Big-endian 64-bit integer decoding with TypedArray

int64.js

// Unmarshals an Uint8Array as a signed 64-bit integer, big-endian.
function decodeInt64(data, i, allowImprecise) {
	var v = 0, j = i + 7, m = 1;
	if (data[i] & 128) {
		// two's complement
                for (var carry = 1; j >= i; --j, m *= 256) {
                        var b = (data[j] ^ 255) + carry;
                        carry = b >> 8;
                        v += (b & 255) * m;
                }
                v = -v;
		if (! allowImprecise && v < Number.MIN_SAFE_INTEGER)
			throw '64-bit integer exceeds MIN_SAFE_INTEGER';
        } else {
                for (; j >= i; --j, m *= 256)
                        v += data[j] * m;
		if (! allowImprecise && v > Number.MAX_SAFE_INTEGER)
			throw '64-bit integer exceeds MAX_SAFE_INTEGER';
        }
	return v;
}