Synchronous calls in two nested loops

Question

Synchronous calls in two nested loops

How can I have Synchronous calls in two nested 'for' loops in Node.JS: Here is my Asynchronous call; I don't know how I can have synchronous version of my code, which goes to next iteration when create_db is done!

    var new_items = [];
    for (var key in config.models) {
       var selected_field = config.models[key];
       for (var item in config.models[key].fields) {
          var selected_item = config.models[key].fields[item];
          new_items.push({AttributeName: selected_item.name, AttributeType: selected_item.type});     
       }
       CreateDB(selected_field.name, new_items);       
       var selected_item = config.models[key].fields[item];

   }

EDDITED: In create_db I'm using call back function!

function CreateDB(name, new_items) {

ddb.createTable(name, {hash: ['id', ddb.schemaTypes().number],
    range: ['time', ddb.schemaTypes().string],
    AttributeDefinitions: new_items
},
{read: 1, write: 1}, function(err, details) {
    console.log("The DB is now created!");
});

}

Thanks

javascript
node.js

Answer 1

You're going to need to modify your CreateDB function so that it takes a callback and then call that callback in .createTables callback:

function CreateDB(name, new_items, callback) {
    ddb.createTable(name, {hash: ['id', ddb.schemaTypes().number],
        range: ['time', ddb.schemaTypes().string],
        AttributeDefinitions: new_items
    },
    {read: 1, write: 1}, function(err, details) {
        console.log("The DB is now created!");
        callback(err, details);
    });
}

Then you can use something like the async module's each method. Something like:

var async = require('async');

async.each(Object.keys(config.models), function(key, callback) {
   var new_items = [];
   var selected_field = config.models[key];
   for (var item in config.models[key].fields) {
      var selected_item = config.models[key].fields[item];
      new_items.push({AttributeName: selected_item.name, AttributeType: selected_item.type});     
   }
   CreateDB(selected_field.name, new_items, callback);
}, function(err) {
    // All done with CreateDB calls
});

Answer 2

You might try something like Futures , most especially their sequence code, here : sequence

Hope this helps!

Edit: I believe you can also use a more built-in approach with es6-promise. However, I have no knowledge on this... :)

Answer 3

I would recommend using the async module. It has utility functions that make what I think you're trying to do really easy.

async.each(Object.keys(config.models), function(key, done) {
    var selected_field = config.models[key];

    // I used the 'map' function to make it a bit easier to read
    var new_items = selected_field.fields.map(function(item) {
        return {
            AttributeName: item.name
            AttributeType: item.type
        };
    });

    CreateDB(selected_field, new_items, done);

}, function(err) {
    // this gets called when all db calls are finished
    console.log("The DB is now created!");
});

function CreateDB(name, new_items, callback) {
    ddb.createTable(name, {
        hash: ['id', ddb.schemaTypes().number],
        range: ['time', ddb.schemaTypes().string],
        AttributeDefinitions: new_items
    }, {
        read: 1,
        write: 1
    }, callback);
}