Node.js - Array undefined but logging on console

Question

What I'm trying to do is to download a csv file, read it line by line and to add the splitted line (on ',') to tmparray. This code works and prints all the element in the array.

var request = require('request');
var fs = require('fs');
readline = require('readline');
try {
    request('https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/2.5_day.csv').pipe(fs.createWriteStream("MyCsv.txt"));
} catch (e) {
    console.error(e);
}
var inputFile = 'MyCsv.csv';
var tmparray;
//read the file
var rd = readline.createInterface({
    input: fs.createReadStream('/home/nome/Node/MyCsv.csv')
});
try {
    //read line by line 
    rd.on('line', (line) => {
        tmparray += line.split(",");

        //print the elements
        tmparray.forEach((element) => {
            console.log(element);
        }, this);
    });
} catch (e) {
    console.error(e);
}

What I want to do is to print the array after I assigned it. I've tried this:

var request = require('request');
var fs = require('fs');
readline = require('readline');
try {
    request('https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/2.5_day.csv').pipe(fs.createWriteStream("MyCsv.txt"));
} catch (e) {
    console.error(e);
}
var inputFile = 'MyCsv.csv';
var tmparray;
//read the file
var rd = readline.createInterface({
    input: fs.createReadStream('/home/nome/Node/MyCsv.csv')
});
try {
    //read line by line 
    rd.on('line', (line) => {
        tmparray += line.split(",");


    });
} catch (e) {
    console.error(e);
} finally {
    console.log(tmparray); // undefined
    // or this: console.log(tmparray[0]) can't read the property '0' of undefined

}

but the array is printed as if it is undefined

Andrew Taylor · Accepted Answer · 2017-11-23 19:18:36Z

2

The problem is that rd.on(...) is asynchronous.

That means that you are telling rd that when it reads a line, it should add it to tmparray — but that doesn't actually happen yet. It happens moments later, after you console.log(tmparray).

You should say rd.on('close', () => console.log(tmparray)) to tell Node "when you have finished reading rd, then log the data".

There are a couple of other issues in the code but they should be easier to find once this is fixed. Looking at it, I think line isn't an event on readable streams so you should say rd.on('data', ...) instead; and you're trying to build up an array using the + operator which doesn't work. It will probably convert everything to strings though, so it should log something fairly reasonable for now.

edited Nov 23, 2017 at 19:18

answered Nov 22, 2017 at 15:12

Andrew Taylor

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1 Comment

JustARandomProgrammer Over a year ago

Will try tomorrow. Makes sense

YouneL · Accepted Answer · 2017-11-22 15:57:02Z

2

Why not use the csv package it will give you the same result, Here is an example of transforming csv file into array:

const csv = require('csv')
    , request = require('request');

var url = 'https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/2.5_day.csv';

request(url, function (err, response, data) {

    if (err) throw err;

    csv.parse(data, function (err, data) {

        if (err) throw err;

        // here you get your array
        console.log(data);

    });

});

answered Nov 22, 2017 at 15:57

YouneL

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