string - Extract specific data from JavaScript .getAttribute()

There are a couple ways you could do this.

1.) Using substr and indexOf to extract it

var str = "www.something.com/user=123123123";
str.substr(str.indexOf('=') + 1, str.length);

2.) Using regex

var str = var str = "www.something.com/user=123123123";
// You can make this more specific for your query string, hence the '=' and group
str.match(/=(\d+)/)[1];  

You could also split on the = character and take the second value in the resulting array. Your best bet is probably regex since it is much more robust. Splitting on a character or using substr and indexOf is likely to fail if your query string becomes more complex. Regex can also capture multiple groups if you need it to.