Is there a way to know that 2 javascript variable point to the same memory address ?
var my_var = {
id: 1,
attribute: "myAttribute"
}
var copy = my_var;
//someting like
if(copy === my_var) return true;
4 Answers
You can't alias variables like you can in C. In javascript, something like
var x = 1;
var y = x
y = 4;
// x is still 1
will always be the case.
However, objects are always passed by reference
var x = { one: 1, two: 2 };
var y = x;
y.one = 100;
// x.one is now 100
4 Comments
freakish
-1: In case of objects "equality" means "the same memory reference".
bendecoste
You are correct, removed that from the post as I think it is exaplined better above.
freakish
Also the first piece of code would produce the same result in C as well. Actually variable assignment works exactly the same as in C. The difference is in: 1) passing variables to functions, 2) there is no reference/pointer operator in JavaScript (i.e. you can't work with memory directly).
bendecoste
(thumbsup), I guess I should have included a C example with pointer operations, they were excluded because javascript doesn't have them .. wording is a little unclear, I suppose.
Is there a way to know if 2 javascript variable point to the same memory address ?
Generally the answer is no. Primitive types (like numbers) are being passed around by value. So we have
> var x = 1;
> var y = 1;
> x === y;
true
even though they don't refer to the same memory location (well, this is an implementation detail, but they are not pointing to the same memory address at least in V8).
But when both sides are objects then yes: use == or ===. If both sides are objects then each operator checks whether they point at the same memory address.
> var x = {test: 1};
> var y = {test: 1};
> x === y;
false
6 Comments
MartianMartian
so in the first example, are x and y pointing to the same 1? please
freakish
@Matian2049 As in the same memory address? That is an implementation detail and depends on the JavaScript engine you are using. For example under V8 (Chrome's JS) the answer is no: that's because primitives live on the stack.
MartianMartian
Thanks for the prompt answer. that's what i meant, if 1 and 1 share same memo addr. that's clear now.
MartianMartian
so i'm debating w/ myself. when var a = 1; var b = a; is 1 copied to b or is be pointing to the first 1 created for a?
freakish
@Matian2049 It is copied. Try this:
var a = 1; var b = a; a = 2; console.log(b); It will print 1. |
In javascript when we declare more than one variable with same value then each and every variable points same memory location.Basically javascript follow the reference counting algorithm.same things happen in Python.
1 Comment
Mehraj Malik
Hw to proof
each and every variable point same memory location by a program?In Python, you can use id(variableName) to get the memory address:
>>> x = "hello world"
>>> id(x)
4347327152
>>> x = 2
>>> id(x)
4305328544
But I don't know the how to achieve it in JavaScript. Otherwise, we can also easily see whether two pointers point to the same address. Hopefully my answer provides some useful information.
6 Comments
temporary_user_name
this is exactly what im trying to find and none of the other answers help at all! i need a javascript version of python's
id built-in
Luillyfe
I do not think this answer deserves such a amount of hateness. For all the people who vote down, this answer could dive to a proper search to the people who is looking for something similar in Javascript.
viery365
Exactly, in python when you do
x = 1 and then x = y, x and y share the same reference to the memory address. I still don't understand whether in javaScript happens the same.
Himanshu
For all the people who voted down, at-least add a reason to do so.
Mathews Mathai
@Himanshu Clearly, this answer doesn't answer the question. "Can be done in python and not sure about Javascript" is more like an acceptable comment and not an answer.
|
===means that 2 vars have same type, in your caseObjectand this will betrue. This will not check for samememory allocation===checks type and value. If both sides are objects it checks whether they point to the same address. That's it.