Start to Use the JavaScript

As a very useful feature, it can change the html / css
 

<p id="target">Greetings</p>
    ...
const el = document.getElementById("target");
el.style.color = "#777";
el.style.fontSize = "13px";

Let's create a function
 

function sum(x, y){
    return x + y;
}

console.log(sum(1, 2));