How can I check if string contains characters & whitespace, not just whitespace?

Well, if you are using jQuery, it's simpler.

if ($.trim(val).length === 0){
   // string is invalid
} 

Simplest answer if your browser supports the trim() function

if (myString && !myString.trim()) {
    //First condition to check if string is not empty
    //Second condition checks if string contains just whitespace
}

if (/^\s+$/.test(myString))
{
      //string contains only whitespace
}

this checks for 1 or more whitespace characters, if you it to also match an empty string then replace + with *.

Instead of checking the entire string to see if there's only whitespace, just check to see if there's at least one character of non whitespace:

if (/\S/.test(myString)) {
    // string is not empty and not just whitespace
}