Lesson 8: How does the listenForTransactionMine function have access to the transactionReceipt, which is not declared?

[yuyangezheng]

async function fund() {
    const ethAmount = ".05"
    console.log(`Funding with ${ethAmount} ...`)
    if (typeof window.ethereum !== "undefined") {
        const provider = new ethers.providers.Web3Provider(window.ethereum)
        const signer = provider.getSigner()
        const contract = new ethers.Contract(contractAddress, abi, signer)
        try {
            const transactionResponse = await contract.fund({
                value: ethers.utils.parseEther(ethAmount),
            })
            await listenForTransactionMine(transactionResponse, provider)
        } catch (error) {
            console.log(error)
        }
    }
}

function listenForTransactionMine(transactionResponse, provider) {
    console.log(`Mining ${transactionResponse.hash}`)
    //return new Promise()
    provider.once(transactionResponse.hash, (transactionReceipt) => {
        // What is the transactionReceipt above?????
        console.log(
            `Completed with  ${transactionReceipt.confirmations} confirmations`,
        )
    })
}

In the above code, I understand that within the fund function, transactionResponse is defined as await contract.fund({value: ethers.utils.parseEther(ethAmount),}) and we call the listenForTransactionMine function within the fund function with the response and provider as parameters. What I don't understand is that without defining transactionReceipt, how are we able to pass it into the anonymous function as a parameter inside the listenForTransactionMine function?

I would have expected this code to output something along the lines of "transactionReceipt is not defined." Is there something under the hood of the once function that is producing the transactionReceipt that I don't understand?

Any help is appreciated！

[Nlferu]

Hi @yuyangezheng

In the provided code, the listenForTransactionMine function utilizes an event listener attached to the provider to listen for a specific transaction's confirmation. When the transaction is confirmed, the listener function is triggered with the transactionReceipt as an argument.

Here's how it works:

1. provider.once(transactionResponse.hash, ...) sets up an event listener on the Ethereum provider for the specific transaction hash transactionResponse.hash.

2. When the transaction is mined and confirmed, the provider emits an event associated with the transaction hash.

3. The listener function passed to provider.once is then executed with the transactionReceipt as an argument. This transactionReceipt contains details about the confirmed transaction, such as the block number, gas used, status, etc.

So, the transactionReceipt parameter in the anonymous function within listenForTransactionMine is automatically provided by the Ethereum provider when the transaction associated with transactionResponse.hash is confirmed. You don't need to explicitly define transactionReceipt; it's part of the event emitted by the provider.