How can implement the function as asynchronous in nodejs

Question

Let me explain my issue. I have a foreach loop and a switch condition inside it. In every switch case, I have concat the results in to a global array.

var resultArray = [];
Panels.forEach(function (panel, index) {
 switch( panel.CategoryId ){
   case 'math': 
      resultArray = resultArray.concat(SOMEVALUE COMES FROM DB);
   break;
   case 'physics': 
      resultArray = resultArray.concat(SOMEVALUE COMES FROM DB);
   break;
   case 'zoology': 
      resultArray = resultArray.concat(SOMEVALUE COMES FROM DB);
   break;
 }

 return resultArray;
})

But the output doesn't contain the values from all cases. I know its because of the asynchronous nature of nodejs. But how can we implement this function in to asynchronous?

Any ideas would be appreciable

Is the query performed before or after switch condition?

Matteo Rubini
– Matteo Rubini

2015-03-03 09:50:38 +00:00
Commented Mar 3, 2015 at 9:50 — Matteo Rubini
– Matteo Rubini, Commented Mar 3, 2015 at 9:50
@MatteoRubini Query performed inside the case condition

Manu
– Manu

2015-03-03 09:55:52 +00:00
Commented Mar 3, 2015 at 9:55 — Manu
– Manu, Commented Mar 3, 2015 at 9:55

Matteo Rubini · Accepted Answer · 2015-03-03 10:13:43Z

0

To create a similar-sync program flow you must use a callback pattern in your design:

functionPerformingQuery(function(result){
    handleResult(result);
});

main-idea is:

create a function accepting a callback-function

call the callback-function when operation is completed

define callback-function inside your code to access all private variables

more standardized method is promise()

https://www.promisejs.org/

answered Mar 3, 2015 at 10:13

Matteo Rubini

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Comments

Vadym Kaptan · Accepted Answer · 2015-03-03 10:16:31Z

0

in the foreach loop, there is no need in return. try it: var resultArray = []; Panels.forEach(function (panel, index) { switch( panel.CategoryId ){ case 'math': resultArray = resultArray.concat(SOMEVALUE COMES FROM DB); break; case 'physics': resultArray = resultArray.concat(SOMEVALUE COMES FROM DB); break; case 'zoology': resultArray = resultArray.concat(SOMEVALUE COMES FROM DB); break; } }) console.log(resultArray);

answered Mar 3, 2015 at 10:16

Vadym Kaptan

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Comments

Aethiss · Accepted Answer · 2015-03-03 11:10:24Z

on my opinion, cuz the request on db is async, you need perform the result using a promise Jquery Promise

var resultArray = [];
Panels.forEach(function (panel, index) {
 switch( panel.CategoryId ){
   case 'math': 
      myDBfunction().then(function(result) {
         resultArray = resultArray.concat(result);
      });
   break;
   case 'physics': 
      myDBfunction().then(function(result) {
         resultArray = resultArray.concat(result);
      });
   break;
   case 'zoology': 
      myDBfunction().then(function(result) {
         resultArray = resultArray.concat(result);
      });
   break;
 }

 return resultArray;
})

function myDBfunction() {

    var deferred = new jQuery.Deferred();

    // do some with DB (not real code)
    mysql.query("SELECT * ....", function(dbError, dbResult) {
      deferred.resolve(dbResult);
    })

  return deferred.promise()
}

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