Readability of JavaScript shorthand for converting strings to numbers with default value
Seems fine syntactically and perfectly idomatic, but always, always supply a radix argument to parseInt:

var numberToAdd = parseInt($('#number-to-add').val(), 10) || 500;


To quote MDN:


  radix: An integer that represents the radix of the [...] string. Always specify this parameter to eliminate reader confusion and to guarantee predictable behavior. Different implementations produce different results when a radix is not specified.
  [...]
  If the input string begins with "0", radix is eight (octal) or 10 (decimal).  Exactly which radix is chosen is implementation-dependent.  ECMAScript 5 specifies that 10 (decimal) is used, but not all browsers support this yet.  For this reason always specify a radix when using parseInt.


(emphasis in the original)