All numbers in JavaScript are actually IEEE-754 compliant floating-point doubles. These have a 53-bit mantissa which should mean that any integer value with a magnitude of approximately 9 quadrillion or less — more specifically, 9,007,199,254,740,991 — will be represented accurately.
NOTICE: in 2018 main browsers and NodeJS are working also with the new Javascript’s primitive-type, BigInt, solving the problems with integer value magnitude.