Extracting the exponent and mantissa of a Javascript Number

Using the new ArrayBuffer access arrays, it is actually possible to retrieve the exact mantissa and exponent, by extracting them from the Uint8Array. If you need more speed, consider reusing the Float64Array.

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],
    }
}

I’ve also created some test cases. 0 fails, since there is another representation for 2^-1023.

var tests = [1, -1, .123, -.123, 1.5, -1.5, 1e100, -1e100, 
                    1e-100, -1e-100, Infinity, -Infinity];

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

    console.log("Testing: " + x + " " + value);
    console.assert(x === value);
});

console.log("Tests passed");