I'm having some troubles to understand what is copied and what is referenced in javascript. What is clear to me is that an object is referenced when it's an object and copied when it's a variable:
var reference = myObject // myObject = {string:'text'} -> referenced
var copy = myVar // myVar = 'text' -> copied
Now what if I want to create an object of objects/var?
var newObject = {anObject: myObject, aVar: myVar}
Will they be copied or referenced? If they are copied, is there a way to make them referenced (at least the object: myObject)?
Edit (Angularjs specific): I wanted to make sure that everything answered is also true with AngularJS and the $rootScope variables (even though I guess the behavior should be identical)
Objects are Passed by Reference In JavaScript
object references are
values. Because of this, objects will behave like they are passed by
reference: If a function changes an object property, it changes the
original value. Changes to object properties are visible (reflected)
outside the function.
if you assign an object to property of another object it is still assigned by reference. while you change the new object value it reflect in base.
eg:
var myObj = {
a: 1
}
var testObj = {
ref: myObj
}
console.log(myObj.a); //1
//change the value from second object
testObj.ref.a = "new val";
console.log(myObj.a); //new val
Primitive values are copied and Non primitive values such as objects and arrays are referenced.
I think "myObject" will be referenced and myVar will be copied. To get the copy of myObject, you can clone it and assign it to the other variable.
Using JQuery:
{
anObject: $.extend({},myObject),
aVar: myVar
}
Related
I'm currently setting up JavaScript classes by extending a function prototype with my object of functions / variables like so....
//class shorthand
var clss = function(args){
var c = function() {};
_.extend(c.prototype, args); //using underscore.js
return c;
};
//class definition
var ThisIsMyClass = clss({
varExample: 5,
test: function() {
console.log(this.varExample);
},
alter: function(){
this.varExample = 8;
}
});
//class initialisers
var hello = new ThisIsMyClass();
var hi = new ThisIsMyClass();
var ayup = new ThisIsMyClass();
My question is that everything resides inside the ThisIsMyClass.prototype and the functions can be called and the variables read, but when the values are changed, they then appear outside of the prototype for that object (and also remaining in the prototype with its original value)?
after running this code
//class initialisers
hello.varExample = 6;
hi.alter();
//look at object structure
console.log(hello);
console.log(ayup);
console.log(hi);
//trace the values
hello.test();
ayup.test();
hi.test();
The console looks like this
Is prototype just a reference to the structure, and then when any changes are made, it copies them to object itself?
When you read a property on an object, the interpreter first looks on the actual object itself for that property. If it finds it there, it returns that value. If it does not find the property on the actual object, then it looks on the prototype. If it finds the property on the prototype, it returns that value.
When you set a property on an object, it always sets the property on the actual object (not on the prototype). So, once you've set the property on an object, any reading of that property will come from the one you set on the actual object, not the one on the prototype. Setting a property directly onto the object essentially "hides" or "overrides" the value set on the prototype. You can think of the prototype as the default value for the property, but as soon as you set one on the object itself, the prototype value is no longer used.
function foo() {}
foo.prototype.greet = function() {
console.log("Hello");
}
var x = new foo();
x.greet(); // will log "Hello" - finds method on the prototype
// set new method - will set this property directly on the object itself
// overriding what was on the prototype
x.greet = function() {
console.log("Goodbye");
}
x.greet(); // will log "Goodbye" - finds method on the object
Demo: http://jsfiddle.net/jfriend00/h6akb/
In the implementation of a javascript object, the object has both a set of properties on the object itself and a reference to its prototype. When looking up a property, it is looked for first on the object itself. If it is not found there, then the interpreter gets the reference to the prototype and looks for the property on the prototype object. The prototype object itself is just yet another object, so the interpreter looks for the property directly on the prototype object and, if found returns it. If not found, it checks to see if the prototype object itself has a prototype and so on (allowing for an inheritance chain).
There's a small test. I expect both obj\d object have their a menmber changed in the end. However the change1 style doesn't work, I wonder why, or why not, should it behave like this?
<button id="btn1" onclick="test()">Change</button>
<script>
var obj1 = { a : {strblahblah: "blah"} };
var obj2 = { a : {strblahblah: "blah"} };
function test(){
change1(obj1.a);
change2(obj2);
alert("obj1: " + obj1.toSource() + "\r\nobj2: " + obj2.toSource());
}
function change1(obj){
obj = {str: "change1"};
}
function change2(obj){
obj.a = {str: "change2"};
}
</script>
Result(after you click the button):
obj1: ({a:{strblahblah:"blah"}})
obj2: ({a:{str:"change2"}})
What's happening in change1 is that obj initially holds a reference to the object in obj1.a. However, the line:
obj = {str: "change1"};
doesn't change obj1.a. Instead it creates a new object ({str: "change1"}) and changes obj so that it now points to this object instead of obj1.a.
In contrast, change2 has obj initially holding a reference to the object in obj2, and has the line:
obj.a = {str: "change2"};
which accesses the internal structure of the referenced object (i.e. obj2) and hence makes actual changes to that object.
When you pass obj1.a to change1(), you are sending the value of obj1.a which itself is another object. But when you send the obj2 to change2(), its taking the reference of the object, and when you assigned the value the original value got changes, where as in the first case this had not happened.
As thg437 suggested, this best explains the case,
Is JavaScript a pass-by-reference or pass-by-value language?
From MDN:
The parameters of a function call are the function's arguments.
Arguments are passed to functions by value. If the function changes
the value of an argument, this change is not reflected globally or in
the calling function. However, object references are values, too, and
they are special: if the function changes the referred object's
properties, that change is visible outside the function
In change1 you are passing obj.a, which is actually a value (because it is an object reference, not the object itself). This means that you receive a copy of that reference in change1 and then modify it to point to a new object. All you've done is modify the copy of the reference that was passed into the function so that it's effectively pointing to something else in memory.
In change2 you are passing obj which is an object reference as well, but you are then modifying the a property of that reference, so you're getting the object that's pointed to by the reference and modifying its a property. This object that is modified is the same one that is pointed to by both the copy of the object reference you received by value in the function and the reference obj1 at the top of the code.
This statement obj = {str: "change1"};
causing obj to refer to something other than the passed argument,
where as obj.a = {str: "change2"};
causing "a" property in passed argument to be changed to something else.
It is expected, in 1st case you're just changing reference of input and in other case you're actually modifying input parameter.
When a variable refers to an object the value is a reference to the object (Referred from: Javascript by reference vs. by value )
function fun1(obj) {
obj.first = 1; //it affects the object ->obj
}
var myObj1 = {first: 0};
fun1(myObj1); //sending the object to the function
console.log(myObj1.first); // 1
But I would like to change the variable from an object, for example
function fun2(obj) {
obj = 1; }
var myObj2 = {first: 0};
fun2(myObj2.first);
console.log(myObj2.first);
Is there any way to achieve this?
Is there any way to achieve this?
Not directly. All you can do is pass an object and have the function modify a property on it, as in your first example. JavaScript does not have pass-by-reference, just pass-by-value. (The value may be an object reference, of course, but it's still a value.) There's no way in JavaScript to directly modify the variable/property you're passing into the function, because what the function receives is a copy of the value of that variable/property, not a reference to that variable/property.
Just to be clear about something: In your first code block, you said you were "sending the object to the function." That's incorrect. You send a reference to the object to the function.
This is the key thing to understand: Variables, properties, and function arguments (collectively, "variables") contain values, and those values are copied when you use assignment or when you pass them into functions. The value that refers to an object is called an object reference, because the value isn't the object, it's a reference to (pointer to) the object elsewhere in memory. The reference is copied when you pass it into a function, the object is not.
Don't confuse the "reference" in "object reference" with the "reference" in "pass-by-reference," they're completely different things. (In pass-by-reference, the reference is to the variable, not an object. JavaScript doesn't have pass-by-reference.)
I'm reading that in JavaScript, a common point of confusion arises because variables of primitives are passed by value, and variables of objects are passed by reference, while in function arguments, both primitives and references are passed by value.
In the course of my tinkering, I've made up the following code, but am having trouble wrapping my head around it.
> function setName2(obj) {
... obj.name="matt";
... obj = new Object();
... obj.name="obama";
... }
If I set
var person = new Object();
person.name = "michelle";
Then run
> setName2(person);
I get
> person.name;
'matt'
Which makes sense because the new object created is a pointer to a local object, hence not affecting the property of the global 'person'.
However, what if I first set
var obj = new Object();
obj.name = "michelle";
Then run
> setName2(obj);
?
I get the same outcome. Does this mean that the compiler recognizes the two variables of the same name (obj global and obj local) as references to different locations within the heap, each having some different pointer association, or is there a different explanation for this phenomenon?
JavaScript does not have pass-by-reference; everything is passed by value. However, some values are themselves references to objects. This distinction (pass-by-reference vs. is-a-reference) leads to much confusion.
An example to clear things up:
function f(o) { ... }
var obj = { foo: 42 };
f(obj);
No matter what you do in f, obj will always refer to the same object because the argument is not passed by reference. However the value obj, which is copied into o, is itself a reference to an object. This is why any property changes made to o inside the function will be visible on obj after it returns.
while in function arguments, both primitives and references are passed by value.
This is not true. There is nothing special about function arguments.
function setName2(obj) {
This accepts a reference to an object as an argument.
obj.name="matt";
This modifies the name property of the object that reference points to.
obj = new Object();
This replaces the reference to the original object with a reference to a new object.
obj.name="obama";
This modifies the name property of the new object. The original object is unchanged.
The confusion comes from the fact that "passed by reference" is misinterpreted by people or used in a wrong sense.
Parameters are passed by value. This means that changing the value inside the method doesn't change the original value.
In case of primitives, the value of a primitive is its value.
In case of objects, the value of an object is a reference to it. You can access and change object's content but you can't change the value of the reference itself.
In other programming languages, like C++ or C#, "passing by reference" means that you pass:
- a reference to a primitive type
- a reference to a reference to an object
In such case, not only the content of an object can be changed but also a reference itself can be changed.
There is NO passing by reference in Javascript.
Javascript uses pass-by-value.
The confusion is that objects are hold by reference variables (kind of pointers). In fact most common languages (java, javascript, etc.) do not have a real pass-by-reference behaviour. Another way to understand this could be pass-reference-by-value, although, formally, there is not such a thing.
That means when you pass an object as a parameter, you are actually passing a reference to the object by-value.
function setName2(obj) {
...
}
setName2(person);
here the contents of person (a reference, or "pointer" if you like) is copied by-value to a new local variable: obj.
obj and person are different variables that hold a reference to the same object.
So, doing obj = new Object(); makes obj to point to the new object. But person is unaffected since it is still a completely different variable.
I don't know where you read that, but it's absolutely not true. Objects are passed by reference, full stop. Whether or not it's a function parameter is completely irrelevant.
I'm sure I'm missing something obvious here, but I'd expect the argument to the changeMe method to be passed "by reference" - in other words, that changes to the parameter inside the function will change the variable outside the function.
The following was run in jsfiddle, using Chrome's F12 developer tools to show the console output. http://jsfiddle.net/fzEpa/
var object1 = { Property1: 'Value1' };
changeMe(object1);
console.log(object1);
function changeMe(refToObject) {
console.log(refToObject);
refToObject = { Property1: 'Value2' };
console.log(refToObject);
}
It is passed by reference, but it is a reference to the object, not to the object1 variable (which is also a reference to the object).
You are overwriting the reference to the object with a reference to a new object.
This leaves the original reference to the original object intact.
To modify the object, you would do something like this:
function changeMe(refToObject) {
refToObject.Property1 = 'Value2';
}
A reference to the object is passed as the argument value. However:
refToObject = { Property1: 'Value2' };
At this point you lose the reference to the object formerly referenced by refToObject as you're assigning this variable to reference a different object.
Now if you were to edit the refToObject's properties instead of discarding the former object reference, your code would work as exepected (as firstly explained in #Quentin's answer).
If you're familiar with C++ this would be equal to doing something like this:
void f(int* ref) {
ref = new int(3);
}
int* a = new int(5);
f(a);
printf("%d",a); //Prints 5
you are trying to redefine the Property 1,so it wouldnt work. in order to work with pass by reference ypu have to do it this way refToObject.Property1='Value2' inside your ChangeMe() function . refer this for better understanding Pass Variables by Reference in Javascript