Solidityで文字列連結 ref: https://qiita.com/onokatio/items/4d74229bd6015379e379

file1.js

string memory str1 = "abc";
string memory str2 = "def";

bytes memory strbyte1 = bytes(str1);
bytes memory strbyte2 = bytes(str2);

file2.js

bytes memory str = new bytes(strbyte1.length + strbyte2.length);

file3.js

uint8 point = 0;

for(uint8 j = 0; j < strbyte1.length;j++){
    str[point] = strbyte1[j];
    point++;
}
for(uint8 k = 0; k < strbyte2.length;k++){
    str[point] = strbyte2[k];
    point++;
}

file4.js

return string(str);

MyStringTest.sol

pragma solidity ^0.4.19;

contract MyStringTest {

    function strConnect() public constant returns(string){

        string memory str1 = "abc";
        string memory str2 = "def";
        bytes memory strbyte1 = bytes(str1);
        bytes memory strbyte2 = bytes(str2);

        bytes memory str = new bytes(strbyte1.length + strbyte2.length);

        uint8 point = 0;

        for(uint8 j = 0; j < strbyte1.length;j++){
            str[point] = strbyte1[j];
            point++;
        }
        for(uint8 k = 0; k < strbyte2.length;k++){
            str[point] = strbyte2[k];
            point++;
        }
        return string(str);
    }
}