两个对象：div1 和 div2，其中div1是div2的父元素，div2只能在div1的范围内拖拽
图中，红点是鼠标的位置，两个绿色箭头相减的结果就是disX，最后oEvent.clientX - disX 就是绿色箭头的部分，这个长度就是判断是否“出格”的依据，也就是这个短的绿色箭头范围应该在0 ~ div2.offsetWidth - div1.offsetWidth之间！
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>客户区可见范围限制拖拽</title> <style type="text/css"> * { padding: 0; margin: 0; } #div1 { width: 500px; height: 500px; background: orange; position: relative; left: 100px; top: 30px; } #div2 { width: 100px; height: 100px; background: black; position: absolute; border: 1px solid blue; } </style> </head> <body> <div> <div></div> </div> <script type="text/javascript"> var oDiv1 = document.getElementById('div1'); var oDiv2 = document.getElementById('div2'); function getStyle(obj, attr) { if (obj.currentStyle) { return obj.currentStyle[attr]; } else { return getComputedStyle(obj, null)[attr]; } } oDiv2.onmousedown = function(ev) { var oEvent = ev || event; // var disX = oEvent.clientX - oDiv2.offsetLeft; // var disY = oEvent.clientY - oDiv2.offsetTop; var disX = oEvent.clientX - parseInt(getStyle(oDiv2, 'left')); var disY = oEvent.clientY - parseInt(getStyle(oDiv2, 'top')); document.onmousemove = function(ev) { var oEvent = ev || event; var l = oEvent.clientX - disX; var t = oEvent.clientY - disY; if (l < 0) { l = 0; } else if (l > oDiv1.offsetWidth - /*parseInt(getStyle(oDiv2,'width'))*/oDiv2.offsetWidth) { l = oDiv1.offsetWidth - oDiv2.offsetWidth; } if (t < 0) { t = 0; } else if (t > oDiv1.offsetHeight - oDiv2.offsetHeight) { t = oDiv1.offsetHeight - oDiv2.offsetHeight; } oDiv2.style.left = l + 'px'; oDiv2.style.top = t + 'px'; }; document.onmouseup = function() { document.onmousemove = null;//如果不取消，鼠标弹起div依旧会随着鼠标移动 document.onmouseup = null; }; }; </script> </body> </html>
基于上述原理，我们来做一个自定义滚动条，通过拖拽滚动条的位置来控制另一个对象的大小，比如一幅图。
<!DOCTYPE html> <html> <head> <meta charset="utf-8"> <title>自定义滚动条</title> <style type="text/css"> #div1 { width: 600px; height: 20px; background: orange; position: relative; margin: 50px auto; } #div2 { width: 20px; height: 20px; background: green; position: absolute; } #div3 { width: 0; height: 0; margin: 20px auto; } #div3 img { width: 100%; height: 100%; } </style> </head> <body> <div> <div></div> </div> <div> <img src="https://timgsa.baidu.com/141128%2F201411281041075742.jpg"> </div> <script type="text/javascript"> var oDiv1 = document.getElementById('div1'); var oDiv2 = document.getElementById('div2'); var oDiv3 = document.getElementById('div3'); oDiv2.onmousedown = function(ev) { var oEvent = ev || event; var disX = oEvent.clientX - oDiv2.offsetLeft; document.onmousemove = function(ev) { var oEvent = ev || event; var l = oEvent.clientX - disX; if (l < 0) { l = 0; } else if (l > oDiv1.offsetWidth - oDiv2.offsetWidth) { l = oDiv1.offsetWidth - oDiv2.offsetWidth; } oDiv2.style.left = l + 'px';//l范围：[0,580] //document.title = l / 580; //范围：[0,1] var ratio = oDiv1.offsetWidth - oDiv2.offsetWidth; var scale = l / ratio; var w = 600 * scale; var h = 370 * scale; console.log(w); oDiv3.style.cssText = ';width:' + w + 'px;height:' + h +'px;'; }; document.onmouseup = function() { document.onmousemove = null; document.onmouseup = null; }; }; </script> </body> </html>