This question already has answers here:
How to access the correct `this` inside a callback
(13 answers)
Closed 6 years ago.
My problem is as simple as the title.. I have some code which makes an AJAX call. This code is similar to this (JSFiddle):
function Test() {
this.name = "U don't wanna know my name..";
}
Test.prototype.ajax = function() {
$.ajax("url/path", data, function() {
alert(this.name);
});
};
var test = new Test();
test.ajax();
In this case this is undefined. I could place the following code before the ajax call and use that in stead of this:
var diz = this;
I was wondering if there's another way of using this without creating a new variable for it.
In this case this is undefined.
this.name is undefined (assuming you meant that), because this is specific to a function's context. Inside that ajax's callback handler this no more belonged to Test, it belonged to that callback function.
was wondering if there's another way of using this without creating a
new variable for it.
I don't think that without saving the reference to parent's this (Test's this) you can access this that belonged to a more global scope from a function's scope.
Related
This question already has answers here:
How to access the correct `this` inside a callback
(13 answers)
Closed 3 years ago.
Edit:
The question referred to as the duplicate doesn't really answer why arrow functions shouldn't be used to access a non-lexical this. It just answers that arrow functions automatically bind this. My situation is that using an arrow function rather than a normal function cause me to lose the correct reference of this. If you need this outside of the current scope, use a normal function.
I've searched around for a solution to my problem with no luck. Even if I was pointed in the right direction as to what I needed to do would be awesome! My problem essentially is that this.Favkey is undefined here:
const setFavorite = val => {
console.log(this);
this.Favorite = val;
AsyncStorage.setItem(this.Favkey, JSON.stringify(val));
};
This function is getting assigned to a particular object like so:
for (const obj of objArray) {
obj.Favkey = `c${obj['-id=']}`;
obj.Favorite = await getFavorite(obj.Favkey);
obj.SetFavorite = setFavorite;
}
And then I am calling this later on a button:
onPress={val => props.myObj.SetFavorite(val)}
In the first block of code, I want this to be the specific obj that I am attempting to enclose the function on. But this.Favkey in setFavorite is undefined. What is printed out instead on console.log(this) is what I think is the whole prototype of Object. So I'm not quite sure what I'm doing wrong here.
Don't use an arrow function - it loses the binding to this which is what you're trying to access. Just use a normal function:
const setFavorite = function(val) {...};
This question already has answers here:
How does the "this" keyword in Javascript act within an object literal? [duplicate]
(4 answers)
Closed 4 years ago.
Was doing some dirty things to Array.prototype when I ran into this:
Array.prototype.hook_pop = function(callback) {
var base_pop = this.pop.bind(this); //<-- this works
var base_pop = this.pop; //<-- this doesn't work
this.pop = function() {
var ret = base_pop();
callback(ret, this);
return ret;
}
}
Initially I tried using the non-working option and got an error "Uncaught TypeError: Cannot convert undefined or null to object".
The way I've understood it, unless otherwise bound, "this" should point to the object through which the method is called from, in this case the array instance. When called on the same object though, either way, "this" should be the same when being passed to the pop function, whether its bound or not. Why doesn't the second option work?
var ret = base_pop();
In this line you're invoking base_pop() by itself, and not as a method of any object. Because of this, its this value isn't set.
This question already has answers here:
How does the "this" keyword in Javascript act within an object literal? [duplicate]
(4 answers)
Closed 5 years ago.
Im reading through the YDKJS books and I thought I understood lexical this for the most part.
However there is a section with code that shows this:
var obj = {
id: "awesome",
cool: function coolFn() {
console.log( this.id );
}
};
var id = "not awesome";
obj.cool(); // awesome
setTimeout( obj.cool, 100 ); // not awesome
Ok so the first obj.cool() makes sense of course....but why is the setTimeout printing not awesome.....I mean it's still calling obj.cool() which this refers to it's own objects id?
Or does setTimeout get called as another function that calls obj.cool()? but even in that case (Which I tried calling obj.cool() inside another function that also had an id property and it still printed the right one......so why would the this change with setTimeout?
Since the OBJ.COOL function is passed by reference, you are not actually passing the context object with it. The function gets new invocation context and executes on the WINDOW object, which now has a property called ID which was defined earlier.
This question already has answers here:
How does the "this" keyword work, and when should it be used?
(22 answers)
Closed 6 years ago.
In java script when we make a new constructor function we use "this.property name". We use "this" to refer the object which currently in use. But in a general function we doesn't use "this" keyword. According to my understanding if we use "this" in function it should point to the current function. However when we used, it was not producing the expected result. Why? Example
function greet(name){ console.log("Hello " + this.name);
}
Output is "Hello" then blank.
Because in general function, we are by default referring 'window' object so anything we make it becomes window level object or variable.
Like,
function fun(){
this.title = "window";
}
fun();
or window.fun(); //both are same. Since we call window.fun, this.title means window.fun.
If you create like this:
var obj = {
}
**Now to make title at obj level, you can do like this:
fun.call(obj);
Now you can call obj.title.**
Read this about this
In most cases, the value of this is determined by how a function is called.
When you use the new keyword in javascript an implicit object is created and returned from the function call. Inside of the function this refers to the newly created object. Calling a function without new does not have the same behavior.
See: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new
This question already has answers here:
Access outside variable in loop from Javascript closure [duplicate]
(5 answers)
Closed 8 years ago.
I use the following function to bind all members of a certain class to a function that changes the current page. Heres the function:
function _bind_menu_items() {
var menuItems = fja.getElementsByClass("menuItem"),
index,
elementID,
divIdToBind;
for (index in menuItems) {
elementID = menuItems[index].id;
divIdToBind = elementID.replace("Item", "");
menuItems[index].onclick = function() {
fja.menu.changeActivePage(divIdToBind);
};
}
}
I’ve checked that everything works as expected down to the actual assignment of the onclick property. The error I’m getting is that every div belonging to the menuItem class seems to call the same onclick function... as if the divIdToBind string is the exact same for every onclick that is assigned... How can I fix this?
Number one mistake for Javascript beginners. you're missing that the anonymous functions which gets bound to your onclick handler, closes over its parent context. Since all closures from the same parent context share the same scope chain, all of this functions will reference the last value which gets passed into divIdToBind.
To solve that issue, the most common workaround is to create another function(-context):
menuItems[index].onclick = (function(id) {
return function() {
fja.menu.changeActivePage(id);
};
}(divIdToBind));
Now we do create another function which gets executed immediately. We pass it in the value from divIdToBind and all it does is to return another function (just to create a new context)