So, I have a script that fetches data from a SQL database and I'm trying to build a JS wrapper for it. I'm using the following functions to call that script and use information from the DB as soon as it's ready.
var User = function() {
this.email = null;
//Async call to get user values
this.current(function(response) {
this.email = response.email;
//The value is assigned/usable at this point
});
};
User.prototype.current = function(callback) {
$.post("php/db_functions.php", {object: "CurrentUser"}).done(function(result) {
callback(JSON.parse(result)[0]);
});
};.
Everything seems to work fine, but if I try to access the value from the object after I've created it, it returns undefined, like so:
var me = new User();
//And then, way after the async call and when me.email should be defined
me.email //Returns undefined
Why can I use it in the callback, but not afterwards?
In a function, the context variable this points to either the global window object or to undefined in the strict mode, unless specified otherwise by the caller. Therefore, you need to either capture the value of this in a local variable:
//Async call to get user values
var that = this;
this.current(function(response) {
that.email = response.email;
});
or call the function in the desired context using either the call or the apply method:
User.prototype.current = function(callback) {
var that = this;
$.post("php/db_functions.php", {object: "CurrentUser"}).done(function(result) {
callback.call(that, JSON.parse(result)[0]);
});
};.
Also, as others have mentioned, there is no guarantee the AJAX request will have finished by the time the User contructor returns.
This is a timing bug since the variable is not assigned until the async call returns. You can't access email right away.
Related
I've a question about subsequent function calls or somehow. I've this code here:
let socket = new WebSocket("localhost:8181");
socket.subscribe("abc").bind("test", function (response) {
console.log(response);
});
subscribe(channel) {
//Here I do some things
return this;
}
bind(eventName, callback) {
//Here I need the "abc" value
}
Currently I'm calling a second function after another to first subscribe something and then bind it. The problem is that I need the value passed to the subscribe function in the following bind function.
Currently I'm returning this because otherwise .bind() would be undefined because of the scope bla bla so because of this I can't just return the channel. Does someone has an idea how I can get this done without changing the function call structure?
You can't return this and also pass a variable. I think your best bet may be wrap your subscribe call in a closure so that you can keep local state.
function createAndSubscribe(abc) {
// You can create other local variables here
let otherLocal = 'foo';
let socket = new WebSocket("localhost:8181");
socket.subscribe(abc).bind("test", function (response) {
// Use abc or foo here
});
}
createAndSubscribe('abc');
Every time you call the function createAndSubscribe, it creates a unique closure that holds the value of abc and any other variables that you create.
I have created a local class in a JavaScript file with following content:
class CustomChromeStorage {
//#region userName
get userName() {
let isCurrentValueSet = false;
chrome.storage.sync.get('userName', function (obj) {
this._userName = obj;
isCurrentValueSet = true;
});
while (true) {
if (isCurrentValueSet) {
return this._userName;
}
}
}
set userName(newValue) {
this._userName = newValue;
chrome.storage.sync.set({ 'userName': newValue }, function () {
});
}
remove_userName() {
this._userName = null;
chrome.storage.sync.remove('userName', function () {
});
}
//#endregion userName
My Idea to do such type of code is when I write somewhere else in my code like:
alert(new CustomChromeStorage().userName);
Then my code simply fetches username from chrome storage and show it via an alert. In order to fetch a value from chrome storage we need to provide a callback with as parameter the value. I know this is good practice for asynchronous process but it sometimes becomes cumbersome for me to handle all the callbacks.
I want that when I fetch value from chrome storage via my custom class to execute current code asyncronously. This is why I have written infinite while loop inside getter method of that property but the problem is when I try to alert username via custom chrome storage class my total program execution becomes hang.
The reason behind it is that I initially set isCurrentValueSet = false which never gets true inside while loop.
If anybody have any idea why it does not set to true inside while loop then please let me know.
The obj returned from sync.get is {userName: value} - use obj.userName.
The reason isCurrentValueSet doesn't get set to true is because the function is asynchronous - when the callback executes, it doesn't have access to the class variable isCurrentValueSet.
What you're trying to achieve is just wrong. It's a fact that storage requests are asynchronous for the good of the user and browser performance. You have to learn to design around it and it's easy enough when you get used to it.
You can retrieve multiple variables in one hit so if you have a section of code that needs several variables, just do:
chrome.storage.sync.get({a:"",b:"",c:0,d:[]}, function(result) {
a = result.a
b = result.b
c = result.c
d = result.d
your code
});
By passing an object in, you can request multiple variables and define defaults for if they don't yet exist in storage. Of course you don't have to extract the variables.
While this issue occurred to me specifically with KnockoutJS, my question is more like a general javascript question.
It is good to understand however that ko.observable() and ko.observableArray() return a method so when assigning a value to them, you need to call the target as method instead of simply assigning a value to them. The code that I'm working with should also support plain objects and arrays, which I why I need to resolve to a method to call to assign a value to the target.
Think of these 2 examples:
Non-working one (this context changed in called method):
// Assigning value to the target object
var target;
// target can be of any of thr following types
target = ko.observableArray(); // knockout observable array (function)
// target = ko.observable(); // knockout observable (function)
// target = {}; // generic object
// target = []; // generic array
//#region resolve method to call
var method;
if (isObservable(target)) {
// if it is a knockout observable array, we need to call the target's push method
// if it is a konckout observable, we need to call the target as a method
method = target.push || target;
} else {
// if target is a generic array, we need to use the array's push prototype
// if target is a generic object, we need to wrap a function to assign the value
method = target.push || function(item){ target = item; };
}
//#endregion
// call resolved method
method(entity);
Working one (this context is fine):
if (isObservable(target)) {
if (target.push) {
target.push(entity);
} else {
target(entity);
};
} else {
if (target.push) {
target.push(entity);
} else {
target = entity;
};
}
Now, to the actual question:
In the first approach, later in the execution chain when using a knockout observable knockout refers to this context within itself, trying to access the observable itself (namely this.t() in case someone is wondering). In this particular case due to the way of callin, this has changed to window object instead of pointing to the original observable.
In the latter case, knockout's this context is just normal.
Can any of you javascript gurus tell me how on earth my way of calling can change the 'this' context of the function being called?
Ok, I know someone wants a fiddle so here goes :)
Method 1 (Uncaught TypeError: Object [object global] has no method 'peek')
Method 2 (Works fine)
P.S. I'm not trying to fix the code, I'm trying to understand why my code changes the this context.
UPDATE:
Thanks for the quick answers! I must say I hate it when I don't know why (and especially how) something is happening. From your answers I fiddled up this quick fiddle to repro the situation and I think I got it now :)
// So having an object like Foo
function Foo() {
this.dirThis = function () {
console.dir(this);
};
};
// Instantiating a new Foo
var foo = new Foo();
// Foo.dirThis() has it's original context
foo.dirThis(); // First log in console (Foo)
// The calling code is in Window context
console.dir(this); // Second log in console (Window)
// Passing a reference to the target function from another context
// changes the target function's context
var anotherFoo = foo.dirThis;
// So, when being called through anotherFoo,
// Window object gets logged
// instead of Foo's original context
anotherFoo(); // 3rd log
// So, to elaborate, if I create a type AnotherFoo
function AnotherFoo(dirThis){
this.dirThis = dirThis;
}
// And and instantiate it
var newFoo = new AnotherFoo(foo.dirThis);
newFoo.dirThis(); // Should dir AnotherFoo (4th in log)
If you're after a way to choose the 'this' that will get used at the time of call,
you should use bind, that's exactly done for that.
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind
So if SomeObject has a push method, then storing it like this won't work :
var thePushMethod = someObject.push;
since you loose the context of the function when writing this.
Now if you do :
var thePushMethod = someObject.push.bind(someObject);
the context is now stored inside thePushMethod, that you just call with
thePushMethod();
Notice that you can bind also the arguments, so for instance you might write :
var pushOneLater = someObject.push.bind(someObject, 1 );
// then, later :
pushOneLater(); // will push one into someObject
Consider this example,
function Person () {
this.fname = "Welcome";
this.myFunc = function() {
return this.fname;
}
};
var a = new Person();
console.log(a.myFunc());
var b = a.myFunc;
console.log(b());
Output
Welcome
undefined
When you make a call to a.myFunc(), the current object (this) is set as a. So, the first example works fine.
But in the second case, var b = a.myFunc; you are getting only the reference to the function and when you are calling it, you are not invoking on any specific object, so the window object is assigned. Thats why it prints undefined.
To fix this problem, you can explicitly pass the this argument with call function, like this
console.log(b.call(a));
So, for your case, you might have to do this
method.call(target, entity);
Check the fixed fiddle
I've been using jQuery for a couple of years now with very limited understanding of vanilla javascript. Scope, the object model, and many of the design patterns that I see used in javascript baffle me. I'm trying to implement a class that will eventually be used in a scheduling plugin that I need to write and I'm having a hard time understanding why data stored in one of my class members doesn't seem to be available. I'm not sure if the issue is with scope or some other behavior that I don't understand.
I have the following code with 2 questions in the comments at the appropriate places. The first question is whether or not my scope workaround in my getJSON call is the correct way of handling the scope issue inside getJSON. My second question is why I can't directly access schedule.data.
function Schedule() {
this.year = null;
this.month = null;
this.day = null;
this.start_datetime = null;
this.start_timestamp = null;
this.end_datetime = null;
this.end_timestamp = null;
this.data = [];
return this;
}
Schedule.prototype.init = function() {
var url = '/tripsys/new_admin/employee_schedule/get_employee_schedule_data/' + this.start_timestamp + '/' + this.end_timestamp;
var self = this; // 1. trying to work around scope issues. Is this the correct way to handle the scope problems here?
$.getJSON(url, function(data) {
self.data = data;
});
}
var schedule = new Schedule();
$(document).ready(function() {
schedule.year = $('#year').text();
schedule.month = $('#month').text();
schedule.day = $('#day').text();
schedule.start_datetime = new Date(schedule.year, schedule.month - 1, schedule.day);
schedule.start_timestamp = Math.round(schedule.start_datetime.getTime() / 1000);
schedule.end_datetime = new Date(schedule.year, schedule.month - 1, schedule.day, 23, 59, 59);
schedule.end_timestamp = Math.round(schedule.end_datetime.getTime() / 1000);
schedule.init();
console.log(schedule); // if I log the whole schedule object the data that I expect to be in the "data" member is there
console.log(schedule.data); // 2. why is the data that I expect to be in the "data" member not there when I access schedule.data directly?
});
Thanks for your insight.
Well point number one is correct in that you need to save the this reference while you still can because when the inner function is called by jQuery, this inside the function will refer to the ajax object.
In the second comment you are logging schedule.data before the ajax request has completed. You can see schedule.data when you log schedule because when you log an object in google chrome, the object properties are retrieved after you manually "expand" the object in chrome console. When you manually "expand" it, at that time the request has already completed.
You can reproduce it like this:
var a = {};
console.log(a); //do not "expand" the object properties yet
console.log(a.property); //undefined
a.property = "value";
//"expand" the already logged object and it will have "value"
Yes, that will work, although it isn't a scope issue as much as it uses variable scope to get around a context issue.
To access schedule.data, you need to wait until the data has arrived. In other words, place the console.log code in the callback.
The issue is that the ajax call has not returned before you log the object. If you want to make the ajax call synchronous and the init function gets a result before you log, use the async param on an ajax jQuery call:
$.ajax({
url: url,
dataType: 'json',
async: false,
success: function(data){
self.data = data;
console.log(data);
}
});
This is probably because in this line schedule.init();, it makes an ajax call which has not completed yet when you then do console.log(schedule.data);. Ajax calls are asynchronous. Calling them only starts the networking operation and then they return immediately. They are not completed until the success handler function has been called (and that's when self.data is assigned).
So, if you want to look at the data for the schedule object that was obtained in the .init() function, you have to wait until that ajax call has completed or do something with the data in the completion function.
I am using now.js and Mongoose in a node project and am having trouble accessing the this.now object inside of a mongoose function. E.g.
everyone.now.joinDoc = function (project_id){
this.now.talk(); //this will work
Project.findOne({'_id':project_id}, function(err, project){
if(project){
this.now.talk(); // this will not work "TypeError: Cannot call method 'bark' of undefined"
};
});
};
Change the code to this:
everyone.now.joinDoc = function (project_id){
this.now.talk(); // this will work
var that = this; // save 'this' to something else so it will be available when 'this' has been changed
Project.findOne({'_id':project_id}, function(err, project){
if(project){
that.now.talk(); // use local variable 'that' which hasn't been changed
};
});
};
Inside your inner function, the this is probably being set to something else. So, to preserve the value you want to access, you assign it to a different local variable that will be available in the inner function.
everyone.now.joinDoc = function (project_id){
this.now.talk(); // this will work
Project.findOne({'_id':project_id}, (function(tunnel, err, project){
if(project){
this.now.talk();
};
}).bind(this, "tunnel")); // overwrite `this` in callback to refer to correct `this`
};
Use Function.prototype.bind to set the value of this to the value you want