CODEWITHSUNDEEP
node.js
user4595537
user4595537

Reputation: 3

Synchronous readline in Nodejs

I am making POST request to Nodejs server (with Express). In POST processing, I need to read data from 3 different files and push them in 3 array. However, it seems like that res.end() occurs before the reading finishes. Can someone help fix this please, this is my process function. (response = [[],[],[]])

app.post('/process_post', urlencodedParser, function(req, res) {

    var readMail = readline.createInterface({
        input : fs.createReadStream(path.join(process.cwd(), '/mail.txt'))
    });
    var readSubject1 = readline.createInterface({
        input : fs.createReadStream(path.join(process.cwd(), '/subject1.txt'))
    });
    var readSubject2 = readline.createInterface({
        input : fs.createReadStream(path.join(process.cwd(), '/subject2.txt'))
    });

        readMail.on('line', function(line) { 
            email.push(line);
        });
        readMail.on('close', function() {
            // resolve(email);
            console.log('finish mail');

        });
        readSubject1.on('line', function(line) { sub1.push(line) }).on('close', function() {
            console.log('finish subject1');
        });
        readSubject2.on('line', function(line) { sub2.push(line) }).on('close', function() {
            console.log('finish subject2');
        });


    var response = [email, sub1, sub2];
    res.end(JSON.stringify(response));
});

Upvotes: 0

Views: 2299

Answers (1)

Semih Gokceoglu
Semih Gokceoglu

Reputation: 1428

You have to use async process. Basically, callbacks are the key of many nodejs methods. I strongly suggest you learn promises, callbacks or some libraries like async . Anyway, I added promises to your code. You should return your res.end function inside then after promise.all.

var p1 = new Promise((resolve, reject) => {
    readMail.on('line', function(line) {
        email.push(line);
    });
    readMail.on('error', function(error) {
        reject(error)
    });
    readMail.on('close', function() {
        resolve(email);
    });
});

var p2 = new Promise((resolve, reject) => {
    readSubject1.on('line', function(line) {
        sub1.push(line)
    }).on('close', function() {
        resolve(sub1);
    }).on('error', function(err) {
        reject(err);
    })
});

var p3 = new Promise((resolve, reject) => {
    readSubject2.on('line', function(line) {
        sub2.push(line)
    }).on('close', function() {
        resolve(sub2);
    }).on('error', function(err) {
        reject(err);
    })
});

Promise.all([p1, p2, p3])
    .then(values => {
        console.log('values [p1, p2, p3]');
        res.end(values);
    })
    .catch(err => {
        console.log(err);
    })

Upvotes: 2

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