let p1 = document.getElementById("p1");;
p1.style.left="100px";
let speed = 1;

function moveText(){
    if (parseInt(p1.style.left) < 0 || parseInt(p1.style.left) > (window.innerWidth - p1.offsetWidth)){
        speed = -speed;
    }
    p1.style.left = (parseInt(p1.style.left) + speed) + "px";
}

window.setInterval(moveText,10);