What could be causing this scope issue and what steps can I take to resolve it?

I am currently working on utilizing the 'csv-parser' module within a node.js environment. After successfully reading my csv document, I have managed to obtain an array named 'results' containing all the necessary information in .json format. Strangely, the first console.log displays all the data as expected, but the second one prints out an empty array (as initially declared). Can anyone shed light on why this scope issue is occurring and provide guidance on how to resolve it? Thank you in advance for your assistance.

const csv = require('csv-parser');
const fs = require('fs');
let results = [];
fs.createReadStream('MyData.csv')
    .pipe(csv())
    .on('data', data => results.push(data))
    .on('end', () => {
        console.log(results) //1st console.log
        console.log('CSV file successfully processed');
    });


console.log(results);//2nd console.log

Answer №1

It's not just about scope, it's more of a timing issue at play here. The second console.log(results) is being executed before any data has been populated into it.

When dealing with streams and the csv() module in Node.js, remember that they are non-blocking and asynchronous. This means they operate independently in the background, triggering events periodically to carry out their tasks. Consequently, the remaining sections of your code continue to run even as these processes are ongoing. As a result, your final

console.log(results)

statement executes long before the stream and .pipe(csv()) operations are complete, leading to an empty output.

When writing code involving asynchronous operations in Node.js, make sure to utilize the results WITHIN the callback indicating the completion of the event. Thus, you should handle the results immediately after your initial console.log(results).

const csv = require('csv-parser');
const fs = require('fs');
let results = [];
fs.createReadStream('MyData.csv')
    .pipe(csv())
    .on('data', data => results.push(data))
    .on('end', () => {
        console.log(results) //1st console.log
        console.log('CSV file successfully processed');
        // Handle results HERE
    });

  // Results cannot be used here

Let's illustrate this concept with a simple example that you can test directly:

console.log("A");

setTimeout(function() {
    console.log("B");
}, 100);

console.log("C");

The output will be:

A
C
B

This behavior occurs because setTimeout() functions similarly in a non-blocking and asynchronous manner. Initiating a setTimeout() call sets the timer running, allowing the subsequent code to proceed forward. Hence, C gets printed prior to B.

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