SOURCE

function checkLen(x,y)
{
if (y.length==x.maxLength)
	{
	var next=x.tabIndex
	if (next<document.getElementById("myForm").length)
		{
		document.getElementById("myForm").elements[next].focus()
		}
	}
}

<p>这段脚本在达到文本框的最大长度时跳到下一个文本框：</p> 
 <form id="myForm"> 
  <input size="3" tabindex="1" maxlength="3" onkeyup="checkLen(this,this.value)"> 
  <input size="2" tabindex="2" maxlength="2" onkeyup="checkLen(this,this.value)"> 
  <input size="3" tabindex="3" maxlength="3" onkeyup="checkLen(this,this.value)"> 
 </form>