1.

  1. <!DOCTYPE html>
  2. <html lang="en">
  3. <head>
  4.     <meta charset="UTF-8">
  5.     <meta name="viewport" content="width=device-width, initial-scale=1.0">
  6.     <title>Document</title>
  7. </head>
  8. <body> 
  9.     <button id="btn">btn</button>
  10.     <div style="display:block;">
  11.         div
  12.     </div>
  13.     <script>
  14.         var div = document.getElementsByTagName("div")[0];
  15.         var btn = document.getElementById("btn");
  16.         btn.onclick = function(){
  17.             var d = div.style.display;
  18.             div.style.display = (d=="block")?"none":"block";
  19.         }
  20.     </script>
  21. </body>
  22. </html>

2.

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body> 
    <button id="btn">btn</button>
    <div style="display:block;">
        div
    </div>
    <script>
        var div = document.getElementsByTagName("div")[0];
        var btn = document.getElementById("btn");
        btn.onclick = function(){
            var d = div.style.display;
            if(d=="block"){
                div.style.display = "none"
            }else{
                div.style.display = "block"
            }
        }
    </script>
</body>
</html>