Libraries in Solidity

The Library

Let’s create a file called MyLibrary.sol

//SPDX-License-Identifer: MIT
pragma solidity ^0.8.22;
library MyLibrary {
function addOne(uint256 _myNumber) internal returns (uint256) {
return _myNumber + 1;
}
}

Noteworthy here is the library keyword, where for a contract creation, we would use contract instead. Also, note that we declare all functions with internal , as the Solidity docs on libraries state, “…A library is embedded into the contract if all library functions are internal. Otherwise, the library must be deployed and then linked before the contract is deployed.”
Read more on the topic here. Now let’s move on to

The Contract

Let’s now use MyLibraryinside a new file called MyContract.sol

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.22;
import { MyLibrary } from "./MyLibrary.sol";
contract MyContract {
using MyLibrary for uint256;
uint256 myNumber = 42;
function getMyNumber() public view returns (uint256) {
return myNumber;
}
function getMyNumberPlusOne() public view returns (uint256) {
return myNumber.plusOne();
}
}

Here is the neat part: using MyLibrary for uint256 tells the compiler to “inject” our functions from MyLibrary for all variables of type uint256 ! And because myNumberis of type uint256, we now can return myNumber.plusOne() ! Note that myNumber will be passed in as the first argument to function plusOne() . So at execution time, think of it as getting called like this: plusOne(myNumber)

But what if our Library function takes in more than one argument? Let’s extend MyLibrary with another function:

//SPDX-License-Identifer: MIT
pragma solidity ^0.8.22;
library MyLibrary {
function addOne(uint256 _myNumber) internal pure returns (uint256) {
return _myNumber + 1;
}
function addTwoNumbers(uint256 _firstNumber, uint256 _secondNumber) internal pure returns (uint256) {
return _firstNumber + _secondNumber;
}
}

And in MyContract we add function addSomethingToMyNumber:

//SPDX-License-Identifier: MIT
pragma solidity ^0.8.22;
import { MyLibrary } from "./MyLibrary.sol";
contract MyContract {
using MyLibrary for uint256;
uint256 myNumber = 42;
function getMyNumber() public view returns (uint256) {
return myNumber;
}
function getMyNumberPlusOne() public view returns (uint256) {
return myNumber.plusOne();
}
function addSomethingToMyNumber(uint256 _addThisNumberToMyNumber) public view returns (uint256) {
return myNumber.addTwoNumbers(_addThisNumberToMyNumber);
}
}

We can now call myNumber.addTwoNumbers(_addThisNumberToMyNumber);
Here, myNumber will still be passed as the first argument to addTwoNumbers() and _addThisNumberToMyNumber would be the second argument. So at execution time, imagine it being called as addTwoNumbers(myNumber, _addThisNumberToMyNumber);

I highly recommend you go ahead and deploy MyContractf.e. onto the Remix VM via Remix IDE and execute getMyNumber and addSomethingToMyNumber to verify that MyLibrary works correctly:

Let me know in the comments if you like this short guide and if you want to see more like this. Also, let me know if you tried it and what your experience was.

Unlimited thanks go out to Patrick Collins for sharing his Blockchain development course for FREE on YouTube, where I took inspiration for this article.