I'm desperately trying to recover the value of a callback function but I have no idea how to do that. I have a function where I execute this code:
if (final.error !== undefined) {
console.log("Initial authentication:", final.error_description, "Please refresh the authentication grant");
extAuthCallback(84);
} else {
tokens.set('access_token', final.access_token)
.set('expires_in', final.expires_in)
.set('refresh_token', final.refresh_token)
.set('refresh_date', moment())
.write()
extAuthCallback(1);
}
});
Who performs this function:
function extAuthCallback(result) {
return result;
}
And which is called by this variable:
let authentication = auth.extAuth(access_token, auth.extAuthCallback);
I would like my `authentication' variable to take the value returned in the callback, and I have no idea how to do that. Returning the callback function to my original function doesn't work.
You could use a promise, would need to use an async function as well though.
function asyncExtAuth(access_token) {
return new Promise(resolve => {
auth.extAuth(access_token, resolve);
});
}
let authentication = await asyncExtAuth(access_token);
Related
I am beginner at javascript. I want to understand more Callback Function because i have spend a lot time to learning this function but i not yet understood clearly.
This is my code. i want to make new function(callback) to continue this code, and show all month using method map
const getmonth = (callback) => {
setTimeout(() => {
let error = true;
let month = ["January","February","March","April","Mey","Juny","July","August","September","October","November","Desember"];
if(!error) {
callback(null, month)
} else {
callback(new error("Data gak ketemu", []))
}
}, 4000)
}
Callback is the function, which is passed to another function as an argument.
For example:
function sayHello(callback) {
console.log('Hi everyone');
setTimeout(function(){
callback(); // execution your function with 3 seconds delay
}, 3000);
}
In your case(I'm not using arrow functions to make it more easy to understand for you):
// Lets create a function, which will just print month which is passed as an argument
const printMonth = function(month) {
console.log(month);
}
// Now we are using map function
// https://developer.mozilla.org/uk/docs/Web/JavaScript/Reference/Global_Objects/Array/map
month.map(function(month) {
console.log(month);
});
The map function accepts another function as an argument(it's called callback) and in this function you can do whatever you want with every single element of array. You can also use an return to return new array with modified elements of it.
callbacks are often used to continue code execution after an asynchronous operation has completed.
more about callback functions
You can create callback function on your code like this.
getmonth((err,data)=>{
if(!err){
data.map(month =>month);
}
return err;
})
This is a simple version of what I'm trying to do in my application. I have an if statement which evaluates the result of a function call and then populates an array if the statement comes back as true. AFTER the if statement is completely finished, I want to run some more code such as the console.log as seen below.
I understand that the if's evaluation is taking too long to finish and javascript just continues to the console.log because of its asynchronicity. How do I make the code wait for the if statement to complete?
var tabs = [];
if (isTrue()) {
tabs.push('some string');
}
console.log(tabs[1]);
function isTrue() {
setTimeout(function() {
return true;
}, 500)
}
You can just wrap your code in a Promise and consume the returned values by calling then on it:
var tabs = [];
isTrue().then(res => {
if (res) {
tabs.push('some string');
}
return tabs;
}).then(arr => {
console.log(arr);
});
function isTrue() {
//Just wrap your existing code in a Promise constructor
return new Promise((resolve, reject) => {
setTimeout(() => {
//Pass whatever value you want to consume later to resolve
resolve(true);
}, 500)
});
}
You could pass a callback to the isTrue() function, something like:
function isTrue(_callback) {
setTimeout(function() {
// code here
// Call the callback when done
if (typeof(_callback) === 'function')
_callback(tabs);
});
}
function showTabs(tabs) {
console.log(tabs[1]);
}
isTrue(showTabs);
Ought to work.
Using modern javascript, you can achieve that using promises and async/await:
const isTrue = () => new Promise(resolve => setTimeout(resolve, 500, true));
// you can only use `await` inside an `async` function
async function main() {
// better use `let` instead of `var` since `let` is block scoped,
// see:
// <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let>
let tabs = [];
if (await isTrue()) {
tabs.push('some string');
}
// array's index start by 0, not 1
console.log(tabs[0]);
}
main();
(this code also use arrow functions for isTrue.)
isTrue() returns undefined. The return true inside of the setTimeout callback will return back to the timeout call, not to the isTrue() call. The code executes immeadiately and there is no asynchronity involved (except for that timer that does nothing).
in my chrome extension I need to use chrome storage. In my background script first I create an object and add it to chrome storage and then I want to get my object from there and to be returned. Something like that:
...
var obj = {};
chrome.storage.local.set(obj, function () { });
...
var data = getData(obj); // I want my object to be returned here
var returnedData = null;
function getData(obj) {
chrome.storage.local.get(obj, function(result) {
returnedData = result; // here it works, I can do something with my object
});
return returnedData; // here it doesn't work
}
As far as I understood from here chrome.storage.local.get is asynchronous with its consequences. But is there any way how to get something from chrome storage and make it to be returned? I mean maybe I should wrap chrome.storage.local.get in another function or so?
Many thanks in advance!
If you want to stay away from global variables and you're okay with modern browser requirements, then you can implement a native JavaScript Promise object. For example, here's a function that returns the stored data for a single given key:
function getData(sKey) {
return new Promise(function(resolve, reject) {
chrome.storage.local.get(sKey, function(items) {
if (chrome.runtime.lastError) {
console.error(chrome.runtime.lastError.message);
reject(chrome.runtime.lastError.message);
} else {
resolve(items[sKey]);
}
});
});
}
// Sample usage given this data:
// { foo: 'bar' }
getData('foo').then(function(item) {
// Returns "bar"
console.log(item);
});
If you need support for IE11 and below, then you'll have to turn to a library like jQuery.
No it's not possible
But there are several ways around this problem
Do everything you want to do with the data returned from .get() inside the callback (or start it from there using function calls). This is what #wernersbacher posted
Take a look at deferreds (jQuery or Q libraries). A deferred's promise can be returned from getData. Inside the .get() callback, you can resolve the deferred. Outside of getData you can use .then() to do something after the deferred resolved
Something like this
function getData(obj) {
var deferred = $.Deferred();
chrome.storage.local.get(obj, function(result) {
deferred.resolve(result);
});
return deferred.promise();
}
$.when(getData(obj)).then(function(data) {
// data has value of result now
};
You have to do it like that:
var returnedData = null;
function setData(value) {
returnedData = value;
}
function getData(obj) {
chrome.storage.local.get(obj, function(result) {
setData(result); // here it works, I can do something with my object
});
return; // here it doesn't work
}
..because you tried to return a value which did not get read from storage yet, so it's null.
Update with Manifest V3 :
Now chrome.storage.local.get() function returns a promise that you can chain or can await in an async function.
const storageCache = { count: 0 };
// Asynchronously retrieve data from storage.local, then cache it.
const initStorageCache = chrome.storage.local.get().then((items) => {
// Copy the data retrieved from storage into storageCache.
Object.assign(storageCache, items);
});
Note : You must omit the callback paramter to get the promise.
Reference : https://developer.chrome.com/docs/extensions/reference/storage/#:~:text=to%20callback.-,get,-function
You need to handle it with callback functions. Here are two examples. You use a single function to set, however you create a separate function for each "On Complete". You could easily modify your callback to pass additional params all the way through to perform your needed task.
function setLocalStorage(key, val) {
var obj = {};
obj[key] = val;
chrome.storage.local.set(obj, function() {
console.log('Set: '+key+'='+obj[key]);
});
}
function getLocalStorage(key, callback) {
chrome.storage.local.get(key, function(items) {
callback(key, items[key]);
});
}
setLocalStorage('myFirstKeyName', 'My Keys Value Is FIRST!');
setLocalStorage('mySecondKeyName', 'My Keys Value Is SECOND!');
getLocalStorage('myFirstKeyName', CallbackA);
getLocalStorage('mySecondKeyName', CallbackB);
// Here are a couple example callback
// functions that get executed on the
// key/val being retrieved.
function CallbackA(key, val) {
console.log('Fired In CallbackA: '+key+'='+val);
}
function CallbackB(key, val) {
console.log('Fired In CallbackA: '+key+'='+val);
}
I have got a javascript code like this:
function justTesting() {
promise.then(function(output) {
return output + 1;
});
}
var test = justTesting();
I have got always an undefined value for the var test. I think that it is because the promises are not resolved yet..there is a way to return a value from a promise?
When you return something from a then() callback, it's a bit magic. If you return a value, the next then() is called with that value. However, if you return something promise-like, the next then() waits on it, and is only called when that promise settles (succeeds/fails).
Source: https://web.dev/promises/#queuing-asynchronous-actions
To use a promise, you have to either call a function that creates a promise or you have to create one yourself. You don't really describe what problem you're really trying to solve, but here's how you would create a promise yourself:
function justTesting(input) {
return new Promise(function(resolve, reject) {
// some async operation here
setTimeout(function() {
// resolve the promise with some value
resolve(input + 10);
}, 500);
});
}
justTesting(29).then(function(val) {
// you access the value from the promise here
log(val);
});
// display output in snippet
function log(x) {
document.write(x);
}
Or, if you already have a function that returns a promise, you can use that function and return its promise:
// function that returns a promise
function delay(t) {
return new Promise(function(resolve) {
setTimeout(function() {
resolve();
}, t);
});
}
function justTesting(input) {
return delay(100).then(function() {
return input + 10;
});
}
justTesting(29).then(function(val) {
// you access the value from the promise here
log(val);
});
// display output in snippet
function log(x) {
document.write(x);
}
What I have done here is that I have returned a promise from the justTesting function. You can then get the result when the function is resolved.
// new answer
function justTesting() {
return new Promise((resolve, reject) => {
if (true) {
return resolve("testing");
} else {
return reject("promise failed");
}
});
}
justTesting()
.then(res => {
let test = res;
// do something with the output :)
})
.catch(err => {
console.log(err);
});
Hope this helps!
// old answer
function justTesting() {
return promise.then(function(output) {
return output + 1;
});
}
justTesting().then((res) => {
var test = res;
// do something with the output :)
}
I prefer to use "await" command and async functions to get rid of confusions of promises,
In this case I would write an asynchronous function first,
this will be used instead of the anonymous function called under "promise.then" part of this question :
async function SubFunction(output){
// Call to database , returns a promise, like an Ajax call etc :
const response = await axios.get( GetApiHost() + '/api/some_endpoint')
// Return :
return response;
}
and then I would call this function from main function :
async function justTesting() {
const lv_result = await SubFunction(output);
return lv_result + 1;
}
Noting that I returned both main function and sub function to async functions here.
Promises don't "return" values, they pass them to a callback (which you supply with .then()).
It's probably trying to say that you're supposed to do resolve(someObject); inside the promise implementation.
Then in your then code you can reference someObject to do what you want.
I think what the original poster wants is to return an unwrapped value from a promise without actually returning another promise. Unless proven otherwise, I'm afraid this is not possible outside of a then() or async/await context. You always get a promise no matter what.
You need to make use of reference data type like array or object.
function foo(u,n){
let result = [];
const userBrands = new Promise((res, rej)=> {
res(['brand 1', 'brand 3']);
})
userBrands.then((ub)=>{
return new Promise((res, rej) =>{
res([...ub, 'brand 4', 'brand 5']);
})
}).then(response => {
return result.push(...response);
});
return result;
};
foo();
You cannot return value after resolving promise. Instead call another function when promise is resolved:
function justTesting() {
promise.then(function(output) {
// instead of return call another function
afterResolve(output + 1);
});
}
function afterResolve(result) {
// do something with result
}
var test = justTesting();
In my case I have one repository like this from temphire (breeze)
define(['durandal/system'], function (system) {
var Repository = (function () {
var repository = function (entityManagerProvider, entityTypeName, resourceName, fetchStrategy) {
.........
this.find = function (predicate) {
var query = breeze.EntityQuery
.from(resourceName)
.where(predicate);
return executeQuery(query);
};
function executeQuery(query) {
return entityManagerProvider.manager()
.executeQuery(query.using(fetchStrategy || breeze.FetchStrategy.FromServer))
.then(function (data) { return data.results; });
}
................
};
return repository;
})();
return {
create: create,
getCtor: Repository
};
function create(entityManagerProvider, entityTypeName, resourceName, fetchStrategy) {
return new Repository(entityManagerProvider, entityTypeName, resourceName, fetchStrategy);
}
});
NOW
HOW CAN DO LIKE SOME THIS
repository.query(predicate).execute();
function query(predicate) {
return query = breeze.EntityQuery
.from(resourceName)
.where(predicate);
};
function executeQuery(query) {
return entityManagerProvider.manager().executeQuery(query.using(fetchStrategy || breeze.FetchStrategy.FromServer)).then(function(data) {
return data.results;
});
}
function execute() -- >
return executeQuery
the first action return query and after to execute
many thanks
I think the problem with what you are trying is that return terminates execution. If you want to do something as well as return in that function, then you need to do it before you return.
If, on the other hand, you really need to return the value and then execute something, then you should have the method that calls the function expecting the return, call the function to get the return value, and then have that calling function execute the thing you want executed. If that execution needs some data from the function that returns the value, then return that information with the value returned, and pass it into the function that does the execution.
Use
executeQueryLocally // This is syn
instead of
executeQuery // This is async
executeQuery sync