a handmade Ethereum event filter that you should never really use

eventFilter.js

const eventFilter = (contractAddress, contractAbi, eventName, _provider) => {
    const provider = _provider
    // this will return an array with an object for each event
    const events = contractAbi.abi.filter(obj => obj.type ? obj.type === "event" : false);
    // getting the Transfer event and pulling it out of its array
    const event = events.filter(event => event.name === eventName)[0];
    // getting the types for the event signature
    const types = event.inputs.map(input => input.type)
    // knowing which types are indexed will be useful later
    let indexedInputs = [];
    let unindexedInputs = [];
    event.inputs.forEach(input => {
        input.indexed ? 
        indexedInputs.push(input) :
        unindexedInputs.push(input)
    });
    // event signature
    const eventSig = `${event.name}(${types.toString()})`;
    // getting the topic
    const eventTopic = ethers.utils.keccak256(eventSig);

    // you could also filter by blocks using fromBlock and toBlock
    const logs = provider.getLogs({
        address: contractAddress,
        topics: [eventTopic] 
    });

    // need to decode the topics and events
    const decoder = new ethers.utils.AbiCoder();
    const decodedLogs = logs.map(log => {
        // remember how we separated indexed and unindexed events?
        // it was because we need to sort them differently here
        const decodedTopics = indexedInputs.map(input => {
            // we use the position of the type in the array as an index for the
            // topic, we need to add 1 since the first topic is the event signature
            const value = decoder.decode(input.type, log.topics[indexedInputs.indexOf(input) + 1]);
            return `${input.name}: ${value}`;
        })
        const decodedDataRaw = decoder.decode(unindexedInputs, log.data);
        const decodedData = unindexedInputs.map((input, i) => {
            return `${input.name}: ${decodedDataRaw[i]}`
        });
        
        return [...decodedTopics, ...decodedData]
    });

    return decodedLogs

}