How to access variables when dealing with file system?

Question

var fs = require('fs');

for(var x = 0; x < 10; x++){
    fs.open("file.txt", 'a',function(err, fd){
        if(!err){
            fs.write(fd, x.toString() + "\n");
        }

    });
}

This prints out a bunch of 10s. I made the program to print out 0-9. For some reason, the code inside fs.open cannot access the x variable correctly. Can anybody explain? Is there an issue with scope?