【案例】使用上下左右键控制元素的移动

<!DOCTYPE html>

<html lang="en">

<head>

        <meta charset="UTF-8">

        <title>使用键盘上下左右键控制小球移动</title>

        <style>

                 *{

                         margin:0;

                         padding:0;

                 }

                 #ball{

                         width: 200px;

                         height: 200px;

                         background: pink;

                         border-radius: 50%;

                         position: absolute;

                 }

        </style>

</head>

<body>

        <div id="ball"></div>

</body>

<script>

        //获取元素

        var ball = document.getElementById('ball');

        //定义小球每次移动的步径

        var step = 1;

        //定义小球水平、垂直方向移动定时器标志

        var runX,runY;

        //为使小球运行更流畅,加入水平移动定时器

        function moveX(step){

                 console.log(runX);

                 if(runX){

                         return;

                 }

                 runX = setInterval(function(){

                         var newLeft = ball.offsetLeft + step;

                         var clientWidth = document.documentElement.clientWidth || document.body.clientWidth;

                         if(newLeft <= 0){

                                  newLeft = 0;

                         }

                         if(newLeft >= clientWidth - ball.offsetWidth){

                                  newLeft = clientWidth - ball.offsetWidth;

                         }

                         ball.style.left = newLeft + 'px';

                 },5);

        }

        //为使小球运行更流畅,加入垂直移动定时器

        function moveY(step){

                 if(runY){

                         return;

                 }

                 runY = setInterval(function(){

                         var newTop = ball.offsetTop + step;

                         var clientHeight = document.documentElement.clientHeight || document.body.clientHeight;

                         if(newTop <= 0){

                                  newTop = 0;

                         }

                         if(newTop >= clientHeight - ball.offsetHeight){

                                  newTop = clientHeight - ball.offsetHeight;

                         }

                         ball.style.top = newTop + 'px';

                 },5);

        }

        //键盘按键按下时小球移动

        window.onkeydown = function(e){

                 //兼容性写法

                 var e = e || window.event;

                 //使用switch结构判断键盘按下的是哪个键

                 switch(e.keyCode){

                         case 37:

                                  moveX(-step);

                         break;

                         case 38:

                                  moveY(-step);

                         break;

                         case 39:

                                  moveX(step);

                         break;

                         case 40:

                                  moveY(step);

                         break;

                 }

        }

        //键盘按键抬起时,小球停止移动

        window.onkeyup = function(){

                 //清除定时器

                 clearInterval(runX);

                 runX = undefined;

                 clearInterval(runY);

                 runY = undefined;

        }      

</script>

</html>

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