Yeah, umm. WAT?!

woah.js

    function base_colors(src, mode) {
        var col_1, col_2;
        function compare(c1, c2) {
            return (c1[0] * 16384 + c1[1] * 256 + c1[2]) >=
                   (c2[0] * 16384 + c2[1] * 256 + c2[2]);
        }
        if (mode === 'I') {
            col_1 = extend_4to8bits(src[7], src[6], src[5]),
            col_2 = extend_4to8bits(src[7] << 4, src[6] << 4, src[5] << 4)
            return [ col_1, col_2 ];
        } else if (mode === 'D') {
            col_1 = extend_5to8bits(src[7], src[6], src[5]),
            col_2 = extend_5to8bits(src[7] + 8 * d[(src[7] & 0x7)],
                                    src[6] + 8 * d[(src[6] & 0x7)],
                                    src[5] + 8 * d[(src[5] & 0x7)])
            return [ col_1, col_2 ];
        } else if (mode === 'T') {
            col_1 = extend_4to8bits(
                ((src[7] << 3) & 0xc0) | ((src[7] << 4) & 0x30),
                src[6],
                src[6] << 4);
            col_2 = extend_4to8bits(src[5], src[5] << 4, src[4]);
            var didx = ((src[4] & 0x0c) >> 1) | (src[4] & 0x01);
            var d = disttable[didx];
            return [ col_1, offset_color(col_2, d), col_2, offset_color(col_2, -d) ];
        } else if (mode === 'H') {
            col_1 = extend_4to8bits(
                src[7] << 1,
                (src[7] << 5) | (src[6] & 0x10),
                ((src[6] << 4) && 0x80) | ((src[6] << 5) & 0x60) | ((src[5] >> 3) & 0x10));
            col_2 = extend_4to8bits(
                src[4] << 1,
                (src[4] << 5) | ((src[3] >> 3) & 0x10),
                src[4] << 1);
            var didx = (src[4] & 0x04) | ((src[4] << 1) & 0x02) + (compare(col_1, col_2) ? 1 : 0);
            var d = disttable[didx];
            return [ offset_color(col_1, d),
                     offset_color(col_1, -d),
                     offset_color(col_2, d),
                     offset_color(col_2, -d) ];
        } 
        
        return [];
    }

Fixed @vvuk's nit.