String.IsNullOrEmpty in JavaScript
Starting with:

return (!value || value == undefined || value == "" || value.length == 0);


Looking at the last condition, if value == "", it's length MUST be 0. Therefore drop it:

return (!value || value == undefined || value == "");


But wait! In JS, an empty string is false. Therefore, drop value == "":

return (!value || value == undefined);


And !undefined is true, so that check isn't needed. So we have:

return (!value);


And we don't need parentheses:

return !value


Q.E.D.