I am trying to assign the ajax callback function variable value to the existing variable.
I have
function test(){
}
test.prototype.click=function(){
this.count;
//call ajax codes.....
//ajax callback function
ajax.callback=function(var1){
//I want to assign returned data to this.count property.
this.count=var1.length
}
}
test.prototype.show=function(){
//wont work, it will show undefined...
alert(this.count);
}
var t=new test();
t.click();
t.show();
I think it's the scope issue but I don't know how to solve this. Any idea? Thanks in advance.
Yeah, using this within another scope causes all kinds of issues, so you need to work around this. One way is to avoid using this entirely by defining your function differently. For instance, you can define count like so:
function test() {
function count() {
}
...
And just use count() without the this. prefix.
You can also set a variable to this and use that to refer to count within your other scope. For instance:
var self = this;
Scoping issues with this can be a pain in the neck and can occur when you do more OO with callbacks. It's good you got introduced to this early on, so now you know to be on guard.
Related
Good day!
Here is my issue. I have a function, that needs to loop at an interval that changes. However if you do the below:
example.prototype.Start = async Function() {
setTimeout(this.Function, this.interval)
}
example.prototype.Function = async Function() {
this.interval++;
setTimeout(this.Function, this.interval);
}
In this example, I believe the "this.interval" should be undefined. Now imagine this.interval can be influenced outside this function too. Which is why I want it as a constructor variable. However, I can't think of a way to get it defined within the loop without getting rid of the context of the variable being the whole constructor. I hope that makes sense.
Edit: Ignore that these are asynchronous functions without reason, in my long code, it has a reason.
So, I've got my app to work. But I don't want to use a global variable.
Here's what I'm trying to do:
var AMLid;
$(".commentButton").on('click', function () {
AMLid = $(this).closest('tr').siblings().find('p.important').text();
alert("id is:" + AMLid);
});
$(".saveButton").on("click", function () {
$(".modal-body #txtAddComment").val(AMLid);
});
I want to get an ID from a selected table row, and pass the ID to the modal dialog, so I then can use the ID as a parameter when clicking a button in the modal dialog.
How do I do the exact same thing, without using a global variable? is it possible?
And what are the cons of using a global variable like this? Does it crash or target the wrong ID's if many people use it simultaneously?
Any help is appreciated.
You can wrap the whole thing in a function
(function(){
var AMLid;
$(".commentButton").on('click', function () {
AMLid = $(this).closest('tr').siblings().find('p.important').text();
alert("id is:" + AMLid);
});
$(".saveButton").on("click", function () {
$(".modal-body #txtAddComment").val(AMLid);
});
})();
You can avoid the use of a global variable by using an Immediately-Invoked Functon Expression, which would look like this:
(function() {
var AMLid;
$(".commentButton").on('click', function () {
AMLid = $(this).closest('tr').siblings().find('p.important').text();
alert("id is:" + AMLid);
});
$(".saveButton").on("click", function () {
$(".modal-body #txtAddComment").val(AMLid);
});
})();
This works because the AMLid is now private to the function; when the function is executed it creates a new execution context which includes that variable, which is then accessible to statements made in the function, but not outside it. And because this creates a closure the variable continues to be accessible by the callbacks attached to those functions. Moreover, as the function is anonymous it itself doesn't have a name polluting the namespace.
The term Immediately-Invoked Functon Expression comes from Ben Alman, and you can read his original blog post discussing the concept here: http://benalman.com/news/2010/11/immediately-invoked-function-expression/
Some cons of using a global include: hard to keep track of variables, other scripts might mess with its value (when they probably shouldn't have access to it), harder to maintain, less clean code. Your worry about it being overwritten if multiple people use it won't be an issue because it's client-side and so there will be one instance per page loaded in any given browser.
Javascript is client-side so actually I can't get the point in your "many people use it simultaneously". One browser for user, so you don't have to worry about multiple client. Each one use his own set of global variable.
If your executions are not linked in any way (they are "onclick") you can just wrap them in a function so you're actually setting a "local/global" variable.
Every function that'll need that AMLid has to be declared inside that function scope.
The only way to keep variables out of the global scope is by wrapping them in a function. If this is all the code you're using in this particular module, it doesn't really make a difference.
Let's start from the code:
function say(name) {
var ghost=function () {
function ghost() {
alert('!');
};
return body;
};
eval("var body=''+"+name+';');
eval(name+('=('+ghost).replace('body', body)+')();');
eval(name+'();');
}
function Baal() {
if ('undefined'===typeof ghost) {
say('Baal');
return;
}
ghost();
}
say('Baal'); // or just Baal();
Looks like that saying the devil's name invoke his presence (well, maybe he needs somebody for spiritual possession) ..
As you can see the ghost doesn't exist along with Baal, but we can invoke it since there're evals in say(name).
say(name) reassigns Baal to its code body as a closure and makes it captured a ghost method, that's how things work. But I'm trying to avoid eval ..
So .. let me reword the question:
How do I make a nonexistent(and not a member or global) method invocable without using eval?
Let me rephrase your question, just to make sure I’ve got it. Given a function, you want to put a new variable in its scope, without that scope being the global scope or a scope shared between the caller and the subject, without using eval (or the equivalent new Function and other hacks depending on the environment).
You can’t.
In the case you just mentioned, you could define one function, base(), that uses arguments.callee.caller.
Don’t do that.
The short answer: You don't.
That scope is not available. If you were to attach the scope then it would be available inside of the scope used. You could then access the method handles. I assume this is not what you were looking for, but here is what that would look like. demo
function say(name){
var methods = {};
methods.Baal = function(){
alert("!");
};
return methods[name];//this could invoke as well: methods[name]()
}
var handle = say('Baal');
handle();
What your evals break down to is something along these lines (although with dynamic content from string building - this is the end result)
function say(name) {
var Baal = (function () {
function ghost() {
alert('!');
};
return function(){
if ('undefined'===typeof ghost) {
say('Baal');
return;
}
ghost();
}
})();
Baal();
}
say('Baal'); // or just Baal();
Note that the meat of what happens here is from the function Baal, namely that it calls a hardcoded ghost() which in turn calls a hardcoded alert. Why go through all of this trouble to access a hardcoded function?
A better way would be to inject this function as a callback which expects some parameters to be injected.
jsFiddle Demo
function say(callback){
var params = "!";
if( typeof callback == "function" ){
callback(params);
}
}
say(function(params){
alert(params);
});
It's very difficult for me to read through your code and figure out what you are trying to accomplish with it, but it appears that you are trying to introduce a variable into the current scope so that you can call it. You cannot do this in javascript with the method that you demonstrated. Scoping only ever "flows down". By that I mean that a variable or function defined within a function will only be available to that function and any other functions defined therein. Your function named ghost will only ever be available within the function where it is defined, regardless of when that function is evaluated.
What you can do, however, is write a function that returns a function. You can then call that function and assign the result to a variable in the scope where you want to expose functionality. Doing that would look something like this.
function defineSpecialAlert() {
return function(name) {
alert(name + "!");
};
}
var newlyDefinedMethod = defineSpecialAlert();
newlyDefinedMethod("Baal");
So if I understand, it seems like you want to create an alias of eval: Something like
#Note this code is not intended as a solution, but demonstrates
#an attempt that is guaranteed to fail.
#
function myAlias(ctx) {
eval.call(ctx, 'var ghost = 42');
}
myAlias(this);
alert(ghost);
Javascript allows many funky sleight-of-hand tricks especially with closures, but this is maybe the one impossible thing that javascript cannot do. I've tried at length to do this exact same thing, and I can tell you that you'll run into nothing but complaints from the browser, saying that eval cannot be re-contexted or aliased in any way.
Hello i have the following issue i am not quite sure how to search for it:
function(){
var sites;
var controller = {
list: function(){
sites = "some value";
}
}
}
So the question is how to access the sites variable from the top defined as
var sites
EDIT:
Here is a more complete part. i am Using marionette.js. i don't want to define the variable attached to the Module (code below) variable but keep it private to the Module, hope that makes sense. Here is the code that works:
Admin.module("Site", function(Module, App, Backbone, Marionette, $, _ ) {
Module.sites = null;
Module.Controller = {
list: function (id) {
Module.sites = App.request("site:entities");
}
};
});
and i would like instead of
Module.sites=null;
to do
var sites;
That sort of thing does make a difference right? Because in the first case i would be defining an accessible variable from outside where as the second case it would be a private one. i am a bit new to javascript so please try to make it simple.
if you are looking for global access, just declare the variable outside the function first, make your changes to the variable inside the function, then you can get the value whenever you need it.
I have found some info on this: sadly what i am trying to do doesn't seem possible.
Can I access a private variable of a Marionette module in a second definition of that module?
So i guess i have to do _variable to make developers know its private.
Disclaimer: I have no experience using Marionette, however, what you're describing sounds very doable.
One of the most powerful (in my opinion) features of JavaScript is closures. What this means is that any function declared from within another function has access to the variables declared in the outer function.
For example:
var func;
function foo() {
var answer = 42;
func = function () {
// I have access to variable answer from in here.
return answer++;
};
}
// By calling foo(), I will assign the function func that has access "answer"
foo();
// Now I can call the func() function and it has access to the "answer"
// variable even though it was in a scope that doesn't exist anymore.
// Outputs:
// 42
// 43
console.log(func());
console.log(func());
What this means is that if you declare var sites from within your module definition function as you described, you should have access to it from within any of your inner anonymous functions. The only exception is if Marionette is re-writing your functions (by using the Function function and toString()), which seems unlikely but possible.
Your original example should would as described, my suspicion is that there is something else going wrong with the code that is unrelated to your scope.
I have a function that looks like this:
function outer() {
function inner_1() {
alert('inner_1');
}
function inner_2() {
alert('inner_2');
}
function inner_3() {
alert('inner_3');
}
inner_1();
inner_2();
inner_3();
}
I need to call outer(), but I want to replace inner_1() with another function.
I have tried this:
new_outer = outer;
new_outer.inner_1 = function() {
alert('my new inner function');
};
If I try to call the newly redefined inner_1 like this:
new_outer.inner_1();
it works as expected ('my new inner function' is alerted).
But if I try to call the outer function:
new_outer();
the old version of inner_1 is called.
I want to redefine inner_1 and the call outer. How can I achieve this?
This seems like a really bad idea so I am not going to post any code. However, if you are pursuing the answer for "educational purposes only", I will just hint that although you cannot easily redefine a function from outside its scope (as per your example), there is nothing stopping you from redefining a function attached to a function object.
I do think, however, there is a better solution to whatever the problem you are trying to solve is.