I am using the Q library of node.js . I am trying to print a report which would print the query name and the result . This is what I have . The code prints "In function undefined . How do I access the value of the promise object from within the "then" function ?
var queries = ["2091 OR 2092 OR 2093",
"2061 OR 2062",
"2139 OR 2140 OR 2141"
];
var promises = new Array();
for (var i=0; i<queries.length; i++) {
promises[i]=performSearch(queries[i]);
promises[i].query = queries[i];
console.log("Outside function ", promises[i].query);
promises[i].then(function(data) {
console.log("In function ", this.query);
processSearchResults(data,this.query);
});
}
Q.allSettled(promises).then(function(results) {
endFunction();
});
This is what I have:
promises[i].then(function(data) {
console.log("In function ", this.query);
processSearchResults(data,this.query);
});
The code prints "In function undefined".
The spec mandates that the callback is called with nothing for the this value, so this will refer to the global (window) object in sloppy mode, which does not have a .query property. In strict mode you'd have gotten an exception when this was undefined.
How do I access the value of the promise object from within the "then"
function ?
There is no special method. You usually will not need to access a promise as an object, it's only a transparent value representing the single result of an asynchronous computation. All you need to do with it is to call its .then() method - and inside the callback, there's no reason to access the promise object, because the information it holds is already accessible to you (data, and the fact that the fulfillment callback was invoked).
So if you want to access your .query property, you'd have to use promises[i] as usual. However, you will need a closure for i so you'd better use map anyway and hold the query string in the closure directly instead of making it a property on a promise object:
var queries = ["2091 OR 2092 OR 2093",
"2061 OR 2062",
"2139 OR 2140 OR 2141"
];
var promises = queries.map(function(query) {
var promise = performSearch(query);
console.log("Outside function ", query);
var processedPromise = promise.then(function(data) {
console.log("In function ", query);
return processSearchResults(data, query);
});
return processedPromise; // I assume you don't want the unprocessed results
});
Q.allSettled(promises).then(function(processedResults) {
endFunction(processedResults);
});
Related
I am very new to node.js and here is my problem
I have a nested set of maps here(I have tried using for-each as well).
All the "get..." methods nested are returning a promise defined by me. I have to loop over the data returned by the methods continuously and use it to call the next "get.." method(there is a dependency). After completing the entire nested loops, the "allobj" array should have my final result and I am trying to return it once all the looping is done. I have not had any luck so far, as an empty array gets returned.
var allobj = new Array();
var catcount = 0;
//categories.forEach(function(category){
var p = categories.map(function(category,index,array){
catcount++;
return gettests(runid,category).then(function(tests){
//console.log(tests.sort().reverse()); //this is sync
//tests.forEach(function(test){
var t = tests.map(function(test,index,array){
return getmetriccategories(runid,category,test).then(function(data){
//console.log(data);
//data.forEach(function(mc){
var d = data.map(function(mc,index,array){
return getmetrics(runid,category,test,mc).then(function(data1){
var x = data1.map(function(metric,index,array){
allobj.push(metric);
return metric;
});
});
});
})
})
})
})
//return when all nested loops are done
res.send(allobj);
Your functions getmetriccategories and getmetrics return Promises, at least, their return values seem to have a then method which is an indicator for that. This means, they work asynchronously. So, the map calls don't return the results directly, but an array of Promises. To wait for all of these Promises to be fulfilled (which means, the asynchronous functions are completed), you can use Promise.all function:
Promise.all(data.map(...))
.then(function (result) {
res.send(result)
})
As you see, Promise.all returns a new Promise, so you can use then to receive the result and send it with res.send.
I have a function,
asdf() {
var a = fooController.getOrCreateFooByBar(param);
console.log("tryna do thing");
console.log(a); //undefined
if (!a.property) {
//blah
}
that dies. getOrCreateFooByBar does a
Model.find({phoneNumber : number}).exec()
.then(function(users) {})
and finds or creates the model, returning it at the end:
.then(function(foo) { return foo}
How can I use the result of this in asdf()? I feel like this is a fairly easy question but I am getting stuck. If I try to do a.exec() or a.then() I get 'a cannot read property of undefined' error.
The main idea about Promises (as opposed to passed callbacks) is that they are actual objects you can pass around and return.
fooController.getOrCreateFooByBar would need to return the Promise it gets from Model.find() (after all of the processing done on it). Then, you'll be able to access it in a in your asdf function.
In turn, asdf() should also return a Promise, which would make asdf() thenable as well. As long as you keep returning Promises from asynchronous functions, you can keep chaining them.
// mock, you should use the real one
const Model = { find() { return Promise.resolve('foo'); } };
function badExample() {
Model.find().then(value => doStuff(value));
}
function goodExample() {
return Model.find().then(value => doStuff(value));
}
function asdf() {
var a = badExample();
var b = goodExample();
// a.then(whatever); // error, a is undefined because badExample doesn't return anything
return b.then(whatever); // works, and asdf can be chained because it returns a promise!
}
asdf().then(valueAfterWhatever => doStuff(valueAfterWhatever));
As a relative beginning in Javascript development, I'm trying to understand this problem I'm encountering in a function I've built that will be a part of the code to connect to a postgreSQL database.
In this function, I'm using the knex query builder method to check if a table exists in a remote database, and this method resolves to a boolean that indicates whether the string you specified matches with a table of the same name in the database. I've a provided a sample example of the knex syntax so people better understand the function.
knex.schema.hasTable('users').then(function(exists) {
if (!exists) {
return knex.schema.createTable('users', function(t) {
t.increments('id').primary();
t.string('first_name', 100);
t.string('last_name', 100);
t.text('bio');
});
}
});
I am trying to make my function return a boolean by using the .every Array method, which checks each array and should every index pass the condition defined, the .every method should return a true value, otherwise false. I've built a function which takes an array of schema keys, or names of tables, and passes it to the .every method. The .every method then uses the knex.schema.hasTable method to return a true or false.
My concern is that through many different configurations of the function, I have not been able to get it to return a correct value. Not only does it return an incorrect value, which may have something to do with .every, which I believe can return "truthey" values, but after defining the function, I will often get a "Function undefined" error when calling it later in the file. Here is a sample of my function - again I think it is moreso my poor understanding of how returns, promises and closures are working together, but if anyone has insight, it would be much appreciated.
var schemaTables = ['posts','users', 'misc'];
// should return Boolean
function checkTable() {
schemaTables.every(function(key) {
return dbInstance.schema.hasTable(key)
.then(function(exists) {
return exists;
});
});
}
console.log(checkTable(), 'checkTable function outside');
// console.log is returning undefined here, although in other situations,
I've seen it return true or false incorrectly.
Your function is not working properly for two reasons:
You are not returning the in the checkTable function declaration, so it will always return undefined.
You should write:
function checkTable() {
return schemaTables.every(function(key) {
return dbInstance.schema.hasTable(key)
.then(function(exists) {
return exists;
});
});
}
Anyway you will not get what you want just adding return. I'll explain why in the second point.
Array.prototype.every is expecting a truthy or falsey value syncronously but what the dbInstance.schema.hasTable returns is a Promise object (and an object, even if empty, is always truthy).
What you have to do now is checking if the tables exist asynchronously, i'll show you how:
var Promise = require("bluebird");
var schemaTables = ['posts', 'users', 'misc'];
function checkTable(tables, callback) {
// I'm mapping every table into a Promise
asyncTables = tables.map(function(table) {
return dbInstance.schema.hasTable(table)
.then(function(exists) {
if (!exists)
return Promise.reject("The table does not exists");
return Promise.resolve("The table exists");
});
});
// If all the tables exist, Promise.all return a promise that is fulfilled
// when all the items in the array are fulfilled.
// If any promise in the array rejects, the returned promise
// is rejected with the rejection reason.
Promise.all(asyncTables)
.then(function(result) {
// i pass a TRUE value to the callback if all the tables exist,
// if not i'm passing FALSE
callback(result.isFulfilled());
});
}
checkTable(schemaTables, function (result) {
// here result will be true or false, you can do whatever you want
// inside the callback with result, but it will be called ASYNCHRONOUSLY
console.log(result);
});
Notice that as i said before, you can't have a function that returns a true or false value synchronously, so the only thing you can do is passing a callback to checkTable that will execute as soon as the result is ready (when all the promises fulfill or when one of them rejects).
Or you can return Promise.all(asyncTables) and call then on checkTable it self, but i'll leave you this as exercise.
For more info about promises check:
The bluebird website
This wonderful article from Nolan Lawson
Thanks Cluk3 for the very comprehensive answer. I actually solved it myself by using the .every method in the async library. But yes, it was primarily due to both my misunderstanding regarding returns and asynchronous vs synchronous.
var checkTablesExist = function () {
// make sure that all tables exist
function checkTable(key, done) {
dbInstance.schema.hasTable(key)
.then(function(exists) {
return done(exists);
});
}
async.every(schemaTables, checkTable,
function(result) {
return result;
});
};
console.log(checkTablesExist());
// will now print true or false correctly
I'm new to Node.js, so apologies if my question sounds somehow stupid.
Here's what I'm heading for:
I'm designing an object of which one property has to be set by database query. For this task I'm requiring promise-mysql. In order to provide the property to other objects, I would like to have a getter that checks weather the property is set or not and in the latter loads the property from the database before returning its value.
My approach so far:
MyObject = function ()
{
this._property = null;
};
MyObject.prototype._loadProperty = function()
{
var self = this;
return pool.query('SELECT * FROM table WHERE condition = ?', [condition]).then(function(rows)
{
return self._property = rows;
}).catch(function(err)
{
console.log(err);
});
};
MyObject.prototype._getProperty = function()
{
return this._property || this._loadProperty();
}
So far the getter is returning a promise object if the property has not been set/loaded yet. I can handle this promise object with ...then() in the calling function, but I would like the getter to hold back the return value until the promise has settled. Is this even possible?
Regards
Greg
Your interface should probably just always return a promise. Then the caller can just do:
this.getProperty().then(...)
Their interface will be the same whether the property was previously cached or not.
In fact, you can even use the promise internally as your caching mechanism.
MyObject = function () {
this._propertyPromise = null;
};
MyObject.prototype._loadProperty = function () {
var self = this;
return pool.query('SELECT * FROM table WHERE condition = ?', [condition]).catch(function (err) {
console.log(err);
throw err; // propagate reject error back to caller
});
};
MyObject.prototype.getProperty = function () {
if (!this._propertyPromise) {
this._propertyPromise = this._loadProperty();
}
return this._propertyPromise;
}
A reason for doing it this way is that you don't want the caller to have to know or do anything differently if the property is being fetched asynchronously or is already cached. The only way to offer the same interface for both is to make the interface always be async and just returning the promise gives you that automatically.
FYI, in the code you showed, you were using .catch() to log an error, but not rethrowing the error. That would "silently" eat the error and it would not be returned back to the caller. The caller would instead see a fulfilled promise with an undefined value. If you want to do your own .catch() for logging reasons, then you must rethrow the error (or return a rejected promise) to make sure it propagates back to the caller. A .catch() all by itself with no return value will just change the promise into a resolved promise because it assumes that you've handled the error.
I need to use bluebird in my code and I have no idea how to use it. My code contains nested loops. When the user logs in, my code will run. It will begin to look for any files under the user, and if there are files then, it will loop through to get the name of the files, since the name is stored in a dictionary. Once it got the name, it will store the name in an array. Once all the names are stored, it will be passed along in res.render().
Here is my code:
router.post('/login', function(req, res){
var username = req.body.username;
var password = req.body.password;
Parse.User.logIn(username, password, {
success: function(user){
var Files = Parse.Object.extend("File");
var object = [];
var query = new Parse.Query(Files);
query.equalTo("user", Parse.User.current());
var temp;
query.find({
success:function(results){
for(var i=0; i< results.length; i++){
var file = results[i].toJSON();
for(var k in file){
if (k ==="javaFile"){
for(var t in file[k]){
if (t === "name"){
temp = file[k][t];
var getname = temp.split("-").pop();
object[i] = getname;
}
}
}
}
}
}
});
console.log(object);
res.render('filename', {title: 'File Name', FIles: object});
console.log(object);
},
error: function(user, error) {
console.log("Invalid username/password");
res.render('logins');
}
})
});
EDIT:The code doesn't work, because on the first and second console.log(object), I get an empty array. I am suppose to get one item in that array, because I have one file saved
JavaScript code is all parsed from top to bottom, but it doesn't necessarily execute in that order with asynchronous code. The problem is that you have the log statements inside of the success callback of your login function, but it's NOT inside of the query's success callback.
You have a few options:
Move the console.log statements inside of the inner success callback so that while they may be parsed at load time, they do not execute until both callbacks have been invoked.
Promisify functions that traditionally rely on and invoke callback functions, and hang then handlers off of the returned value to chain the promises together.
The first option is not using promises at all, but relying solely on callbacks. To flatten your code you will want to promisify the functions and then chain them.
I'm not familiar with the syntax you're using there with the success and error callbacks, nor am I familiar with Parse. Typically you would do something like:
query.find(someArgsHere, function(success, err) {
});
But then you would have to nest another callback inside of that, and another callback inside of that. To "flatten" the pyramid, we make the function return a promise instead, and then we can chain the promises. Assuming that Parse.User.logIn is a callback-style function (as is Parse.Query.find), you might do something like:
var Promise = require('bluebird');
var login = Promise.promisify(Parse.User.logIn);
var find = Promise.promisify(Parse.Query.find);
var outerOutput = [];
return login(yourArgsHere)
.then(function(user) {
return find(user.someValue);
})
.then(function(results) {
var innerOutput = [];
// do something with innerOutput or outerOutput and render it
});
This should look familiar to synchronous code that you might be used to, except instead of saving the returned value into a variable and then passing that variable to your next function call, you use "then" handlers to chain the promises together. You could either create the entire output variable inside of the second then handler, or you can declare the variable output prior to even starting this promise chain, and then it will be in scope for all of those functions. I have shown you both options above, but obviously you don't need to define both of those variables and assign them values. Just pick the option that suits your needs.
You can also use Bluebird's promisifyAll() function to wrap an entire library with equivalent promise-returning functions. They will all have the same name of the functions in the library suffixed with Async. So assuming the Parse library contains callback-style functions named someFunctionName() and someOtherFunc() you could do this:
var Parse = Promise.promisifyAll(require("Parse"));
var promiseyFunction = function() {
return Parse.someFunctionNameAsync()
.then(function(result) {
return Parse.someOtherFuncAsync(result.someProperty);
})
.then(function(otherFuncResult) {
var something;
// do stuff to assign a value to something
return something;
});
}
I have a few pointers. ... Btw tho, are you trying to use Parse's Promises?
You can get rid of those inner nested loops and a few other changes:
Use some syntax like this to be more elegant:
/// You could use a map function like this to get the files into an array of just thier names
var fileNames = matchedFiles.map(function _getJavaFile(item) {
return item && item.javaFile && item.javaFile.name // NOT NULL
&& item.javaFile.name.split('-')[0]; // RETURN first part of name
});
// Example to filter/retrieve only valid file objs (with dashes in name)
var matchedFiles = results.filter(function _hasJavaFile(item) {
return item && item.javaFile && item.javaFile.name // NOT NULL
&& item.javaFile.name.indexOf('-') > -1; // and has a dash
});
And here is an example on using Parse's native promises (add code above to line 4/5 below, note the 'then()' function, that's effectively now your 'callback' handler):
var GameScore = Parse.Object.extend("GameScore");
var query = new Parse.Query(GameScore);
query.select("score", "playerName");
query.find().then(function(results) {
// each of results will only have the selected fields available.
});