Lesson 7: Some questions in "Testing Fund Me" course code

[Dear-MrHe]

In FundMe.test.js file.
First question: priceFeed is a variable of type AggregatorV3Interface in the FundMe contract, why can await fundMe.priceFeed() get the address of priceFeed?

describe("constructor", function() {
    it("sets the aggregator address correctly", async function() {
        const response = await fundMe.priceFeed()
        assert.equal(response, mockV3Aggregator.address)
    })
})

Second question: Why does the index symbol of mapping and array type become () here, shouldn’t it be []? This happens in await fundMe.addressToAmountFunded(deployer) and await fundMe.funders(0).

describe("fund", function() {
    it("Fails if you don't send enough ETH", async function() {
        await expect(fundMe.fund()).to.be.revertedWith("You need to spend more ETH!")
    })
    it("updated the amount funded data structure", async function() {
        await fundMe.fund({value: sendValue})
        const response = await fundMe.addressToAmountFunded(deployer)
        assert.equal(response.toString(), sendValue.toString())
    })
    it("Add funder to array of funders", async function() {
        await fundMe.fund({value: sendValue})
        const funder = await fundMe.funders(0)
        assert.equal(funder, deployer)
    })
})

Can anyone help with the above that confuse me? Thank you very much.

[alymurtazamemon]

@Dear-MrHe

1. JavaScript/typescript does have any knowledge about contract type so when you return a contract type from the smart contract it returns the address of that contract in the form of a string value. Also vice-versa is true.
2. Solidity by default create the getter function for all public variables including mappings and arrays. But for mappings and arrays, the getter function takes the parameter and only returns 1 value at a time.

i.e

function addressToAmountFunded(address _add) external view returns (uint256) {
    return addressToAmountFunded[_add];
}

[Dear-MrHe]

@aaankeet Um? priceFeed is a variable of type AggregatorV3Interface.

[aaankeet]

@Dear-MrHe

Yea and its value will be the address which you have assigned to it.

Its like this AggregatorV3Interface public priceFeed = AggregatorV3Interface("address of the aggregator "). In order to retrieve its value
you would do like this contract.priceFeed(). In this case your contract is fundMe So it would be fundMe.priceFeed() .

If you want to learn more about Interfaces check this out #5587 keep in mind there's lot more to interfaces but this would do it.

Hope this clears your doubts & if not please elaborate on what are you looking for.

[Dear-MrHe]

@aaankeet Thanks, I probably understand.

[alymurtazamemon]

@alymurtazamemon Okay, thank you for your answer, but for the first question, according to your statement, what I want to ask is why can’t I get the address of priceFeed directly through fundMe.priceFeed? Instead, add a () after it.

As I said in point 2, solidity creates the getter function for all public variables, so here you are calling that function not a variable.

[Dear-MrHe]

@alymurtazamemon Ok, thanks.