Some IPv6 utility functions I put together, improvements appreciated

IPv6Utils.js

var ipv4Regex = /^(\d+)\.(\d+)\.(\d+)\.(\d+)$/;

function expandIPv6(ipv6ColonNotation) {
    var pad = "0000"
    var count = (ipv6ColonNotation.match(/:/g)|| []).length;
    if (count == 0 || count > 7) {
        // too few or too many colons 
        return "err";
    }
    
    var blocks = ipv6ColonNotation.split(':');
    var emptyBlock = 0;
    for (i=0; i<blocks.length; i++) {
        if (blocks[i].length < 4 & blocks[i].length > 0) {
            blocks[i] = pad.substring(0, pad.length - blocks[i].length) +  blocks[i];
            console.log(blocks[i]);
        } else if (blocks[i].length === 0 && emptyBlock === 0 ) {            
            for (j=0; j <(7-count); j++) {
               blocks.splice(i, 0, pad);               
            }
            emptyBlock = 1;
         } else if (blocks[i].length === 0 && emptyBlock === 1 ) {
             blocks[i] = pad;
         }
    }
    
    var str = '';
    for (i=0; i<7; i++) {
        str += blocks[i] + ':';
    }
    str += blocks[7]
    
    return str;
}

function ipv4toIPv6(address) {
    var ipv6Str = "0000:0000:0000:0000:0000:ffff:"
    parts = ipv4Regex.exec(address);
    
    parts.shift();
    
    for (i=0; i<parts.length; i++) {
        ipv6Str += parseInt(parts[i]).toString(16);
        if (i === 2) {
            ipv6Str += ':';
        }
    }
    return ipv6Str;
}

// not all IPv6 addresses are convertable
function ipv6ToIPv4(address) {
    var prefix = "0000:0000:0000:0000:0000:ffff:"
    var ipv4Addr = '';
    
    address = expandIPv6(address);
    
    if (!address.startsWith(prefix)) {
        return;
    }
    
    address = address.replace(/:/g,'');
    
    for (i=0; i<4; i++) {
        ipv4Addr += parseInt(address.substring(24 + (i*2), 24 + (i*2) + 2),16)
        if (i<3) {
           ipv4Addr += '.';
        }
    }
    
   return ipv4Addr;
}

function ipv6toBinary(address) {
    var binAddr = 0;
    if (address.indexOf('.') > 0) {
        address = ipv4toIPv6(address);
    }
    address = expandIPv6(address);
    address = address.replace(/:/g,'');
    for (i=0; i<address.length; i+=2) {
        if (i>0) {
           binAddr = binAddr * 256;
        }
        binAddr += parseInt(address.substring(i,i+2), 16);
    }
    return binAddr;
}