JSFiddle - React, Tailwind, and code Playground

HTML

<div id="platform">

</div>

CSS

#platform {
  width: 10px;
  height: 10px;
  position: absolute;
  left: 400px;
  top: 400px;
  background-color: green;
}

JavaScript

const platform = document.getElementById("platform");
let isPlatformMoving = false;
let platformMovingDirection;

document.addEventListener("keydown", ()=>{
    const keyCode = event.keyCode;
    if (keyCode === 37) {
        console.log(platformMovingDirection);
        if(platformMovingDirection === "left"){
            return;
        } else if(platformMovingDirection === "right") {
            clearInterval(isPlatformMoving);
        }
        platformMovingDirection = "left";
        isPlatformMoving = setInterval(movePlatform, 0);

        return;
    } else if (keyCode === 39) {
        console.log(platformMovingDirection);
        if(platformMovingDirection === "right"){
            return;
        } else if(platformMovingDirection === "left") {
            clearInterval(isPlatformMoving);
        }
        platformMovingDirection = "right";
        isPlatformMoving = setInterval(movePlatform, 0);

        return;
    }
});
document.addEventListener("keyup", ()=> {
     const keyCode = event.keyCode;
    if (isPlatformMoving && ((keyCode === 39 && platformMovingDirection === "right") || (keyCode === 37 && platformMovingDirection === "left"))) {
        clearInterval(isPlatformMoving);
        isPlatformMoving = false;
        platformMovingDirection = undefined;
    }
});
function movePlatform() {
    const position = parseFloat(getComputedStyle(platform).getPropertyValue("left"));
    if(platformMovingDirection === "left") {
        platform.style.left = ( position - 1 ) + "px";
    } else if(platformMovingDirection === "right") {
        platform.style.left = ( position + 1 ) + "px";
    }
}