javascript - not - pass by value vs pass by reference




Is JavaScript a pass-by-reference or pass-by-value language? (20)

The primitive types (Number, String, etc.) are passed by value, but Objects are unknown, because they can be both passed-by-value (in case we consider that a variable holding an object is in fact a reference to the object) and passed-by-reference (when we consider that the variable to the object holds the object itself).

Although it doesn't really matter at the end, I want to know what is the correct way to present the arguments passing conventions. Is there an excerpt from JavaScript specification, which defines what should be the semantics regarding this?


sharing what I know of references in javascript

In Javascript, objects are stored as references:

var a = {
  a: 1,
  b: 2,
  c: 3
};
var b = a;

//b.c is referencing to a.c value
console.log(b.c) //output: 3
//changing value of b.c
b.c = 4
//also changes the value of a.c
console.log(a.c) //output: 4

JavaScript passes primitive types by value and object types by reference

Now, people like to bicker endlessly about whether "pass by reference" is the correct way to describe what Java et al. actually do. The point is this:

  1. Passing an object does not copy the object.
  2. An object passed to a function can have its members modified by the function.
  3. A primitive value passed to a function cannot be modified by the function. A copy is made.

In my book that's called passing by reference.

Brian Bi - Which programming languages are pass by reference?


  1. primitive type variable like string,number are always pass as pass by value.
  2. Array and Object is passed as pass by reference or pass by value based on these two condition.

    • if you are changing value of that Object or array with new Object or Array then it is pass by Value.

      object1 = {item: "car"}; array1=[1,2,3];

    here you are assigning new object or array to old one.you are not changing the value of property of old object.so it is pass by value.

    • if you are changing a property value of an object or array then it is pass by Reference.

      object1.key1= "car"; array1[0]=9;

    here you are changing a property value of old object.you are not assigning new object or array to old one.so it is pass by reference.

Code

    function passVar(object1, object2, number1) {

        object1.key1= "laptop";
        object2 = {
            key2: "computer"
        };
        number1 = number1 + 1;
    }

    var object1 = {
        key1: "car"
    };
    var object2 = {
        key2: "bike"
    };
    var number1 = 10;

    passVar(object1, object2, number1);
    console.log(object1.key1);
    console.log(object2.key2);
    console.log(number1);

Output: -
    laptop
    bike
    10

A very detailed explanation about copying, passing and comparing by value and by reference is in this chapter of "JavaScript: The Definitive Guide" book.

Before we leave the topic of manipulating objects and arrays by reference, we need to clear up a point of nomenclature. The phrase "pass by reference" can have several meanings. To some readers, the phrase refers to a function invocation technique that allows a function to assign new values to its arguments and to have those modified values visible outside the function. This is not the way the term is used in this book. Here, we mean simply that a reference to an object or array -- not the object itself -- is passed to a function. A function can use the reference to modify properties of the object or elements of the array. But if the function overwrites the reference with a reference to a new object or array, that modification is not visible outside of the function. Readers familiar with the other meaning of this term may prefer to say that objects and arrays are passed by value, but the value that is passed is actually a reference rather than the object itself.

One thing I still cannot figure out. Check code below. Any thoughts?

function A() {}
A.prototype.foo = function() {
    return 'initial value';
}


function B() {}
B.prototype.bar = A.prototype.foo;

console.log(A.prototype.foo()); //initial value
console.log(B.prototype.bar()); //initial value

A.prototype.foo = function() {
    return 'changed now';
}

console.log(A.prototype.foo()); //changed now
console.log(B.prototype.bar()); //Why still 'initial value'???

Consider the following:

  1. Variables are pointers to values in memory.
  2. Reassigning a variable merely points that pointer at a new value.
  3. Reassigning a variable will never affect other variables that were pointing at that same object

So, forget about "pass by reference/value" don't get hung up on "pass by reference/value" because:

  1. The terms are only used to describe the behavior of a language, not necessarily the actual underlying implementation. As a result of this abstraction, critical details that are essential for a decent explanation are lost, which inevitably leads to the current situation where a single term doesn't adequately describe the actual behavior and supplementary info has to be provided
  2. These concepts were not originally defined with the intent of describing javascript in particular and so I don't feel compelled to use them when they only add to the confusion.

To answer your question: pointers are passed.


// code
var obj = {
    name: 'Fred',
    num: 1
};

// illustration
               'Fred'
              /
             /
(obj) ---- {}
             \
              \
               1


// code
obj.name = 'George';


// illustration
                 'Fred'


(obj) ---- {} ----- 'George'
             \
              \
               1


// code
obj = {};

// illustration
                 'Fred'


(obj)      {} ----- 'George'
  |          \
  |           \
 { }            1


// code
var obj = {
    text: 'Hello world!'
};

/* function parameters get their own pointer to 
 * the arguments that are passed in, just like any other variable */
someFunc(obj);


// illustration
(caller scope)        (someFunc scope)
           \             /
            \           /
             \         /
              \       /
               \     /
                 { }
                  |
                  |
                  |
            'Hello world'

Some final comments:

  • It's tempting to think that primitives are enforced by special rules while objects are not, but primitives are simply the end of the pointer chain.
  • As a final example, consider why a common attempt to clear an array doesn't work as expected.


var a = [1,2];
var b = a;

a = [];
console.log(b); // [1,2]
// doesn't work because `b` is still pointing at the original array

For programming language lawyers, I've went through the following sections of ECMAScript 5.1 (which is easier to read than the latest edition), and go as far as asking it on the ECMAScript mailing list.

TL;DR: Everythings're passed by value, but properties of Objects are references, and the definition of Object is creepily lacking in the standard.

Construction of Argument Lists

Section 11.2.4 "Argument Lists" says the following on producing a argument list consisting of only 1 argument:

The production ArgumentList : AssignmentExpression is evaluated as follows:

  1. Let ref be the result of evaluating AssignmentExpression.
  2. Let arg be GetValue(ref).
  3. Return a List whose sole item is arg.

The section also enumerate cases where argument list has 0 or >1 arguments.

Thus, everything's are passed by reference.

Access of Object Properties

Section 11.2.1 "Property Accessors"

The production MemberExpression : MemberExpression [ Expression ] is evaluated as follows:

  1. Let baseReference be the result of evaluating MemberExpression.
  2. Let baseValue be GetValue(baseReference).
  3. Let propertyNameReference be the result of evaluating Expression.
  4. Let propertyNameValue be GetValue(propertyNameReference).
  5. Call CheckObjectCoercible(baseValue).
  6. Let propertyNameString be ToString(propertyNameValue).
  7. If the syntactic production that is being evaluated is contained in strict mode code, let strict be true, else let strict be false.
  8. Return a value of type Reference whose base value is baseValue and whose referenced name is propertyNameString, and whose strict mode flag is strict.

Thus, properties of Objects are always available as reference.

On Reference

It is described in section 8.7 "The Reference Specification Type", that references are not real types in the language - they're only used to describe the behavior of the delete, the typeof, and the assignment operators.

Definition of "Object"

It is defined in 5.1 edition that "An Object is a collection of properties". Therefore, we can infer, that the value of the object is the collection, but as to what is the value of the collection is poorly defined in the spec, and requires a bit of effort to understand.


I would say it is pass-by-copy -

Consider arguments and variable objects are objects created during the execution context created in the beginning of function invocation - and your actual value/reference passed into the function just get stored in this arguments + variable objects.

Simply speaking, for primitive types, the values get copied in the beginning of function call, for object type, the reference get copied.


I've read through these answers multiple times, but didn't REALLY get it until I learned about the technical definition of "Call by sharing" as termed by Barbara Liskov

The semantics of call by sharing differ from call by reference in that assignments to function arguments within the function aren't visible to the caller (unlike by reference semantics)[citation needed], so e.g. if a variable was passed, it is not possible to simulate an assignment on that variable in the caller's scope. However, since the function has access to the same object as the caller (no copy is made), mutations to those objects, if the objects are mutable, within the function are visible to the caller, which may appear to differ from call by value semantics. Mutations of a mutable object within the function are visible to the caller because the object is not copied or cloned — it is shared.

That is, parameter references are alterable if you go and access the parameter value itself. On the other hand, assignment to a parameter will disappear after evaluation, and is non-accessible to the function caller.


In a low level language, if you want to pass a variable by reference you have to use a specific syntax in the creation of the function:

int myAge = 14;
increaseAgeByRef(myAge);
function increaseAgeByRef(int &age) {
  *age = *age + 1;
}

The &age is a reference to myAge, but if you want the value you have to convert the reference, using *age.

Javascript is a high level language that does this conversion for you. So, although objects are passed by reference, the language converts the reference parameter to the value. You don't need to use &, on the function definition, to pass it by reference, neither *, on the function body, to convert the reference to the value, JS does it for you.

That's why when you try to change an object inside a function, by replacing it's value (i.e. age = {value:5}), the change doesn't persist, but if you change it's properties (i.e. age.value = 5), it does.

Learn more


In some case, this may be helpful to alter anArg:

function alterMyArg(func) {
    // process some data
    // ...
    func(data);
}

alertMyArg(function(d) {anArg = d;});

It's interesting in Javascript. Consider this example:

function changeStuff(a, b, c)
{
  a = a * 10;
  b.item = "changed";
  c = {item: "changed"};
}

var num = 10;
var obj1 = {item: "unchanged"};
var obj2 = {item: "unchanged"};

changeStuff(num, obj1, obj2);

console.log(num);
console.log(obj1.item);    
console.log(obj2.item);

This produces the output:

10
changed
unchanged
  • If it was pure pass by value, then changing obj1.item would have no effect on the obj1 outside of the function.
  • If it was pure pass by reference, then everything would have changed. num would be 100, and obj2.item would read "changed".

Instead, the situation is that the item passed in is passed by value. But the item that is passed by value is itself a reference. Technically, this is called call-by-sharing.

In practical terms, this means that if you change the parameter itself (as with num and obj2), that won't affect the item that was fed into the parameter. But if you change the INTERNALS of the parameter, that will propagate back up (as with obj1).


Javascript is always pass-by-value, everything is of value type. Objects are values, member functions of objects are values themselves (remember that functions are first-class objects in Javascript). Also, regarding the concept that everything in Javascript is an object, this is wrong. Strings, symbols, numbers, booleans, nulls and undefineds are primitives. On occasion they can leverage some member functions and properties inherited from their base prototypes, but this is only for convenience, it does not mean that they are objects themselves. Try the following for reference

x = "test";
alert(x.foo);
x.foo = 12;
alert(x.foo);

In both alerts you will find the value to be undefined.


My simple way to understand this...

  • When calling a function, you are passing the content (reference or value) of the argument variables, not the the variables themselves.

    var var1 = 13;
    var var2 = { prop: 2 };
    
    //13 and var2's content (reference) are being passed here
    foo(var1, var2); 
    
  • Inside the function, parameter variables, inVar1 and inVar2, receive the contents being passed.

    function foo(inVar1, inVar2){
        //changing contents of inVar1 and inVar2 won't affect variables outside
        inVar1 = 20;
        inVar2 = { prop: 7 };
    }
    
  • Since inVar2 received the reference of { prop: 2 }, you can change the value of the object's property.

    function foo(inVar1, inVar2){
        inVar2.prop = 7; 
    }
    

Object outside a function is passed into a function by giving a reference to the outside obejct. When you use that reference to manipulate its object, the object outside is thus affected. However, if inside the function you decided to point the reference to something else, you did not affect the object outside at all, because all you did was re-direct the reference to something else.


Primitives are passed by value and objects are passed by reference. This is quite different from other languages like C, VB or Delphi. I can't say how they handle objects and primitives exactly, but I know of VB and Delphi that it can (and should) be specified.

php does something similar since version 5: all objects are passed by reference, but all primitives may be passed by reference, if preceeded by an ampersand (&). Otherwise primitives are passed by value.

So in javascript, if I pass an object X into a function via a parameter, it will still be X. If you are changing data inside the function (or any other object, but that's not important) that new value is also available outside the function.


Semantics!! Setting concrete definitions will necessarily make some answers and comments incompatible since they are not describing the same thing even when using the same words and phrases, but it is critical to get past the confusion (especially for new programmers).

First of all, there are multiple levels of abstraction that not everyone seems to grasp. Newer programmers who have learned on 4th or 5th generation languages may have difficulty wrapping their mind around concepts familiar to assembly or C programmers not phased by pointers to pointers to pointers. Pass-by-reference does not simply mean the ability to change a referenced object using a function parameter variable.

Variable: Combined concept of a symbol which references a value at a particular location in memory. This term is usually too loaded to be used alone in discussing details.

Symbol: Text string used to refer to variable (i.e. variable's name).

Value: Particular bits stored in memory and referenced using variable's symbol.

Memory location: Where a variable's value is stored. (The location itself is represented by a number separate from the value stored at the location.)

Function parameter: Variable declared in a function definition, used for referencing variables passed to the function.

Function argument: Variable outside the function which is passed to the function by the caller.

Object variable: Variable whose basic underlying value is not the "object" itself, rather its value is a pointer (memory location value) to another location in memory where the object's actual data is stored. In most higher-generation languages, the "pointer" aspect is effectively hidden by automatic de-referencing in various contexts.

Primitive variable: Variable whose value IS the actual value. Even this concept can be complicated by auto-boxing and object-like contexts of various languages, but the general ideas is that the variable's value IS the actual value represented by the variable's symbol rather than a pointer to another memory location.

Function arguments and parameters are not the same thing. Also, a variable's value is not the variable's object (as already pointed out by various people, but apparently ignored). These distinctions are critical to proper understanding.

Pass-by-value or Call-by-sharing (for objects): The function argument's value is COPIED to another memory location which is referenced by the function's parameter symbol (regardless of whether it's on the stack or heap). In other words, the function parameter received a copy of the passed argument's value... AND (critical) the argument's value IS NEVER UPDATED / ALTERED / CHANGED by the calling function. Remember, an object variable's value is NOT the object itself, rather it is the pointer to the object, so passing an object variable by value copies the pointer to the function parameter variable. The function parameter's value points to the exact same object in memory. The object data itself can be altered directly via the function parameter, BUT the function argument's value IS NEVER UPDATED, so it will continue to point to the same object throughout and even after the function call (even if its object's data was altered or if the function parameter is assigned a different object altogether). It is incorrect to conclude that the function argument was passed by reference just because the referenced object is updatable via the function parameter variable.

Call / Pass-by-reference: The function argument's value can/will be updated directly by the corresponding function parameter. If it helps, the function parameter becomes an effective "alias" for the argument--they effectively refer to the same value at the same memory location. If a function argument is an object variable, the ability to change the object's data is no different than the pass-by-value case since the function parameter will still point to the same object as the argument. But in the object variable case, if the function parameter is set to a completely different object, then the argument will likewise also point to the different object--this does not happen in the pass-by-value case.

JavaScript does not pass by reference. If you read closely, you will realize that all contrary opinions misunderstand what is meant by pass-by-value and they falsely conclude that the ability to update an object's data via the function parameter is synonymous to "pass-by-value".

Object clone/copy: A new object is created and the original object's data is copied. This can be a deep copy or shallow copy, but the point is that a new object is created. Creating a copy of an object is a separate concept from pass-by-value. Some languages distinguish between class object and structs (or the like), and may have different behavior for passing variables of the different types. But JavaScript does not do anything like this automatically when passing object variables. But the absence of automatic object cloning does not translate to pass-by-reference.


The most succinct explanation I found was in the AirBNB style guide:

  • Primitives: When you access a primitive type you work directly on its value

    • string
    • number
    • boolean
    • null
    • undefined

E.g.:

var foo = 1,
    bar = foo;

bar = 9;

console.log(foo, bar); // => 1, 9
  • Complex: When you access a complex type you work on a reference to its value

    • object
    • array
    • function

E.g.:

var foo = [1, 2],
    bar = foo;

bar[0] = 9;

console.log(foo[0], bar[0]); // => 9, 9

I.e. effectively primitive types are passed by value, and complex types are passed by reference.


The variable doesn't "hold" the object, it holds a reference. You can assign that reference to another variable, now both reference the same object. It's always pass by value (even when that value is a reference...).

There's no way to alter the value held by a variable passed as a parameter, which would be possible if JS supported passing by reference.


Think of it like this: It's always pass by value. However, the value of an object is not the object itself, but a reference to that object.

Here is an example, passing a number (a primitive type)

function changePrimitive(val) {
    // At this point there are two '10's in memory.
    // Changing one won't affect the other
    val = val * 10;
}
var x = 10;
changePrimitive(x);
// x === 10

Repeating this with an object yields different results:

function changeObject(obj) {
    // At this point there are two references (x and obj) in memory,
    // but these both point to the same object.
    // changing the object will change the underlying object that
    // x and obj both hold a reference to.
    obj.val = obj.val * 10;
}
var x = { val: 10 };
changeObject(x);
// x === { val: 100 }

One more example:

function changeObject(obj) {
    // Again there are two references (x and obj) in memory,
    // these both point to the same object.
    // now we create a completely new object and assign it.
    // obj's reference now points to the new object.
    // x's reference doesn't change.
    obj = { val: 100 };
}
var x = { val: 10 };
changeObject(x);
// x === { val: 10}

This is little more explanation for Pass by value and Pass by reference (Javascript). In this concept, they are talking about passing the variable by reference and passing the variable by reference.

Pass by value (Primitive Type)

var a = 3;
var b = a;

console.log(a); // a = 3
console.log(b); // b = 3

a=4;
console.log(a); // a = 4
console.log(b); // b = 3
  • applies to all primitive type in JS(string, number, boolean, undefined, null).
  • a is allocated a memory (say 0x001) and b creates a copy of the value in memory (say 0x002).
  • So changing the value of a variable doesn't affect the other, as they both reside in two different locations.

Pass by reference (objects)

var c = { "name" : "john" };    
var d = c;

console.log(c); // { "name" : "john" }
console.log(d); // { "name" : "john" }

c.name = "doe"; 

console.log(c); // { "name" : "doe" }    
console.log(d); // { "name" : "doe" }
  • JS engine assigns the object to the variable c, it points to some memory say (0x012)
  • when d=c, in this step d points to the same location (0x012).
  • changing the value of any changes value for both the variable.
  • functions are objects

Special case, Pass by reference (objects)

c = {"name" : "jane"}; 
console.log(c); // { "name" : "jane" }    
console.log(d); // { "name" : "doe" }
  • The equal(=) operator sets up new memory space or address






pass-by-value