function isUpperCase(myString) { 
  return (myString == myString.toUpperCase()); 
} 
function isLowerCase(myString) { 
  return (myString == myString.toLowerCase()); 
} 

你可以使用正则表达式测试和toUpperCase方法:

String.prototype.charAtIsUpper = function (atpos){
      var chr = this.charAt(atpos);
      return /[A-Z]|[\u0080-\u024F]/.test(chr) && chr === chr.toUpperCase();
};
// usage (note: character position is zero based)
'hi There'.charAtIsUpper(3);      //=> true
'BLUE CURAÇAO'.charAtIsUpper(9);  //=> true
'Hello, World!'.charAtIsUpper(5); //=> false

另请参阅

const isUpperCase = (string) => /^[A-Z]*$/.test(string)

然后:

isUpperCase('A') // true
isUpperCase('a') // false

你也可以使用这个，它将检查字符串的小写字母和大写字母

var s = "a"
if(/[a-z]/.test(s)){
  alert ('lower case true');
}

if(/[A-Z]/.test(s)) {
 alert ('upper case true'); 
}

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

function solution(s) {
var c = s[0];

if (c == c.toUpperCase() && !(c >= '0' && c <= '9') &&(c >='A' && c <= 'Z')) {
    return 'upper';
} else if (c == c.toLowerCase() && !(c >= '0' && c <= '9') &&(c >='a' && c <= 'z')){
    return 'lower';
} else if (c >= '0' && c <= '9'){
   return 'digit'
} else {
  return 'other' 
}
}

var str1= (solution('A')) // upper
var str2 = solution('b') // lower
var str3 = solution('1') // digit
var str4 = solution('_') // other
console.log(`${str1} ${str2} ${str3} ${str4}`)