Testing a Payable Function in Solidity

So I'm trying to test a payable function on the following smart contract here using the truffle framework:

contract FundMe {
    using SafeMathChainlink for uint256;

    mapping(address => uint256) public addressToAmountFunded;
    address[] public funders;
    address public owner;
    AggregatorV3Interface public priceFeed;

    constructor(address _priceFeed) public {
        priceFeed = AggregatorV3Interface(_priceFeed);
        owner = msg.sender;
    }

    function fund() public payable {
        uint256 mimimumUSD = 50 * 10**18;
        require(
            getConversionRate(msg.value) >= mimimumUSD,
            "You need to spend more ETH!"
        );
        addressToAmountFunded[msg.sender] += msg.value;
        funders.push(msg.sender);
    }

I specifically want to test the payable function, and I've seen a few things on the internet where people create other contracts with initial balances and then send their testing contract some eth. But I would just like to grab a local ganache wallet and send some eth to the contract and then test that, if someone could show me some test javascript code to wrap my head around this that would be much appreciated!

Solution

For a contract to be able to receive ETH (or any native token - BNB on Binance Smart Chain, TRX on Tron network, ...) without invoking any function, you need to define at least one of these functions receive() (docs) or fallback() (docs).

contract FundMe {

    // intentionally missing the `function` keyword
    receive() external payable {
        // can be empty
    }

    // ... rest of your code
}

Then you can send a regular transaction to the contract address in truffle (docs):

const instance = await MyContract.at(contractAddress);
await instance.send(web3.toWei(1, "ether"));

Note that because receive() and fallback() are not regular functions, you cannot invoke them using the truffle autogenerated methods: myContract.functionName()

If you want to execute a payable function sending it ETH, you can use the transaction params (docs). It's always the last argument, after all of the regular function arguments.

const instance = await MyContract.at(contractAddress);
await instance.fund({
    value: web3.toWei(1, "ether")
});

Note: If the fund() function had 1 argument (let's say a bool), the transaction params would be the 2nd:

await instance.fund(true, {
    value: web3.toWei(1, "ether")
});