Check if a String Contains Uppercase Letters in JavaScript

We can check if a string contains uppercase characters in JavaScript by checking each letter to see if that letter is uppercase in a loop. We will make use of the toUpperCase() and charAt() methods. Here is our function that will check if there are any uppercase letters in a string.

function checkUppercase(str){
    for (var i=0; i<str.length; i++){
      if (str.charAt(i) == str.charAt(i).toUpperCase() && str.charAt(i).match(/[a-z]/i)){
        return true;
      }
    }
    return false;
};

Notice we use a regular expression above to make sure the character is a letter.

Here is our function with a simple example:

function checkUppercase(str){
    for (var i=0; i<str.length; i++){
      if (str.charAt(i) == str.charAt(i).toUpperCase() && str.charAt(i).match(/[a-z]/i)){
        return true;
      }
    }
    return false;
};

console.log(checkUppercase("all letters here are lowercase"));
console.log(checkUppercase("We Have some uppercase Letters in this One."));

#Output:
false
true

When processing strings in a program, it can be useful to know if we have uppercase or lowercase characters. Using JavaScript, we can easily check if a string contains uppercase letter(s) with the help of the JavaScript toUpperCase() method.

To check if a string contains uppercase, we just need to loop over all letters in the string until we find a letter that is equal to that letter after applying the toUpperCase() method and make sure that character is a letter.

Below is our JavaScript function again which will check if a string contains an uppercase letter.

function checkUppercase(str){
    for (var i=0; i<str.length; i++){
      if (str.charAt(i) == str.charAt(i).toUpperCase() && str.charAt(i).match(/[a-z]/i)){
        return true;
      }
    }
    return false;
};

How to Check if a String Contains Lowercase letters in JavaScript

We can also check if a string contains lowercase characters in JavaScript very easily.

To check if a string contains lowercase letters in JavaScript, we can adjust our function that we defined above to use the JavaScript toLowerCase() method, instead of the toUpperCase() method.

Below is a JavaScript function that will check if a string contains a lowercase letter.

function checkLowercase(str){
    for (var i=0; i<str.length; i++){
      if (str.charAt(i) == str.charAt(i).toLowerCase() && str.charAt(i).match(/[a-z]/i)){
        return true;
      }
    }
    return false;
};

function checkLowercase(str){
    for (var i=0; i<str.length; i++){
      if (str.charAt(i) == str.charAt(i).toLowerCase() && str.charAt(i).match(/[a-z]/i)){
        return true;
      }
    }
    return false;
};

console.log(checkLowercase("ALL THE LETTERS ARE UPPERCASE"));
console.log(checkLowercase("We Have some uppercase Letters in this One."));

#Output:
false
true

Hopefully this article has been useful for you to learn how to check if a string contains uppercase letters JavaScript.