var string = " This should become something else too . ";
string = string.replace(/\s+/g, " ");
This code replaces a consecutive set of whitespace characters (\s+) by a single white space. Note that a white-space character also includes tab and newlines. Replace \s by a space if you only want to replace spaces.
If you also want to remove the whitespace at the beginning and end, include:
string = string.replace(/^\s+|\s+$/g, "");
This line removes all white-space characters at the beginning (^) and end ($). The g at the end of the RegExp means: global, ie match and replace all occurences.