fhex

Hex float conversion for JavaScript: toHex for formatting, fromHex for parsing.

Uses the IEEE 754 hexadecimal floating-point format (±0xh.hhhp±d) — the same format used by C's %a printf specifier and Java's Double.toHexString().

Requirements

Node.js >= 20

Usage

import { toHex, fromHex } from 'fhex'

// Formatting
toHex(3.0)    // '0x1.8p+1'
toHex(-10.0)  // '-0x1.4p+3'
toHex(0.5)    // '0x1p-1'

// Parsing
fromHex('0x1.8p+1')   // 3.0
fromHex('-0x1.4p+3')  // -10.0
fromHex('inf')        // Infinity
fromHex('nan')        // NaN

// Round-trip
const hex = toHex(Math.PI)       // '0x1.921fb54442d18p+1'
fromHex(hex) === Math.PI         // true

Bit-level inspection

toHexBits and fromHexBits expose the raw 64-bit IEEE 754 encoding as a hex string. This is useful for inspecting NaN payloads and sign bits that are not observable through normal JavaScript operations.

import { toHex, fromHex, toHexBits, fromHexBits } from 'fhex'

toHexBits(1.0)         // '3ff0000000000000'
toHexBits(NaN)         // '7ff8000000000000'
toHexBits(-0)          // '8000000000000000'

// Construct a NaN with a specific payload
const nan = fromHexBits('7ff0000000000123')
toHex(nan)             // 'nan:0x123'

// Round-trip NaN payloads through hex float format
const parsed = fromHex('nan:0x123')
toHexBits(parsed)      // '7ff0000000000123'

Note: JavaScript engines may canonicalize NaN payloads when values pass through arithmetic operations. The bit-level functions preserve payloads through DataView, but payloads may be lost if the NaN value is used in computations.

Format

Floating point numbers are represented as ±0xh.hhhp±d, where:

• ± is the sign (- for negative, omitted for positive)
• 0x is the hex prefix
• h.hhh is the significand in hexadecimal
• p±d is the exponent in decimal (base 2)

Special values:

• ±0x0p+0 for zero
• ±inf for infinity
• nan for quiet NaN
• nan:0x... for NaN with payload

Parsing features

• Underscores for readability: 0x1_000p+0
• NaN payloads preserved: nan:0x123
• Case-insensitive: INF, NaN, 0X1P+0
• Whitespace trimmed
• Optional exponent: 0x1.8 is equivalent to 0x1.8p+0

License

Apache-2.0