Number.toFixed() not rounding per specification

Tue 2019.08.20

This might be of some use to rule out a floating point issue, or cross check a 'C' snippet, by observing the actual mantissa value used at the point of desired rounding:

by @valderman Feb 2012
https://stackoverflow.com/questions/9383593/extracting-the-exponent-and-mantissa-of-a-javascript-number

Adapted from snippet above for the Espruino tests

function getNumberParts(x) {
    var float = new Float64Array(1),
        bytes = new Uint8Array(float.buffer);

    float[0] = x;

    var sign = bytes[7] >> 7,
        exponent = ((bytes[7] & 0x7f) << 4 | bytes[6] >> 4) - 0x3ff;

    bytes[7] = 0x3f;
    bytes[6] |= 0xf0;

    return {
        sign: sign,
        exponent: exponent,
        mantissa: float[0],
    }
}

tests.forEach(function(x) {
    var parts = getNumberParts(x),
        value = Math.pow(-1, parts.sign) *
                    Math.pow(2, parts.exponent) *
                    parts.mantissa;

    console.log( "sign= " + parts.sign + "  exp= " + parts.exponent + "  man= " + parts.mantissa );
    console.log( "Testing:  arg= " + x + "  calc= " + value );
});

Value under test:

var tests = [1234.505];


sign= 0  exp= 10  man= 1.20557128906

Testing:  arg= 1234.505  calc= 1234.505

=undefined
> 

Still a bit of work to do with the actual manipulation of the mantissa.

Also may be used for and related to post:

Math.round() doesn't follow specification for exact 0.5

References:

nice pictorial on bit layout in F.P. register
https://en.wikipedia.org/wiki/IEEE_754-1985
https://en.wikipedia.org/wiki/Floating-point_arithmetic