How to avoid hardcoding contract address in Solidity - solidity

Question

When looking at other team's smart contracts, I often see code like this:
address constant public token = address(0xabc123...);
Where the hexadecimal number is the address of an earlier-deployed smart contract. Coming from JS and C++ background, I'm not a fan of this because it effectively hardcodes what should be in a configuration file directly into the smart contract code. Several questions come to mind when I see code like this:
What if I want to deploy this to another EVM-compatible network?
What if I want to deploy this to testnet?
I'm still relatively new to Solidity, so it's possible I'm missing some feature of truffle that allows me to insert these strings at the time of deployment, but I didn't see this mentioned in any of the tutorials I went through. I would much rather have something like a JSON configuration file for testnet/mainnet/L2-chain/etc instead of having N versions of the same file with minor differences. How should I handle these cases?

Answer 1

You can have a variable (instead of a constant) containing the token address. And this variable can be set from a constructor.
So you can effectively pass the value from an environment variable to the contract constructor, to the contract storage.
Example:
.env
TOKEN_ADDRESS=0x123
deploy.js using Truffle (docs) for example
MyContract.new(process.env.TOKEN_ADDRESS)
You can also use Hardhat (docs) or any other library allowing you to deploy to any network depending on the config.
MyContract.sol
pragma solidity ^0.8;
contract MyContract {
address token;
constructor(address _token) {
token = _token;
}
}