Many existing answers will check for caps lock on when shift is not pressed but will not check for it if you press shift and get lowercase, or will check for that but will not also check for caps lock being off, or will check for that but will consider non-alpha keys as 'off'. Here is an adapted jQuery solution that will show a warning if an alpha key is pressed with caps (shift or no shift), will turn off the warning if an alpha key is pressed without caps, but will not turn the warning off or on when numbers or other keys are pressed.

$("#password").keypress(function(e) { 
    var s = String.fromCharCode( e.which );
    if ((s.toUpperCase() === s && s.toLowerCase() !== s && !e.shiftKey)|| //caps is on
      (s.toUpperCase() !== s && s.toLowerCase() === s && e.shiftKey)) {
        $("#CapsWarn").show();
    } else if ((s.toLowerCase() === s && s.toUpperCase() !== s && !e.shiftKey)||
      (s.toLowerCase() !== s && s.toUpperCase() === s && e.shiftKey)) { //caps is off
        $("#CapsWarn").hide();
    } //else upper and lower are both same (i.e. not alpha key - so do not hide message if already on but do not turn on if alpha keys not hit yet)
  });

我知道这是一个老话题，但我想我会反馈，以防它能帮助到其他人。这个问题的答案在IE8中似乎都不能用。然而，我确实发现这段代码在IE8中工作。(还没有测试过IE8以下的任何版本)。如果需要，这可以很容易地为jQuery修改。

function capsCheck(e,obj){ 
    kc = e.keyCode?e.keyCode:e.which;  
    sk = e.shiftKey?e.shiftKey:((kc == 16)?true:false);  
    if(((kc >= 65 && kc <= 90) && !sk)||((kc >= 97 && kc <= 122) && sk)){
        document.getElementById('#'+obj.id).style.visibility = 'visible';
    } 
    else document.getElementById('#'+obj.id).style.visibility = 'hidden';
}

函数通过onkeypress事件调用，如下所示:

<input type="password" name="txtPassword" onkeypress="capsCheck(event,this);" />
<div id="capsWarningDiv" style="visibility:hidden">Caps Lock is on.</div> 

你可以试试。增加了一个工作示例。当焦点在输入上时，打开caps lock使led变成红色或绿色。(还没有在mac/linux上测试)

注意:两个版本都适用于我。感谢您在评论中提出建设性意见。

旧版本:https://jsbin.com/mahenes/edit?js,output

另外，这是一个修改后的版本(有人能在mac上测试并确认吗)

新版本:https://jsbin.com/xiconuv/edit?js,output

新版本:

function isCapslock(e) {
  const IS_MAC = /Mac/.test(navigator.platform);

  const charCode = e.charCode;
  const shiftKey = e.shiftKey;

  if (charCode >= 97 && charCode <= 122) {
    capsLock = shiftKey;
  } else if (charCode >= 65 && charCode <= 90
    && !(shiftKey && IS_MAC)) {
    capsLock = !shiftKey;
  }

  return capsLock;
}

旧版本:

function isCapslock(e) {
  e = (e) ? e : window.event;

  var charCode = false;
  if (e.which) {
    charCode = e.which;
  } else if (e.keyCode) {
    charCode = e.keyCode;
  }

  var shifton = false;
  if (e.shiftKey) {
    shifton = e.shiftKey;
  } else if (e.modifiers) {
    shifton = !!(e.modifiers & 4);
  }

  if (charCode >= 97 && charCode <= 122 && shifton) {
    return true;
  }

  if (charCode >= 65 && charCode <= 90 && !shifton) {
    return true;
  }

  return false;
}

对于国际字符，可以根据需要为以下键添加额外的检查。你必须获得你感兴趣的字符的键码范围，可能是通过使用一个键映射数组，它将保存你正在寻址的所有有效用例键…

大写字母A-Z或'Ä'， 'Ö'， 'Ü'， 小写字母a-Z或0-9或'ä'， 'ö'， 'ü'

上面的键只是示例表示。

最近在hashcode.com上有一个类似的问题，我创建了一个jQuery插件来处理它。它还支持对数字的大写锁定识别。(在标准德语键盘布局上，大写锁定对数字有影响)。

你可以在这里查看最新版本:jquery.capsChecker

还有另一个版本，清晰而简单，处理移位的capsLock，并且不受ascii限制，我认为:

document.onkeypress = function (e)
{
    e = e || window.event;
    if (e.charCode === 0 || e.ctrlKey || document.onkeypress.punctuation.indexOf(e.charCode) >= 0)
        return;
    var s = String.fromCharCode(e.charCode); // or e.keyCode for compatibility, but then have to handle MORE non-character keys
    var s2 = e.shiftKey ? s.toUpperCase() : s.toLowerCase();
    var capsLockOn = (s2 !== s);
    document.getElementById('capslockWarning').style.display = capsLockOn ? '' : 'none';
}
document.onkeypress.punctuation = [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,91,92,93,94,95,96,123,124,125,126];

编辑:意义上的capsLockOn被逆转，多，固定。

编辑#2:在进一步检查之后，我做了一些更改，不幸的是，代码更详细了一些，但它可以适当地处理更多的操作。

Using e.charCode instead of e.keyCode and checking for 0 values skips a lot of non-character keypresses, without coding anything specific to a given language or charset. From my understanding, it's slightly less compatible, so older, non-mainstream, or mobile browsers may not behave as this code expects, but it's worth it, for my situation anyway. Checking against a list of known punctuation codes prevents them from being seen as false negatives, since they're not affected by caps lock. Without this, the caps lock indicator gets hidden when you type any of those punctuation characters. By specifying an excluded set, rather than an included one, it should be more compatible with extended characters. This is the ugliest, special-casiest bit, and there's some chance that non-Western languages have different enough punctuation and/or punctuation codes to be a problem, but again it's worth it IMO, at least for my situation.

jQuery:

$('some_element').keypress(function(e){
       if(e.keyCode == 20){
             //caps lock was pressed
       }
});

这个jQuery插件(代码)实现了与Rajesh的回答相同的想法，更简洁一点。