Promises and Async/Await don't seem to do anything in Express/node

January 30, 2023

I’m learning node and express and I’m trying to implement a function that retrieves data from a csv file uploaded by the user.
The data should be processed first and then outputted to the console, but it instead outputs an empty array before processing it.
I followed examples of async/await from tutorials I found online but the issue persists and I’m not sure how to proceed.

app.on("event:file_uploaded", async(userDataPath)=>{
    let result = await retrieveDataFromCsvFile(userDataPath);
    console.log(result); //should be completed after retrieving data
});

function retrieveDataFromCsvFile(path){
    return new Promise((resolve)=>{
        console.log(`RETRIEVING DATA FROM: ${path}`);
        const stream = fs.createReadStream(path);
        const rl = readline.createInterface({input: stream});
        let data = [];
        rl.on("line", (row)=>{
            data.push(row.split(","));
        })
        rl.on("close", ()=>{
            console.log("Data Processed"); //should be completed first
        })
        resolve(data);
    
    })
}

Outputs:

RETRIEVING DATA FROM: <filepath> 
[] 
Data Processed

>Solution :

You need to call resolve when you have the data you want.

At the moment you are:

Registering a line event handler
Registering a close event handler
Calling resolve

Some time later you will get line and close events that trigger their respective event handler functions … but by then you’ve already resolved the promise.

You need to determine what condition means your data is ready. At a guess, I’d expect that to be "When the callback to close is run" and call resolve then.