I am new to node.js.
I am trying to create function, where a randomly generated String is queried to check if it exists or not. If it already exists, the String is randomly generated till it is unique.
let validID = false;
console.log(temp); //temp is the randomly generated String.
while(!validID){
Website.findOne({shortcut: temp},function(err,docs){
if(docs==null){
validID = true;
console.log("The shortcut for the url is" + temp);
}else{
console.log("FOUND");
temp = generateId();
}
});
}
When run, the code is stuck in an infinite while loop.
I tried to see whether the code works with a String value ( not a variable ) passed in as the query inside findOne(). It worked. I am assuming that the fact that temp is a variable is causing the problem. Can variables be passed in as a value in a query? If so, what is the correct way?
Website.findOne operates asynchronously, i.e. the callback-function you passed to it, will be run once the results from the mongodb are fetched. However, node will not be able to actually process this callback, since your callstack never gets emptied due to your while-loop. If you're interested, you can find out more about this here.
One way to solve this is to wrap your Mongo-DB call in a promise, wait for it to resolve, then return if the ID is unique and continue by calling it recursively otherwise (note that this can be highly simplified by using async/await but for understanding how this works using promised are beneficial imo):
function findIdPromise(temp) {
return new Promise((resolve, reject) => {
Website.findOne({
shortcut: temp
}, function (err, docs) {
if (err) {
return reject(err);
}
resolve(docs);
});
});
}
function getNextIsUniqueIdPromise(shortcut) {
return findIdPromise()
.then(docs => {
if (docs == null) {
return shortcut;
}
return getNextIsUniqueIdPromise(generateId());
});
}
// call it initially with
getNextIsUniqueIdPromise(firstShortcutToCheck)
.then(shortcut => {
console.log("The shortcut for the url is" + shortcut):
})
.catch(err => {
console.log("an error occured", err):
});
Related
I'm writing a Windows Node.js server app (using ES6 btw).
The first thing I want to do - in the top-level code - is sit in a while loop, calling an async function which searches for a particular registry key/value. This function is 'proven' - it returns the value data if found, or else throws:
async GetRegValue(): Promise<string> { ... }
I need to sit in a while loop until the registry item exists, and then grab the value data. (With a delay between retries).
I think I know how to wait for an async call to complete (one way or the other) before progressing with the rest of the start-up, but I can't figure out how to sit in a loop waiting for it to succeed.
Any advice please on how to achieve this?
(I'm fairly new to typescript, and still struggling to get my head round all async/await scenarios!)
Thanks
EDIT
Thanks guys. I know I was 'vague' about my code - I didn't want to put my real/psuedo code attempts, since they have all probably overlooked the points you can hopefully help me understand.
So I just kept it as a textual description... I'll try though:
async GetRegValue(): Promise<string> {
const val: RegistryItem = await this.GetKeyValue(this.KEY_SW, this.VAL_CONN);
return val.value
}
private async GetKeyValue(key: string, name: string): Promise<RegistryItem> {
return await new Promise((resolve, reject) => {
new this.Registry({
hive: this.Hive, key
}).get(name, (err, items) => {
if (err) {
reject(new Error('Registry get failed'));
}
else {
resolve( items );
}
});
})
.catch(err => { throw err });
}
So I want to do something like:
let keyObtained = false
let val
while (keyObtained == false)
{
// Call GetRegValue until val returned, in which case break from loop
// If exception then pause (e.g. ~100ms), then loop again
}
}
// Don't execute here till while loop has exited
// Then use 'val' for the subsequent statements
As I say, GetRegValue() works fine in other places I use it, but here I'm trying to pause further execution (and retry) until it does come back with a value
You can probably just use recursion. Here is an example on how you can keep calling the GetRegValue function until is resolves using the retryReg function below.
If the catch case is hit, it will just call GetRegValue over and over until it resolves successfully.
you should add a counter in the catch() where if you tried x amount of times you give up.
Keep in mind I mocked the whole GetRegValue function, but given what you stated this would still work for you.
let test = 0;
function GetRegValue() {
return new Promise((resolve, reject) => {
setTimeout(function() {
test++;
if (test === 4) {
return resolve({
reg: "reg value"
});
}
reject({
msg: "not ready"
});
}, 1000);
});
}
function retryReg() {
GetRegValue()
.then(registryObj => {
console.log(`got registry obj: ${JSON.stringify(registryObj)}`)
})
.catch(fail => {
console.log(`registry object is not ready: ${JSON.stringify(fail)}`);
retryReg();
});
}
retryReg();
I don't see why you need this line:
.catch(err => { throw err });
The loop condition of while isn't much use in this case, as you don't really need a state variable or expression to determine if the loop should continue:
let val;
while (true)
{
try {
val = await GetRegValue(/* args */);
break;
} catch (x) {
console.log(x); // or something better
}
await delay(100);
}
If the assignment to val succeeds, we make it to the break; statement and so we leave the loop successfully. Otherwise we jump to the catch block and log the error, wait 100 ms and try again.
It might be better to use a for loop and so set a sensible limit on how many times to retry.
Note that delay is available in an npm package of the same name. It's roughly the same as:
await new Promise(res => setTimeout(res, 100));
I have quite a simple problem, but I can't find an elegant solution to fix this.
In the following code, I have two nested calls to a mongo DB. I use Monk to manage my calls.
The problem is : the for loop (1) loops before the nested insertion can happen. So the next find (2) instruction does not find the last inserted action.
The call order is 1-2-2-2-3-3-3 (for an actionList of size 3). So all my data is inserted.
My objective is to have the call order 1-2-3-2-3-2-3
Do you have any clue of how to manage such a problem, without making a big find on my database and manage my list server-side ? (Get all data, make myself the search, that is quite horrible to do, insert elements I want, then push it all to the db...)
for (var action of actionList)//(1)
{
collectionActions.find(//(2)
{eventid : action.eventid},
function(e,actionsFound)
{
if (actionsFound.length == 0)
{
collectionActions.insert(action, function(err, result)//(3)
{
console.log("insert action : " + action._id);
})
}
}
)
}
The native Promise object has an all method that could be leveraged to help.
Assuming find is a compliant promise, the following code would queue up all of the actions in an array through map and which would return a promise for each action that eventually returns messages to the final then for all.
A couple of notes: your code as it stands swallows all of the errors that might occur (I'm not sure that is want you want); this also assumes that insert returns a promise.
Promise.all([
// Iterate over actionList
actionList.map(function(action) {
// returns a promise with a then already attached
return collectionActions.find({
eventid: action.eventid
}).then(function(e, actionsFound) {
if (actionsFound.length == 0) {
// returns another promise that will resolve up to outer promises
return collectionActions.insert(action, function(err, result) {
// Finally resolve a value for outer promises
return 'insert action : ' + action._id;
});
} else {
// A different value to resolve with if the above promise
// is not required
return 'some other message for ' + action._id;
}
});
})
]).then(function(results) {
// Log out all values resolved by promises
console.log(results);
});
UPDATE: After the clarification of the question it sounds like you just need to chain the promises together rather than run them in parallel.
// Iterate over actionList
actionList.reduce(function(promise, action) {
// Chain promises together
return promise.then(function(results) {
return collectionActions.find({
eventid: action.eventid
}).then(function(e, actionsFound) {
if (actionsFound.length == 0) {
// returns another promise that will resolve up to outer promises
return collectionActions.insert(action, function(err, result) {
// Finally resolve a value for outer promises
return results.push('insert action : ' + action.sourceName);
});
} else {
// A different value to resolve with if the above promise
// is not required
return results.push('some other message for ' + action.sourceName);
}
});
});
}, Promise.resolve([])).then(function(results) {
// Log out all values resolved by promises
console.log(results);
});
I finally got my solution, by using a recursive function.
var currentIndex = 0;
var searchAndInsert = function(actionList)
{
var action = actionList[currentIndex];
if (typeof actionList[currentIndex] != "undefined")
{
collectionActions.find(
{eventid : action.eventid},
function(e,actions)
{
console.log("find ended")
if (actions.length == 0)
{
collectionActions.insert(action, function(err, result)
{
console.log("insert action : " + action.sourceName);
currentIndex++;
if (typeof actionList[currentIndex] != "undefined")
searchAndInsert(actionList);
})
}
else
{
currentIndex++;
if (typeof actionList[currentIndex] != "undefined")
searchAndInsert(actionList);
}
}
)
}
};
Followup from this question > Stopping response if document isn't found since it was recommended I use Promise.
So basic premise, I want node to return "Can't find ID" message if we can't find the id in our database.
v1.post("/", function(req, res) {
// If the project_id isn't provided, return with an error.
if ( !("project_id" in req.body) ) {
return res.send("You need to provide Project ID");
}
// Check if the Project ID is in the file.
helper.documentExists( ProjectsData, {project_id: req.body.project_id} )
.then(function(c) {
if ( c == 0 ) {
return res.send("The provided Project Id does not exist in our database.");
} else {
var gameDataObj = req.body;
GameData.addGameId(gameDataObj, function (err, doc) {
if (err) {
if (err.name == "ValidationError") {
return res.send("Please send all the required details.");
}
throw err;
};
res.json(doc);
})
};
});
});
And helper.documentExists
module.exports = {
documentExists: function(collection, query) {
return collection.count( query ).exec();
},
};
But the script continues to run after this and prints the "required data not found".
Output:
required data not found
1
I am using native ES6 Promises.
var mongoose = require("mongoose");
mongoose.Promise = global.Promise;
EDIT: Included the entire get route. (will fix those throw err later)
#######POINT 1#########
ProjectsData.count( {project_id: req.body.project_id} )
.then(function(c) {
#######POINT 3#########
if ( c == 0 ) {
console.log("1");
return res.send("The provided Project Id does not exist in our database.");
console.log("2");
}
});
#######POINT 2#########
//some other logic
console.log("required data not found");
Following async workflow: after POINT 1, the promise is created and your handler is attached. Now POINT 2 will continue, while (at some future clock the promise is resolved and you reach POINT 3.
With my limited understanding of your workflow/purpose I'd say simply put POINT 2 code in the else{} of the if at POINT 3 (as you rightly guessed in the comments).
EDIT: thanks to #jfriend00 for pointing out a serious mistake in the previous version of my answer.
Your code essentially results in this:
ProjectsData.count().then(...);
console.log("required data not found");
So, of course the second console.log() is going to run and print. Nothing that happens in the .then() handler runs until long after the console.log() has already run. And, even then, it can't stop other code from running. Promises don't make the interpreter "wait". They just provide structure for you to coordinate your asynchronous operations.
If you want to branch with promises, then you have to branch inside the .then() handler, not after it.
You don't show enough of the rest of what you're doing to know how to recommend a complete solution. We need to see the rest of your request in order to help you with the proper branching based on asynchronous results.
You probably need something like this:
ProjectsData.count( {project_id: req.body.project_id} ).then(function(c) {
if ( c == 0 ) {
return res.send("The provided Project Id does not exist in our database.");
} else {
// put other logic here
}
}).catch(function(err) {
// handle error here
});
For a Node API I must generate a random alphanumeric key, this should be unique and SHORT (I can't use neither uuid or Mongo ObjectID).
I thought this logic:
Generate key,
Query MongoDB for key existance
If key exists, repeat the process,
If key doesn't exist, assign it and respond to client.
I tried then:
do {
key = randomKey(8);
newGroup.key = key;
Group.findOne({numberId: key}).then(function (foundGroup) {
console.log("cb");
if (! foundGroup) {
console.log("not found")
notUnique = false;
}
}).catch(function (err) {
return response.send(500);
});
} while (notUnique);
But only I got is an infinite loop, notUnique is never switching to true. Just in case, this was tested against an empy database.
How could I achieve it?
You can do this easily with async module:
var async = require('async')
async.forever(
function(next) {
key = randomKey(8);
Group.findOne({numberId: key}).then(function (foundGroup) {
console.log("cb");
if (! foundGroup) {
console.log("not found")
notUnique = false;
next(new Error(), key) // break forever loop with empty error message and the key
}
else
return next() //continue loop
}).catch(function (err) {
next(err); //break loop
return response.send(500);
});
},
function(err, key) {
if(key){
newGroup.key = key;
}
}
);
Since you're already using promises, I'd do something like this: Create a function that recursively returns a promise, creating a promise chain, and eventually throws an error when a condition is/isn't met. You then just need to catch that final error.
edit: updated to return the key
function find(){
var key = randomKey(8);
return Group.findOne({numberId: key })
.then(function(foundGroup) {
if(foundGroup) return find(); // recursion
else throw key;
}).catch(function(err){ // in case Group.findOne throws
throw key;
});
}
find().catch(function(key){
return response.send(key);
});
The find function here will keep calling itself recursively as long as it keeps finding a valid object. And since it returns a promise they will all be chained automatically.
If and eventually when it doesn't find the object, or Group.findOne throws an error, the key is thrown.
find().catch will catch that eventual error, which will be the key.
I'm writing a NodeJS v0.10 application with MariaSQL.
i want to create a function that returns the id of a row, and if the row doesn't exist, to create it and then return the id.
this is what I have so far:
TuxDb.prototype.createIfNEDrinkCompany = function(drinkCompany) {
this.client.query("insert into drink_company(drink_company_name) values(:drink_company) on duplicate key update drink_company_id=drink_company_id",
{'drink_company' : drinkCompany})
.on('result',function(res) {
res.on('end',function(info){
if (info.insertId > 0) {
return info.insertId;
} else {
this.client.query("select drink_company_id from drink_company where drink_company_name = :drink_company",{'drink_company' : drinkCompany})
.on('result',function(res){
res.on('row',function(row){
return row.drink_company_id;
});
});
}
});
});
}
now the problem is that since it's asynchronous, the function ends before the value is returned.
how can I resolve this issue ?
The standard way in nodejs of dealing with async code is to provide a callback function as a last argument to your method and call it whenever your asynchronous finishes. The callback function standard signature is (err, data) - you can read more about here: Understanding callbacks in Javascript and node.js
Rewriting your code:
TuxDb.prototype.createIfNEDrinkCompany = function(drinkCompany, callback) {
this.client.query("insert into drink_company(drink_company_name) values(:drink_company) on duplicate key update drink_company_id=drink_company_id",
{'drink_company' : drinkCompany})
.on('result',function(res) {
res.on('end',function(info){
if (info.insertId > 0) {
callback(null, row.drink_company_id);
} else {
this.client.query("select drink_company_id from drink_company where drink_company_name = :drink_company",{'drink_company' : drinkCompany})
.on('result',function(res){
res.on('row',function(row){
callback(null, row.drink_company_id);
});
});
}
});
});
}
and then in the code calling your method
db.createIfNEDrinkCompany(drinkCompany, function(err, id){
// do something with id here
})