I'm creating a Phonegap application that will perform differently on first run. The way that I am detecting the first run is by seeing of one of the database tables exists. As you can probably tell from the code below, I am checking for the error that is (probably) indicating that the table already exists, thus proving that this is not the application's first run.
function databaseExists(){
var exists;
database.transaction(function(tx){
tx.executeSql('CREATE TABLE GLOBAL (uid, property, value)');
}, function(err){
exists = true;
}, function(){
exists = false;
});
return exists;
}
My problem, however, is that the asynchronous execution of the Javascript code means that the function returns its value before the success (or error) function has set it's value.
This function is called in the initialising stage of the application:
if (databaseExists()){
// Do Something
}
And therefore must return the value rather than execute the function in the success callback of the transaction.
Is there a way to force the execution to wait until the database transaction is complete or return the value through the database.transaction object?
Thanks in advance,
Jon
You need to write it in callback form:
var dataBaseExists(yep, nope) {
database.transaction(function(tx) {
tx.executeSql('CREATE TABLE GLOBAL (uid, property, value)');
}, function(){
if (yep) {
yep.apply(this, arguments);
}
}, function(){
if (nope) {
nope.apply(this, arguments);
}
});
};
var itDoes = function() {
console.log("great");
};
var itDoesNot = function() {
console.log("what a pity");
};
databaseExists(itDoes, itDoesNot);
You need callbacks, but if don't need checking existment of your tables, you can do that easily with localStorage.
e.g.
if(localStorage.getItem('init') === null){
//init
localStorage.setItem('init', true);
}
You will avoid dealing with database.
and maybe this gonna be helpful "CREATE TABLE IF NOT EXISTS..."
I know there's gonna be programmers don't like my solution, but I love it!
var myfEspereti=false;
function Espereti(pStatus)
{
if (pStatus==="wait")
{
myfEspereti = true;
while(myfEspereti)
{
}
}
else if (pStatus==="go")
{
myfEspereti=false;
}
}
and then call Espereti ("wait") when you want to wait for an async call. Inside the async call, when it's finish, call Espereti ("go") and that's it!
Related
Coming from a Java background I am now trying to wrap my mind around the asynchronous nature of Javascript. I use promises in my code to do this and until now everything works like a charm, but now I am having a conceptual question and didn't find a definitive answer even after reading the Promise/A+ spec multiple times.
My requirements are this: I have a method that modifies a shared object, stores the update in a PouchDB and reads it back afterwards in order to get an updated revision id field from the db (optimistic locking). Storing and updating the data in Pouch is asynchronous (I am omitting storing "this" to call the methods from within the promises for brevity):
var _doc = ...;
var _pouch = new PouchDB(...);
function setValue(key, value) {
_doc[key] = value;
_pouch.put(_doc)
.then(function() {
return _pouch.get(_doc._id);
})
.then(function(updatedDoc) {
_doc = updatedDoc;
});
}
Now, I want to make sure that no other key is set on _doc while it is being written to the db before it has been read again. Is it (a) even possible that another setValue() call is executing a put() (with an outdated revision id) while the get() call from Pouch has not been executed (given the message-queue-approach that JS is using) and (b) if it is possible, is the following solution fail-safe (it is working in my tests, but since I don't know if my tests are considering all possibilities...; storing "this" is again omitted):
var _doc = ...;
var _pouch = new PouchDB(...);
var _updatePromise;
function setValue(key, value) {
if (_updatePromise == null) {
setValueInternal(key, value);
}
else {
// make sure the previous setValue() call is executed completely before
// starting another one...
_updatePromise.then(function() {
setValueInternal(key, value);
});
}
}
function setValueInternal(key, value) {
_doc[key] = value;
_updatePromise = new Promise(function(done, reject) {
_pouch.put(_doc)
.then(function() {
return _pouch.get(_doc._id);
})
.then(function(updatedDoc) {
_doc = updatedDoc;
_updatePromise = null;
done();
})
catch(function(error) {
_updatePromise = null;
reject(error);
});
});
}
I think it should work correctly if fulfilling a promise (calling done()) will synchronously call the next then() function, but I am unable to find a definitive answer whether this is the case.
Any clarification is greatly appreciated and thanks for your help.
Chaining promises as you're attempting to do here does indeed work as expected, but I do not believe there is any guarantee that done is called synchronously. I think your code would work, but you have some anti-patterns in it. I would recommend simplifying to avoid explicit creation of the promises.
Also think about: If you call setValue 4 times in a row, how many round-trips to the server should that make? Doing it this way is going to make it take 4. Did you want to batch them into 1 or 2?
One Round Trip Per setValue:
var _doc = ...;
var _pouch = new PouchDB(...);
var _updatePromise = Promise.resolve();
function setValue(key, value) {
// make sure the previous setValue() call is executed completely before
// starting another one...
_updatePromise = _updatePromise.then(function() {
_doc[key] = value;
return _pouch.put(_doc)
.then(function() {
return _pouch.get(_doc._id);
})
.then(function(updatedDoc) {
_doc = updatedDoc;
});
});
}
I'm attempting to simply save a string to Chrome's Local Storage, but I can't seem to do it. It could be that I've got the wrong idea about function argument implementation (after 3 years, I think I might be getting it) but any help getting this code to work would be appreciated. I wanted to make it as dynamic as possible, allowing me to take the id of any text input field I write and change the stored string appropriately.
Here's the code I've got so far:
function setData() {
dataToStore = document.getElementById('txtStore').value;
return dataToStore;
}
function storeData(data) {
localStorage.setItem('input', data);
}
btnStore.addEventListener('click', storeData(setData), false);
Is my implementation of function arguments a bit backwards? I really thought I had it this time..
When you write:
btnStore.addEventListener('click', storeData(setData), false);
storeData(setData) is executing immediately at the beginning (when you bind the click event). Therefore, it will pass the initial value of your input which is probably empty.
So you should call setData, when you store the Data:
function setData() {
dataToStore = document.getElementById('txtStore').value;
return dataToStore;
}
function storeData() {
localStorage.setItem('input', setData());
}
btnStore.addEventListener('click', storeData, false);
you would have to do this:
function setData() {
dataToStore = document.getElementById('txtStore').value;
return dataToStore;
}
function storeData(dataFunct) {
localStorage.setItem('input', dataFunct());
}
btnStore.addEventListener('click', storeData(setData), false);
you can use without argument
function setData() {
dataToStore = document.getElementById('txtStore').value;
return dataToStore;
}
function storeData() {
var data = setData();
localStorage.setItem('input', data);
}
btnStore.addEventListener('click', storeData(), false);
You're saving the value in setData, which is a function, not the value returned from the function. You have to invoke the function to get the return value.
function storeData(data) {
vare returnedData = data()
localStorage.setItem('input', data);
}
with your current code you'd have to use the above snippet. But it would make more sense to retrieve the data before invoking storeData, and not pass the function.
You need to differentiate between function invocations and function references.
btnStore.addEventListener('click', storeData(setData), false);
An event is assigned a callback - i.e. reference to a function - but you're passing it the return value of setData, which is a string, not a function.
function setData() {
dataToStore = document.getElementById('txtStore').value;
return dataToStore;
}
function storeData() {
localStorage.setItem('input', setData());
}
btnStore.addEventListener('click', storeData, false);
You should simplify it a bit using more descriptive names for your functions:
function getDataFromElement(id){
dataToStore = document.getElementById(id).value;
return dataToStore;
}
function storeData(data) {
localStorage.setItem('inputData', data);
}
function retrieveData() {
return localStorage.getItem('inputData');
}
btnStore.addEventListener('click', storeData(getDataFromElement('txtStore')), false);
This way is more generic and you can even reuse it for other elements, improve it turning that 'inputData' into a parameter.
You could also add a check to verify that local storage is available before using it:
function canStorage(){
return (typeof(Storage) !== "undefined") ? true : false;
}
It is a good idea to batch your reads and writes, say one read per page and one write per page unload. It does depends on your use case, however, storing data in memory ie. data structure until some less frequent commit action.
To write data to Window.localStorage.
localStorage.setItem(k, v);
To read data from storage
localStorage.getItem(k);
Update:
A snippet of a module I wrote in gist which provides basic functions such as;
get
set
delete
flush
has
and a few chainable functions.
Hope this helps.
I am trying to return whether a user already exists in a MongoDB. Running console.log within collection.find() prints the correct amount (greater than 0). However, when userExists is called, it always returns false (0).
How do I make Javascript wait for these functions to complete before returning a value? I've read about jQuery's $.Deffered(), but this feels dirty to me, and it didn't work.
function userExists(db, uid){
var collection = db.get('users');
var ret = 0;
collection.find({"uid":uid},{},function(e,docs){
ret = docs.length
});
return ret > 0?true:false;
}
As some have noted, collection.find is asynchronous, so when you reach the next line in userExists (the line where you've got return ret > 0?true:false;), it's too early and the value of ret hasn't been set. Anywhere outside of the callback to collection.find (and any functions it calls in turn), the query hasn't happened yet.
There is (basically) no way to "pause" userExists until after the query, so you need to change your whole approach. What you need is the Continuation Pattern. This means that whatever you're doing with the result of collection.find has to happen in the callback.
I don't know what you're trying to do with ret, so this might mean big changes to how your code is organized. Here's an outline that I hope will give you the general idea:
function processResultAndDisplayToTheUser(ret) {
//everything that needs to happen after the value of ret is set
if (ret > 0) {
doSomething();
} else {
doSomethingElse();
}
}
function userExists(db, uid){
var collection = db.get('users');
//ret doesn't exist here
collection.find({"uid":uid}, {}, function(e,docs){
var ret = docs.length;
//ret finally exists, so pass it to a function and use it
processResultAndDisplayToTheUser(ret);
});
//ret doesn't exist here either
}
//ret still doesn't exist here
I took the hint and ended up restructuring my code. I created a function addUpdateUser(), did the count there, then ran the addUser() or updateUser() functions accordingly.
addUpdateUser(db, {
"uid" : uid,
});
function addUpdateUser(db, userInfo){
var collection = db.get('users');
collection.find({"uid":userInfo.uid},{},function(e,docs){
if(docs.length > 0){
updateUser(db, userInfo)
}else{
addUser(db, userInfo)
}
});
}
since collection.find is asynchronous method that doesn't return immediately you need to change your code to,
you can pass a callback function
function userExists(db, uid,callback){
var collection = db.get('users');
collection.find({"uid":uid},{},function(e,docs){
callback(docs.length);
});
}
now you can call this userExists function as
userExists(db, uid,function(ret){
//do something here
})
This question already has answers here:
How do I return the response from an asynchronous call?
(41 answers)
Closed 8 years ago.
I am in the process of relearning Javascript and last week when writing this code for a university assignment I think that there is probably a much better way of executing this code
app.get('/member/all', function(req, res) {
connection.query('CALL GetAllMembers()', function(err,rows){
connection.query('CALL CountMembers()', function(err, allMembers){
console.log(err);
connection.query('CALL CountAllIndMembers()', function(err,indMembers){
console.log(err);
connection.query('CALL CountInactiveMembers()', function(err,inactiveMembers){
console.log(err);
connection.query('CALL CountAllMembersInGroups()', function(err,groupMembers){
console.log(err);
res.render('members', {members : rows[0], title : "All Members", groupMembers : groupMembers[0][0].AllGrpMembers,
inactiveMembers : inactiveMembers[0][0].AllInactiveMembers, indMembers : indMembers[0][0].AllIndMembers,
allMembers : allMembers[0][0].AllMembers, statistics : true});
});
});
});
});
});
});
});
When I was trying to declare variables under the app.get such as var allMembers... when the callback was executed I was unable to set allMembers = rowsFromTheCallback. It seemed that it was a local variable to that callback. I'm sure this is something to do with the variable scope and/or hoisting. Just wanted to ask you guys if there would be a better way to do this as even though this function works. It is very ugly to look at haha!
Thanks in advance
Jack
As far as scope goes, all the inner functions should be able to read and write to the outer variable unless it is shadowed by an inner variable declaration or function parameter.
The problem you are having might be related to the async-ness of the code. See this code:
function delay(n, cb){
setTimeout(function(){ bs(delay) }, delay);
}
function main(){
var allMembers = 17;
delay(500, function(){
console.log(allMembers); // This looks at the outer "allMembers"
allMembers = 18;
delay(200, function(allMembers){ // <-- SHADOW
console.log(allMembers); // This looks at the allMembers from "delay 200"'s callback
allMembers = 42;
});
delay(300, function(){
console.log(allMembers); //This is the outside "allMembers" again
});
});
return allMembers; // Still 17!
}
main();
main will return before the setTimeouts have even fired so its going to return the original value of that variable. In order to wait for the inner callbacks to run, the only way is to make main take a callback to signa when its done, instead of just returning.
function main(onResult){
delay(500, function(){
//...
onResult(allMembers);
});
// <-- no return value
});
main(function(allM){
console.log(allM);
});
See async library: https://github.com/caolan/async
async.series([
getAllMembers,
countMembers,
...
], function(err, results) {
// err contains an error if any of the functions fails. No more functions will be run.
// results is an array containing results of each function if all the functions executed without errors
}));
function getAllMembers(callback) {
connection.query('CALL CountMembers()', callback);
}
function countMembers(callback) {
...
}
If the execution order of the functions does not matter, async.parallel can be used instead of async.series.
There is power in using a library to handle and encapsulate "Continuation Passing Style" (CPS) interactions with your asynchronous calls. The following code isn't from a library, but I'm going to walk through it and use it as an example of one way to implement CPS.
Setting up a scope appropriate queue is the first step. This example uses about the most simple method for doing so:
var nextList = [];
After that we need a method to handle our first case, the need to queue tasks to be performed in the future. In this case I was focused on performing them in order so I named it next.
function next() {
var todo,
current,
task,
args = {};
if (arguments.length > 0) { // if called with parameters process them
// if parameters aren't in an array wrap them
if (!Array.isArray(arguments['0'])) {
todo = [arguments];
} else { // we were passed an array
todo = [];
arguments['0'].forEach(function (item) {
// for each item we were passed add it to todo
todo.push(item);
});
}
nextList = todo.concat(nextList);
// append the new items to the end of our list
}
if (nextList.length > 0) { // if there are still things to do
current = Array.prototype.slice.apply(nextList.shift());
task = current[0];
args = current.slice(1);
task.apply(null, args); // execute the next item in the list
}
}
This allows us to make calls like:
.map(function (filepath) {
tasks.push(
[
handleAsset,
{
'path': filepath,
}
]
);
});
tasks.push([done]);
next(tasks);
This will call handleAsset, which is async, once for each file, in order. This will allows you to take your code and change each of the nested calls into a separate function in the form:
function memberAll() {
app.get('/member/all', function(req, res) {
if (err) {
handleError(err, 'memberAll');
} else {
next(getAllMembers, 'parameters to that call if needed');
}
});
}
where handleError is a common error handler, and the next call allows you to pass on relevant parameters to the next function that is needed. Importantly in the success side of the if statement you could either:
conditionally call one of several functions
call next with an array of calls to make, for instance if you had functions for processFolder and processFile you could expect that processing a folder might involve processing other folders and files and that the number would vary
do nothing except call next() with no parameters and end the current branch
Embellishments can include writing a clean function for emptying the nextList, adding items to nextList without calling an item from the list, etc. The alternative at this point is to either use an existing library for this or to continue writing your own.
I'm relatively new to coding in JavaScript, and I've came across a problem. I like to nest functions to keep things orderly, but how would I exit from a parent function from inside a child function?
example:
function foo1() {
function foo2() {
//return foo1() and foo2()?
}
foo2();
}
See update under the fold
You can't. You can only return from the child function, and then return from the parent function.
I should note that in your example, nothing ever calls foo2 (As of your edit, something does). Let's look at a more real example (and one that comes up a lot): Let's say we want know if an array contains an entry matching some criterion. A first stab might be:
function doesArrayContainEntry(someArray) {
someArray.forEach(function(entry) {
if (entryMatchesCondition(entry)) {
return true; // Yes it does <-- This is wrong
}
});
return false; // No it doesn't
}
You can't directly do that. Instead, you have to return from your anonymous iterator function in a way to stop the forEach loop. Since forEach doesn't offer a way to do that, you use some, which does:
function doesArrayContainEntry(someArray) {
return someArray.some(function(entry) {
if (entryMatchesCondition(entry)) {
return true; // Yes it does
}
});
}
some returns true (and stops looping) if any call to the iterator function returns true; it returns false if no call to the iterator returned true.
Again, that's just one common example.
You've referred to setInterval below, which tells me that you're almost certainly doing this in a browser environment.
If so, your play function almost certainly has already returned by the time you want to do what you're talking about, assuming the game has any interaction with the user other than alert and confirm. This is because of the asynchronous nature of the environment.
For example:
function play() {
var health = 100;
function handleEvent() {
// Handle the event, impacting health
if (health < 0 {
// Here's where you probably wanted to call die()
}
}
hookUpSomeEvent(handleEvent);
}
The thing is, that play will run and return almost immediately. Then the browser waits for the event you hooked up to occur, and if it does, it triggers the code in handleEvent. But play has long-since returned.
Make a note whether the parent function should also return.
function foo1() {
bool shouldReturn = false;
function foo2() {
shouldReturn = true; // put some logic here to tell if foo1() should also return
return;
}
if (shouldReturn) {
return;
} else {
// continue
}
}
It only says that you can't return the parent function in the child function, but we can do a callback and make it happen.
function foo1(cb = () => null) {
function foo2() {
cb();
}
foo2();
}
foo1(() => {
// do something
});
We can use Promises for this:
const fun1 = async () => {
const shouldReturn = await new Promise((resolve, reject) => {
// in-game logic...
resolve(true)
})
if(shouldReturn) return;
}
if you wanna return from the parent function, then just resolve with true
Based on your comment, something like this might work as a main game loop.
function play() {
var stillPlaying = true;
while(stillPlaying) {
... play game ...
stillPlaying = false; // set this when some condition has determined you are done
}
}