how to access a local variable value outside the scope of its function in nodejs-CodePudding

I want to compare the data of two files and for that, I'm reading that file using the fs module but since I want to compare the values so I thought to store the value in an external variable but when I do console.log(budget_details) I get nothing in console. Please someone help. Please point me out if my approach is wrong and if we don't need to do that in nodejs. I'm new to nodejs.

import csv from 'csv-parser'
import fs from 'fs';

let budget_details
const budgetProcessing = (budget_file_path) => {
    try{
        fs.createReadStream(budget_file_path)
            .pipe(csv())
            .on('data', (row) => {
                budget_details = row
            })
            .on('end', () => {
                console.log('CSV file successfully processed');
            });
    }
    catch(error){
        console.log(error)
    }
}

budgetProcessing('budget.csv')

console.log(budget_details)

CodePudding user response：

Let's first explain why you don't get the expected result, it doesnt have to do with scope actually:

import csv from 'csv-parser'
import fs from 'fs';

let budget_details
const budgetProcessing = (budget_file_path) => {
    try{
        fs.createReadStream(budget_file_path)
            .pipe(csv())
            .on('data', (row) => {
                budget_details = row
            })
            .on('end', () => {
                console.log('CSV file successfully processed');
            });
     }
    catch(error){
        console.log(error)
    }
}

budgetProcessing('budget.csv')

console.log(budget_details)

fs.createReadStream is not itslef exactly asynchronous but then we pipe the returned stream to csv-parser which does event based parsing, so even if you call budgetProcessing before the console.log(budget_details) the stream reading has most likely not runned yet and budget_details is still undefined.

To fix this, you could move this console.log(budget_details) where it is set like so:

    let budget_details
    const budgetProcessing = (budget_file_path) => {
        try{
            fs.createReadStream(budget_file_path)
                .pipe(csv())
                .on('data', (row) => {
                    budget_details = row
                    console.log(budget_details)
                })
                .on('end', () => {
                     console.log('CSV file successfully processed');
                });
         }
         catch(error){
            console.log(error)
        }
    }

    budgetProcessing('budget.csv')

but then the variable itself wouldnt serve any real purpose so instead you could do this:

    const budgetProcessing = (budget_file_path, callback) => {
        try{
            fs.createReadStream(budget_file_path)
                .pipe(csv())
                .on('data', (row) => {
                    callback(row)
                })
                .on('end', () => {
                     console.log('CSV file successfully processed');
                });
                
         }
         catch(error){
            console.log(error)
        }
    }

    budgetProcessing('budget.csv', (budget_details) => {
        console.log(budget_details) // or anything with budget_details
    })

Lastly, I want to make clear that the callback will be called for each row of the csv as specified in csv-parser's documentation

CodePudding user response：

your code is not asynchronous. Anything with 'on', which takes a function, would indicate that it is event driven. You need something like:

import csv from 'csv-parser'
import fs from 'fs';

let budget_details
const budgetProcessing = (budget_file_path) => new Promise((resolve, reject) => {
  try {
    fs.createReadStream(budget_file_path)
      .pipe(csv())
      .on('data', (row) => {
        budget_details = row
      })
      .on('end', () => {
        console.log('CSV file successfully processed');
        resolve()
      });
  } catch (error) {
    console.log(error)
    reject(error)
  }
})

budgetProcessing('budget.csv')
  .then(() => console.log(budget_details))