nan-box — for {{name}}

sign · 1 bit exponent · 11 bits mantissa · 52 bits

— —

IEEE 754 binary64

sign—

biased exponent—

unbiased exponent—

mantissa (hex)—

decoded value—

tagged-value interpretations

JavaScriptCore—

LuaJIT 2—

SpiderMonkey-ish—

payload (low 48)—

payload (low 47)—

paste hex or a number

doubles

edges

JavaScriptCore

LuaJIT

SpiderMonkey

quick reference

a NaN is any binary64 value where the exponent field is all-ones (0x7FF) and the mantissa is non-zero. that leaves 1 sign bit + 52 mantissa bits = 53 bits of free space inside the NaN encoding.

quiet NaN: MSB of mantissa is 1. signaling NaN: MSB of mantissa is 0 (mantissa still non-zero). all production NaN-boxing engines use quiet NaNs only, because x87 silently quiets sNaNs on load.

JavaScriptCore stores pointers in the low 48 bits and uses the 0x0002000000000000 offset for doubles. LuaJIT stores doubles verbatim and tags non-doubles by placing a value of 0xFFF8–0xFFFF in the top 16 bits, which sits safely inside the NaN-encoding window above any plausible x86-64 user-space pointer. SpiderMonkey's historical punbox64 scheme carves a 4-bit type tag out of bits 47–50 once the top 16 bits sit at or above 0xFFF0.

don't have a hex string handy? type a plain number into the "or a number" field — 3.14, -0, Infinity, NaN — and it encodes to bits the same way DataView would. every distinct value you visit (by bit, preset, or typed) gets kept in recently viewed below the presets, up to twelve, saved to this browser so it survives a reload.