Convert uppercase string to lowercase string in JS manually

Trying to write a function which returns the calling string value converted to lowercase. The trick is I can't use toLocaleLowerCase().

Below is what I have so far.

function charChange(char){
    for (var i=0; i<char.length; i++){
        var char2=charCodeAt(char[i])+32;
        var char3=String.fromCharCode(char2);
        if (char3 !== charCodeAt(97-122){
            alert("Please enter only letters in the function")
        }
    }
    return char;
}

To convert the uppercase letters of a string to lowercase manually (if you're not doing it manually, you should just use String.prototype.toLowerCase()), you must:

1. Write the boilerplate function stuff:

   function strLowerCase(str) {
       let newStr = "";
       // TODO
       return newStr;
   }
   

2. Loop over each character of the original string, and get its code point:

   function strLowerCase(str) {
       let newStr = "";
       for(let i = 0; i < str.length; i++) {
           let code = str.charCodeAt(i);
           // TODO
       } return newStr;
   }
   

3. Check if the character is an uppercase letter. In ASCII, characters with code points between 65 and 90 (inclusive) are uppercase letters.

   function strLowerCase(str) {
       let newStr = "";
       for(let i = 0; i < str.length; i++) {
           let code = str.charCodeAt(i);
           if(code >= 65 && code <= 90) {
               // TODO
           } // TODO
       } return newStr;
   }
   

4. If the character is uppercase, add 32 to its code point to make it lowercase (yes, this was a deliberate design decision by the creators of ASCII). Regardless, append the new character to the new string.

   function strLowerCase(str) {
       let newStr = "";
       for(let i = 0; i < str.length; i++) {
           let code = str.charCodeAt(i);
           if(code >= 65 && code <= 90) {
               code += 32;
           } newStr += String.fromCharCode(code);
       } return newStr;
   }
   

5. Test your new function:

   strLowerCase("AAAAAAABBBBBBBCCCCCZZZZZZZZZaaaaaaaaaaa&$*(@&(*&*#@!");
   // "aaaaaaabbbbbbbccccczzzzzzzzzaaaaaaaaaaa&$*(@&(*&*#@!"