mysql/node.js how to make/fake synchronous?

Question

My problem is as follows :

I have many Mysql requests to do in Node, and it's done asynchronously.

In the following example, I would like to wait for the checkExists function to finish one way or another (and populate my input variable) before the function doStuffWithInput starts. I don't see any other way than pasting doStuffWithInput multiple times in the various possible callbacks (after each 'input=keys;') ... I'm sure there is a better way though. Any ideas?

var input;

db.checkExists_weekParents(id,function(count){ //check table existence/number of rows
    if(count!==='err'){ //if doesnt exist, create table
        db.create_weekParents(id,function(info){ 
            if(info!=='err'){ //as table is empty, create input from a full dataset
                db.makeFull_weekParents(id,function(keys){
                    input = keys;       
                });
            }
        });
    }else{ //if exists, check number of entries and create input keys as a subset of the full dataset
        db.makeDiff_weekParents(id,function(keys){
            if(keys.length!==0){
                input = keys;
            }else{ //if the table already has full dataset, we need to export and start again.
                db.export_weekParents(id,function(info){
                    db.create_weekParents(id,function(info){
                        if(info!=='err'){
                            db.makeFull_weekParents(id,function(keys){
                                input = keys;       
                            });
                        }
                    });
                });
            }
        });
    }
});

Once all this is done, we have lots of stuff to do (spawn child processes, more db operations, etc...)

doStuffWithInput(input,function(output){
    //Tons of stuff here
    console.log(output);
})

I really hope this is clear enough, I'll clarify if needed.

EDIT

Trying to rewrite using promises seems the best way to go, and I imagine it can be a great example for others like me struggling with pyramid of doom. So far I have :

    var Q = require('q');   
    function getInput(){
        var dfd = Q.defer();
        db.check_weekParents(id,function(count){
            console.log('count '+count);
            if(count==='err'){
                db.create_weekParents(id,function(info){
                    if(info!=='err'){
                        console.log('created table');
                        db.makeDiff_weekParents(id,function(keys){
                            input = keys;
                            dfd.resolve(input);
                        });
                    }
                });
            }else{
                db.makeDiff_weekParents(id,function(keys){
                    input=keys;
                    dfd.resolve(input);
                });
            }
        });
        return dfd.promise;
    }
    getInput().then(function (input) {
        console.log(input);
    });

It is magic!!

Explosion Pills · Accepted Answer

You can use promises rather than callbacks. There are many possibilities in node, and the mysql library you are using may even support them. For example with Q:

function getInput(){
    var dfd = Q.defer();
    if(count!==='err'){
        db.create_weekParents(id,function(info){
            /* after everything completes */
            dfd.resolve(input);

/* snip */

    return dfd.promise;
}

Then you can do

getInput().then(function (input) {
    doStuffWithInput(input ...
});

mysql/node.js how to make/fake synchronous?

Answers (2)

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