Convert a string with a hex representation of an IEEE-754 double into JavaScript numeric variable

Question

Suppose I have a hex number "4072508200000000" and I want the floating point number that it represents (293.03173828125000) in IEEE-754 double format to be put into a JavaScript variable.

I can think of a way that uses some masking and a call to pow(), but is there a simpler solution?

A client-side solution is needed.

This may help. It's a website that lets you enter a hex encoding of an IEEE-754 and get an analysis of mantissa and exponent.

http://babbage.cs.qc.edu/IEEE-754/64bit.html

Because people always tend to ask "why?," here's why: I'm trying to fill out an existing but incomplete implementation of Google's Procol Buffers (protobuf).

Answer 1

The new Typed Arrays mechanism allows you to do this (and is probably an ideal mechanism for implementing protocol buffers):

var buffer = new ArrayBuffer(8);
var bytes = new Uint8Array(buffer);
var doubles = new Float64Array(buffer); // not supported in Chrome

bytes[7] = 0x40; // Load the hex string "40 72 50 82 00 00 00 00" 
bytes[6] = 0x72;
bytes[5] = 0x50;
bytes[4] = 0x82;
bytes[3] = 0x00;
bytes[2] = 0x00;
bytes[1] = 0x00;
bytes[0] = 0x00;

my_double = doubles[0];

document.write(my_double);  // 293.03173828125

This assumes a little-endian machine.

Unfortunately Chrome does not have Float64Array, although it does have Float32Array. The above example does work in Firefox 4.0.1.