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

CodePudding user response：

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

At the moment you are:

1. Registering a line event handler
2. Registering a close event handler
3. 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.

CodePudding user response：

As you can see your console logs are in the wrong order so that should tell you that the promise is resolved first and the file is processed later. You should change the close event to

    rl.on("close", ()=>{
        console.log("Data Processed"); //should be completed first
    resolve(data);
    })

this would ensure that your promise is resolved after the function is done processing the file.