node js：使用相同的参数名在另一个函数内执行一个函数

Question

I have a beginners question regarding node.js / javascript syntax and asynchronous calls. I'm using node-dbi to pull some information out of a MySQL Server.

I have 2 tables. Client and Zone .

I need to write a function that does this:

for (i=0;<zone.count;i++){
  for (j=0;j<client.count;j++){
     //loop through the entire client table for each zone record & run a condition
  }
}

Here is what my syntax in node-dbi looks like:

db.fetchAll('SELECT * from Zone', null, function(err, result){
    if (result) {
        db.fetchAll('SELECT * from Client', null, function(err, result){
            if (result) {
                //do something to all client records for each zone record
            }
        });
    }
});

As it's immediately obvious, my result and err variables are clashing.. Can someone please explain the syntax to solve this asynchronous function?

Thanks!

Answer 1

Give each variable a name specific to that function:

db.fetchAll('SELECT * from Zone', null, function(zoneErr, zoneResult){
    if (zoneResult) {
        db.fetchAll('SELECT * from Client', null, function(clientErr, clientResult){
            if (clientResult) {
                //do something to all client records for each zone record
            }
        });
    }
});

Also, you should refactor that a bit to make it more readable and remove that deep nesting.

Here's one way to think about it:

var getZones = function() {
  var result;
  db.fetchAll('SELECT * from Zone', null, function(zoneErr, zoneResult){
     if (zoneResult) {
       callback();
       result = true; // assumes you need to keep track of success of failure of result
     } else {
       result = false;
     }
  }
  return result;
};


var getClients { ... same code as above for clients };

getZones(getClients);

But that refactoring is off the cuff - it may not fit in your situation.

The problem should be solved by the first piece of code

Answer 2

Either rename the err and result into errZone,resultZone /errClient,resultClient as Squadrons suggested, or use async https://npmjs.org/package/async

// WARNING UNTESTED CODE
// npm install async
// or add async into your package.json dependencies
var async = require('async');
async.parallel({ 
        zone: function (callback) {
            db.fetchAll('SELECT * from Zone', null, function (err, result) {
                if (err || !result) {
                    callback(err || 'No Zone results found', result)
                }
            });
        },
        client: function (callback) {
            db.fetchAll('SELECT * from Client', null, function (err, result) {
                if (err || !result) {
                    callback(err || 'No Client results found', result)
                }    
            });
        }
    },
    function (err, results) {
        if (err) throw err;
        for (var i = 0; i < results.zone.count; i++) {
            for (var j = 0; j < results.client.count; j++) {
                //loop through the entire client table for each zone record & run a     condition
            }
        }
    });