I have created a small Javascript utility to provide this functionality. I don't claim it is perfect, so let me know how you fair. If people like it, I'll make this the official answer to this question.

CharFunk: https://github.com/joelarson4/CharFunk

• CharFunk.getDirectionality(ch) - Used to find the directionality of the character
• CharFunk.isAllLettersOrDigits(string) - Returns true if the string argument is composed of all letters and digits
• CharFunk.isDigit(ch) - Returns true if provided a length 1 string that is a digit
• CharFunk.isLetter(ch) - Returns true if provided a length 1 string that is a letter
• CharFunk.isLetterNumber(ch) - Returns true if provided a length 1 string that is in the Unicode "Nl" category
• CharFunk.isLetterOrDigit(ch) - Returns true if provided a length 1 string that is a letter or a digit
• CharFunk.isLowerCase(ch) - Returns true if provided a length 1 string that is lowercase
• CharFunk.isMirrored(ch) - Returns true if provided a length 1 string that is a mirrored character
• CharFunk.isUpperCase(ch) - Returns true if provided a length 1 string that is uppercase
• CharFunk.isValidFirstForName(ch) - Returns true if provided a length 1 string that is a valid leading character for a JavaScript identifier
• CharFunk.isValidMidForName(ch) - Returns true if provided a length 1 string that is a valid non-leading character for a ECMAScript identifier
• CharFunk.isValidName(string,checkReserved) - Returns true if the string is a valid ECMAScript identifier
• CharFunk.isWhitespace(ch) - Returns true if provided a length 1 string that is a whitespace character
• CharFunk.indexOf(string,callback) - Returns first matching index that returns a true return from the callback
• CharFunk.lastIndexOf(string,callback) - Returns last matching index that returns a true return from the callback
• CharFunk.matchesAll(string,callback) - Returns true if all characters in the provided string result in a true return from the callback
• CharFunk.replaceMatches(string,callback,ch) - Returns a new string with all matched characters replaced

I don't believe there is a built-in function for that. But it's easy enough to write with a regex

function isLetter(str) {
  return str.length === 1 && str.match(/[a-z]/i);
}

You could use comparison operators to see if it is in the range of digit characters:

var c = justPrices[i].substr(commapos+2,1);
if (c >= '0' && c <= '9') {
    // it is a number
} else {
    // it isn't
}