Node JS中的非阻塞

Question

I define a method that will get a value from the Database, I named it as getParam () as the following:

function getParam( tableName,  paramName, id){
var param=0;
var query = client.query('SELECT '+ paramName + ' AS myparam FROM  ' + tableName + ' where contest_id = ' + id);
  query.on('row', function(row) {
    param= row.myparam;
          });

return param;   

}

But I have a problem; the callback function param= row.myparam; it returns param as undefined then it executes the callback function function(row) { param= row.myparam;} .

However I want the callback function to run first then it returns the value in param. How can I do it since Node JS is asynchronous?

Answer 1

Unless your database client has a synchronous mode - which is a bad idea in node anyways - you have to do it asynchronous, either with a callback or a promise:

function getParam( tableName,  paramName, id, callback){
  var param=0;
  var query = client.query('SELECT '+ paramName + ' AS myparam FROM  ' + tableName + ' where contest_id = ' + id);
  query.on('row', function(row) {
      param= row.myparam;
      // call the callback from here
      callback(param);
  });
  // removed your return statement, since there is nothing to return yet
  // return param;   
}

And call it:

getParam("mytable","myparam","256", function(param) {
   // do what you want with param here
});

If you prefer a promise, then you can do it promise-style (any of several libraries is fine):

function getParam( tableName,  paramName, id){
  var param=0, defer = makeDeferSomehow();
  var query = client.query('SELECT '+ paramName + ' AS myparam FROM  ' + tableName + ' where contest_id = ' + id);
  query.on('row', function(row) {
      param= row.myparam;
      // call the callback from here
      defer.resolve(param);
  });
  // removed your return statement, since there is nothing to return yet
  // return param;   
  // instead return the promise
  return defer.promise;
}

And then you would call it:

getParam("mytable","myparam","256").then(function(param) {
   // do what you want with param here
});