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) {...};
Related
This question already has an answer here:
How to inspect a JavaScript Bound Function
(1 answer)
Closed 2 years ago.
Given a function,
function main() {
// some logic
}
Lets assume the function main is bind with const obj = { name: "John Doe" }
like const fn = main.bind(obj);
Now the question is, Is there a way to get the fn function binding?
Note: i know binding can be accessed using the this keyword inside the main function but is there any way to access this value outside the context. is there any magic (hypothetical) method like fn.getContext().
Thank you for your time.
No there is not. While the new function object has an internal [[BoundThis]] slot, that slot is not accessible via a user-facing API.
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:
How does the "this" keyword work, and when should it be used?
(22 answers)
Closed 6 years ago.
Suppose I have an object:
var o = {
prop: 3,
method: function() {return this.prop}
}
I was expecting this
(o.method)()
to return undefined, however it returned 3 meaning that this is set to o inside method. Why is it so? If you evaluate (o.method) separately, it evaluates to a standalone function, so I expected this to reference global object. Why, for example, the difference exists here:
(o.method)() vs (o.method || true)()
I know that o.method() will use o as context, the question is specifically about accessing the function first like this (o.method) and then calling it.
That's just how JavaScript's rules work. Unless you do some contortions, this usually means the thing before the . when you access the method prior to calling it. In this case, that's o.
The following statements are identical:
(o.method)();
o.method();
o.method.call(o);
o["method"]();
However, if you put the method on something else, it'll take on the meaning of the thing it's on:
var p = {prop: 42, method: o.method};
p.method(); // returns 42
var method = o.method;
var prop = 13;
method(); // returns 13
Note: As JavaScript grew to be much more than it was originally designed for, people realized that this probably wasn't the most intuitive way for this to work, so in ES6 if you use "Arrow Functions" (aka Lambda Functions) it won't rebind this.
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.