CODEWITHSUNDEEP
javascriptnode.jsasynchronouscallback
Harsha Reaper
Harsha Reaper

Reputation: 129

How to execute the callback at the end of the function calling it?

In my Node js application I have a function that reads line by line from a file, stores the contents of the file in an array and then calls a callback function with the array passed to the callback.

function readFile(callback)
{
    var fs = require('fs'), readline = require('readline');

    var rd = readline.createInterface({
        input: fs.createReadStream('./file.txt'),
        output: process.stdout,
        terminal: false
    });

    var data = [];

    rd.on('line', function(line) {
        data.push(line);
    });

    callback(data);
}

The problem I'm facing is that readFile is running the callback before the file is read and the data array is filled. So the callback is running with an empty array. How can I make it run once the array is fully filled? Thanks.

Upvotes: 0

Views: 1022

Answers (1)

Jaromanda X
Jaromanda X

Reputation: 1

readline has a close event which is fired on "end"

So, your code should be

function readFile(callback) {
    var fs = require('fs'), readline = require('readline');

    var rd = readline.createInterface({
        input: fs.createReadStream('./file.txt'),
        output: process.stdout,
        terminal: false
    });

    var data = [];

    rd.on('line', function(line) {
        data.push(line);
    }).on('close', function () {
        callback(data);
    });
}

in ES2015:

function readFile(callback) {
    var fs = require('fs'), readline = require('readline');

    var rd = readline.createInterface({
        input: fs.createReadStream('./file.txt'),
        output: process.stdout,
        terminal: false
    });

    var data = [];

    rd.on('line', line => data.push(line)).on('close', () => callback(data));
}

or even

function readFile(callback) {
    var fs = require('fs'), 
        readline = require('readline'),
        data = [];

    readline.createInterface({
        input: fs.createReadStream('./file.txt'),
        output: process.stdout,
        terminal: false
    })
    .on('line', line => data.push(line))
    .on('close', () => callback(data));
}

but I'm not sure which of the two ES2015 codes is more readable

Upvotes: 1

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