trim in javascript ? what this code is doing?

/^\s+|\s+$/g searches for whitespace from either the beginning or the end of the string. The expression can be split into two parts, ^\s+ and \s+$ which are separated by | (OR). The first part starts from the beginning of the string (^) and includes as many whitespace characters it can (\s+). The second part does the same but in reverse and for the end using the dollar sign ($).

In plain english, the regular expression would go like this:

Find as many whitespace characters from the beginning of the string as possible or as many whitespace characters from the end as possible.

Note that \s matches spaces, tabs and line breaks.

The /g part at the end enables global searching, which allows multiple replacements (eg. not just the beginning, but the end of the string also).

^ is the beginning of the string, and $ is the end. \s means a whitespace character (which in JavaScript specifically means tab, vertical tab, form feed, space, non-break space, byte order mark, Unicode space separator (category Zs), line feed, carriage return, line separator, or paragraph separator), and + means 1 or more. | is alternation, a choice between two possibilities. g is the global flag. So the regex means the beginning, then one or more whitespace, or one or more whitespace, then the end. Then, we replace all matches (since it's global) with the empty string.

You might be interested in this blog post, which analyzes in more detail than you probably need :) the pros and cons of various trim functions.