Issue with variable scope in javascript/NodeJS

Question

Im having issues with accessing the value of a variable in NodeJS. Here is some example code and the results I get.

for (var z=0, zMessageCount = Description.length; z<zMessageCount; z++){
    console.log(z);
    if(SomeOtherColumnValue[z] > 9){
        client.query('SELECT * FROM my_table WHERE some_column=' + ColumnValue[z], function (err, results) {
            if(results < 1){
                console.log(z);
            }
        })
    }
}

Here is the issue I am having. In for "for" loop the value of z is going from 0 to 14. However when I try to access it from the client.query function its value is 15. It is not adding 1 to itself for every loop. Is there something I am missing here?

z should be fine in that scope. Hmm, wild guess but what does client.query actually do with the string you pass it? I have a feeling it sends it to another non-blocking callback function and it's being executed after your loop finishes, rather than write away. What library is this? — Bartek
– Bartek, Commented Sep 19, 2011 at 4:07
client.query is part of node-mysql. It does appear as if it is... Is there a different way I should be doing this? — DvideBy0
– DvideBy0, Commented Sep 19, 2011 at 4:10
Might be wrong, but you could try var columnName = ColumnName[z]; and pass that into client.query. Defining columnName after your if statement. This might help from the z being passed as a reference into client.query — Bartek
– Bartek, Commented Sep 19, 2011 at 5:12
The anonymous function (the third param in client.query) creates a closure which allows it to see the value of 'z' as it continues to get incremented. +1 to generalhenry's solution below. — Kai
– Kai, Commented Sep 19, 2011 at 5:21

generalhenry · Accepted Answer · 2011-09-19 05:14:08Z

4

You need to wrap z in a function to provide scope.

for (var z=0, zMessageCount = Description.length; z<zMessageCount; z++){
    console.log(z);
    if(SomeOtherColumnValue[z] > 9){
      zQuery(z);  
    }
}

function zQuery (z) {
  client.query('SELECT * FROM my_table WHERE some_column=' + ColumnValue[z], function (err, results) {
      if(results < 1){
         console.log(z);
      }
   });
}

answered Sep 19, 2011 at 5:14

generalhenry

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DvideBy0 Over a year ago

That worked. Thank you very much. I spent hours trying to figure out where I went wrong.

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