default set it to false and when KEYUP is clicked - set it to true. And check also your flag variable in your else if condition. Here is the code:

let isKeyupPressed = false;

window.addEventListener('keydown', event => {

  if (event.key == "ArrowLeft"|| event.key == "a") {
    // xloc -= 10;
  } else if (event.key == "ArrowRight" || event.key == "d" ) {
    // xloc += 10;
  } else if (event.key == "ArrowDown" || event.key == "s" ) {
    // yloc += 20;
  } else if (!isKeyupPressed && (event.key == "ArrowUp" || event.key == "w")) {
    // yloc -= 50;
    isKeyupPressed = true;
    console.log('Arrow Up invoke? ', event.key); // it will log only once
  }
  
  console.log(event.key);
})

I'm not sure I understand what you want to do, but can something like this resolve your problem ?

let isFirst = true;
onEvent("screenGamemode3", "keydown", function(event) {
  if (event.key == "Left"|| event.key == "a") {
    xloc = xloc - 10;
  } else if (event.key == "Right" || event.key == "d" ) {
    xloc = xloc + 10;
  } else if (event.key == "Down" || event.key == "s" ) {
    yloc = yloc + 20;
  }
    else if ((event.key == "Up" || event.key == "w") && isFirst == true) {
    yloc = yloc - 50;

    }
});