function isCapital(ch){
    return ch.charCodeAt() >= 65 && ch.charCodeAt() <= 90;
}

if (character == character.toLowerCase())
{
  // The character is lowercase
}
else
{
  // The character is uppercase
}

在现代浏览器中，您可以使用regexp和unicode属性测试。

/\p{Lu}/u.test("A") // is true
/\p{Lu}/u.test("Å") // is true
/\p{Lu}/u.test("a1å") // is false

更多信息:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Unicode_Property_Escapes

一般类别列表如下:

https://unicode.org/reports/tr18/#General_Category_Property

josh和maleki的答案如果字符或整个字符串都是数字，则在大写和小写都返回true。使结果为假结果。 使用josh的例子

var character = '5';
if (character == character.toUpperCase()) {
 alert ('upper case true');
}
if (character == character.toLowerCase()){
 alert ('lower case true');
}

另一种方法是首先测试它是否为数字，否则测试它是否为大写或小写 例子

var strings = 'this iS a TeSt 523 Now!';
var i=0;
var character='';
while (i <= strings.length){
    character = strings.charAt(i);
    if (!isNaN(character * 1)){
        alert('character is numeric');
    }else{
        if (character == character.toUpperCase()) {
            alert ('upper case true');
        }
        if (character == character.toLowerCase()){
            alert ('lower case true');
        }
    }
    i++;
}

function checkCharType (charToCheck) {
    // body... 
    var returnValue = "O";
    var charCode = charToCheck.charCodeAt(0);

    if(charCode >= "A".charCodeAt(0) && charCode <= "Z".charCodeAt(0)){

        returnValue = "U";

    }else if (charCode >= "a".charCodeAt(0) &&
                charCode <= "z".charCodeAt(0) ){
        returnValue = "L";
    }else if (charCode >= "0".charCodeAt(0) &&
            charCode <= "9".charCodeAt(0)  ) {
        returnValue = "N";
    }
    return returnValue;
}

var myString = prompt("Enter Some text: ", "Hello world !");

switch (checkCharType(myString)) {
    case "U":
        // statements_1
        document.write("First character was upper case");
        break;

    case "L":
        document.write("First character was a lower case");
        break;
    case "N":
        document.write("First character was a number");
        break
    default:
        // statements_def
        document.write("First character was not a character or a number");
        break;
}

Define a Function checkCharType().By declaring the variable returnValue and initialising it to the Character "O" to indicate it's Some other value. U for uppercase; L for Lowercase ; N for number Use the charCodeAt() method to get the character code of the first character. Using if Statement , which check within what range of values the character code falls. If it falls between the character codes for A and Z, Its Uppercase, character code between a and z ,Its Lowercase. and so on. "A".charCode(0) var myChar = new String("A"); myChar.charCodeAt(0); "A" : number code "65“ Check the String

其他答案的问题是，一些字符，如数字或标点符号，在检查小写/大写时也返回true。

我发现这个方法非常有效:

function isLowerCase(str)
{
    return str == str.toLowerCase() && str != str.toUpperCase();
}

这适用于标点符号、数字和字母:

assert(isLowerCase("a"))
assert(!isLowerCase("Ü"))
assert(!isLowerCase("4"))
assert(!isLowerCase("_"))

要检查一个字母，只需使用isLowerCase(str[charIndex])