Convert bitcoin `nBits` field into a hex string value for `target` using only string functions

nbits2target.js

    // Returns a 64 char (256 bit) hex string of the target
    // Based on https://developer.bitcoin.org/reference/block_chain.html#target-nbits

    function nbits2target( nbits ) {
        const significand = nbits & 0x00ffffff;
        const exponent = nbits >>> (8*3);
        
        // (all `*2` are becuase calcuations are in bytes, but in string 1 byte = 2 letter places)

        const fixed6SigString =  (significand.toString(16)).padStart( 3*2 , "0");
        //  a 3 digit (6 byte) hex string with a leading fixed point

        const paddedSigString = ("00").repeat(32) + fixed6SigString + ("00").repeat(32) ;
        // padded string has a fixed (hexa)decimal point after byte 32
        
        const expString = paddedSigString.slice( exponent*2, (32+exponent)*2);
        // Now we move the point to the right exp bytes
        
        return expString;
    }