I have determined by experience that 9007199254740995
    (which is 2^53+3) is the smallest not representable
    integer in Javascript.

Here's something to drop in your browser console as an amusing illustration of this

     9007199254740994 === 
    (9007199254740995 - 1)

and

     9007199254740995 === 
    (9007199254740995 - 1)