Solidity – How to Get the Array Length in Ethers.js and Hardhat

ethers.jshardhatsolidity

I have a function that pushes a value into a public array.
Now in my hardhat tests, I would like to check that before the first time calling my function, the array has a length of 0, and afterward, it's 1.
However, somehow I always have to pass an index to the getter function.

Is there any way to get the length of a solidity array from ethers?

untested pseudo-code below:

Solidity

contract A {
  address[] public functionCaller;

  function test() external {
    functionCaller.push(msg.sender);
  }
}

tests/index.ts

// ...
await a.test()

// what to call here?
expect(await a.functionCaller().length).to.equal(1); // throws error

The error is:

Argument of type 'undefined' is not assignable to parameter of type 'BigNumberish'.

This makes sense since the function signature to get the array length seems to be different. Every help is appreciated. Thanks!

Best Answer

The length of a public array is not exposed by default in solidity.

You can either define a specific solidity function for it :

function getLength() public view returns (uint256) {
   return functionCaller.length;
}

And call that function in your test.

Or directly from ethers.js, read the storage slot of your array (in your example it's in storage slot 0) which holds its length (see the documentation). Be careful though, changing your contract's storage layout may change the storage slot of the array, thus, invalidating your test.

describe("Validate length on push", function () {
  it("Should increment the length after a push is done", async function () {
    const contract = await ethers.getContractFactory("A");
    const instance = await contract.deploy();
    await instance.deployed();

    // Read storage slot 0 holding the length of the array
    const initialLength = ethers.BigNumber.from(await ethers.provider.getStorageAt(instance.address, 0));

    expect(initialLength.eq(0)).to.equal(true);

    await instance.test();

    // Read storage slot 0 holding the length of the array
    const newLength = ethers.BigNumber.from(await ethers.provider.getStorageAt(instance.address, 0));

    expect(newLength.eq(1)).to.equal(true);
  });
});

I hope that answers your question.

Related Solutions

Solidity: How to Access the Length of a Public Array?

Solidity creates getter functions for public state variables automatically. For example, the following will work:

contract A {
  uint256 public foo = 1;
}

contract B {
  A a;

  function getFoo() public view returns (uint256) {
    return a.foo();
  }
}

In the case of arrays however, you can only get a single element from the array, rather than the entire array. From the Solidity documentation:

If you have a public state variable of array type, then you can only retrieve single elements of the array via the generated getter function. This mechanism exists to avoid high gas costs when returning an entire array. You can use arguments to specify which individual element to return, for example data(0). If you want to return an entire array in one call, then you need to write a function.

Basically, while you can do the following:

contract A {
  uint256[] public foo;
}

contract B {
  A a;

  function getFoo() public view returns (uint256) {
    return a.foo(0); // Get item at index 0 from the array
  }
}

You cannot use a.foo() to read the entire array, and you will have to use a getter function like this for example:

contract A {
  uint256[] public foo;

  function getFooLength() public view returns (uint256) {
    return foo.length;
  }
}

contract B {
  A a;

  function getFoo() public view returns (uint256) {
    return a.getFooLength();
  }
}

Hardhat – Best Practices for Writing Tests Using Chai in Solidity

I got an answer from a member of this repo which explain everything :

If you do await expect(...something...), the promise returned inside the expect will be captured by Chai. If you then don't assert anything on that promise, then the result will be discarded and the test will pass.

I wasn't aware of that, now I found how to verify that a transaction is NOT reverted (what I was looking for), I need to use .not.to.be.reverted : https://www.chaijs.com/api/bdd/#method_not

Best Answer

Related Solutions

Solidity: How to Access the Length of a Public Array?

Hardhat – Best Practices for Writing Tests Using Chai in Solidity

Related Topic