onfocus 获取焦点
    onblur 遗失焦点

    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.     <input id="app" type="text">
    10.     <script>
    11.         /* 
    12.         onfocus  获取焦点
    13.         onblur   遗失焦点
    14.          */
    15.        var app = document.getElementById("app");
    16.        app.onfocus = function(){
    17.            this.style.backgroundColor = "red"
    18.        }
    19.        app.onblur =  function(){
    20.            this.style.backgroundColor = "yellow"
    21.        }
    22.     </script>
    23. </body>
    24. </html>

    1.PNG