无法使用mysql和节点js返回选择查询值

Question

I am new in node js, Now I am trying to set a return value of select query in mysql using node js....I am using node-mysql package...

example code

var mysql =  require('mysql');
var connection =  mysql.createConnection({
    host : "localhost",
    user : "root",
    password: "root123",
    database: "testdb"
});

var retValue = undefined;

var query = connection.query('SELECT * FROM tblData;');
query
    .on('error', function(err) {
        // Handle error, an 'end' event will be emitted after this as well
    })
    .on('fields', function(fields) {
        // the field packets for the rows to follow
    })
    .on('result', function(row) {
        // Pausing the connnection is useful if your processing involves I/O
        connection.pause();
        processRow(row, function() {
            retValue = row;
        });
    })
    .on('end', function(row) {

    });

    connection.end();

    function processRow(rows)
    {
        retValue = rows;
    }

    console.log(retValue);

retValue is always undefined. I know it is asynchronous call. anyone tell me how to set value for this variable.

Thanks Deepak

Answer 1

Since database query is an asynchronous opreation, your variable retValue has not been set yet at the time you call console.log(retValue) .

var retValue;

var query = connection.query('SELECT * FROM tblData;');
query
    .on('error', function(err) {
        // Handle error, an 'end' event will be emitted after this as well
    })
    .on('fields', function(fields) {
        // the field packets for the rows to follow
    })
    .on('result', function(row) {
        // Pausing the connnection is useful if your processing involves I/O
        connection.pause();
        processRow(row);
        console.log(retValue); //retValue is now set
    })
    .on('end', function(row) {

    });

    connection.end();

    function processRow(rows)
    {
        retValue = rows;
    }

    console.log(retValue); // undefined, retValue has not been set yet

Answer 2

The retvalue will available only in scope, that your put in result secion. So you can make something like that to log your result:

var connection =  mysql.createConnection({
    host : "localhost",
    user : "root",
    password: "root123",
    database: "testdb"
});

var retValue = undefined;

var query = connection.query('SELECT * FROM tblData;', function(err, results, fields) {
    console.log(results);
    // Here continue logic you needed with results from table
});

Answer 3

Move the console.log inside function processRow(rows) so that it looks like

function processRow(rows)
{
    retValue = rows;
    console.log(retValue);
}

also change

processRow(row, function() {
    retValue = row;
});

To

processRow(row);

The second parameter was unused.

Answer 4

i can't get the result out of scope...i want to assign and return the value like this

fucntion() getResults{ ... ... var retValue = undefined;

var query = connection.query('SELECT * FROM tblData;', function(err, results, fields) { retValue = results; });

return retValue };

var ret = getResults(); console.log(ret);