CODEWITHSUNDEEP
javascriptmysqlnode.js
Pavel Trefilov
Pavel Trefilov

Reputation: 11

Node.js - mysql query inside foreach

I have this structure of code:

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    rows.forEach(function(item) {
        connection.query(queryItem,function(err,rows) {
            //doing something
            result = rows[0].field;
            //and want to push it to an array
            response.push(result);
        });
    });
    console.log(response); //empty
});

I know that forEach is blocking but query is non-blocking. I tried to use promises:

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    rows.forEach(function(item) {
        var promise = new Promise(function(resolve,reject) {
            connection.query(queryItem,function(err,rows) {
                //doing something
                result = rows[0].field;
                //and want to push it to an array
                resolve(result);
            });
        });
        promise.then(function(result) {
            console.log(result); //ok
            response.push(result) //not ok, result is empty
        });
    });
    console.log(response); //empty
});

But it's not helped. How can I push value into array from non-blocking function and use it after?

Upvotes: 1

Views: 4984

Answers (1)

Kevin B
Kevin B

Reputation: 95017

Creating promises is a step in the right direction, but you need to take it a step further by aggregating the promises into an array and passing to Promise.all, and then waiting for them all to finish before trying to access response.

connection.query(query1,function(err,rows) {
    var response = [];
    //doing something with rows
    Promise.all(rows.map(function(item) {
        var promise = new Promise(function(resolve,reject) {
            connection.query(queryItem,function(err,rows) {
                //doing something
                result = rows[0].field;
                //and want to push it to an array
                resolve(result);
            });
        });
        return promise.then(function(result) {
            console.log(result); //ok
            response.push(result) //ok
        });
    }).then(function () {
        console.log(response); //not empty
    });
});

Upvotes: 4

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