The string, although modified in the function, beeing part of an array is changed in the global scope. It is more complicated, but it is possible. The assignment in a function, either for an object or an array has no effect outside the scope of the function. This is why giving an object as argument is sometimes called "call-sharing" rather than "call by reference". In the above snippet, because 2 is a scalar primitive, a holds one initial copy of that value, and b is assigned another copy of the value.
When changing b , you are in no way changing the value in a. But both c and d are separate references to the same shared value [1,2,3] , which is a compound value. It's important to note that neither c nor d more "owns" the [1,2,3] value -- both are just equal peer references to the value. So, when using either reference to modify.
To do that, b would have to be a pointer to a rather than a reference to the array -- but no such capability exists in JS! When we pass in the argument a , it assigns a copy of the a reference to x. Now, inside the function, we can use that reference to mutate the value itself push 4. To effectively pass a compound value like an array by value-copy, you need to manually make a copy of it, so that the reference passed doesn't still point to the original.
For example:. Here, obj acts as a wrapper for the scalar primitive property a. When passed to foo.. We now can use the wrapper reference to access the shared object, and update its property. After the function finishes, obj. 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. 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. 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 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.
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". 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.
In JavaScript, when assigning an object to a variable, the value assigned to the variable is a reference to the object:. Now, people like to bicker endlessly about whether "pass by reference" is the correct way to describe what Java et al.
The point is this:. There is no "pass by reference" available in JavaScript. Observation: If there isn't any way for an observer to examine the underlying memory of the engine, there is no way to determine whether an immutable value gets copied or a reference gets passed. JavaScript is more or less agnostic to the underlying memory model. JavaScript has values. Two variables can hold the same value or more accurate: two environment records can bind the same value.
The only type of values that can be mutated are objects through their abstract [[Get]] and [[Set]] operations. If you forget about computers and memory, this is all you need to describe JavaScript's behaviour, and it allows you to understand the specification. Now you might ask yourself how two variables can hold the same value on a computer. You might then look into the source code of a JavaScript engine and you'll most likely find something which a programmer of the language the engine was written in would call a reference.
So in fact you can say that JavaScript is "pass by value", whereas the value can be shared, and you can say that JavaScript is "pass by reference", which might be a useful logical abstraction for programmers from low level languages, or you might call the behaviour "call by sharing". As there is no such thing as a reference in JavaScript, all of these are neither wrong nor on point.
Therefore I don't think the answer is particularly useful to search for. It is a container for an object and the name of a property, and it is an intermediate value e. The term reference also sometimes appears in the specification in unrelated sections.
When calling a function, you are passing the content reference or value of the argument variables, not the the variables themselves. Inside the function, parameter variables, inVar1 and inVar2 , receive the contents being passed. Passing arguments to a function in JavaScript is analogous to passing parameters by pointer value in C:.
TL;DR : Everythings're passed by value, but properties of Objects are references, and the definition of Object is creepily lacking in the standard. Section It is described in section 8. It is defined in 5. 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.
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, 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.
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:. So, although objects are passed by reference, the language converts the reference parameter to the value. That's why when you try to change an object inside a function, by replacing it's value i.
Learn more. If you want normal function parameter behavior like in other languages passing copy of a value then just clone the object before passing into a function:. Basic types are passed by value i. Complex types objects are passed as "pointer to the object". So the actual stuff you are passing is a pointer which is passed by value it's an address, a numerical value like any other. Obviously if you try to modify a property of the object inside the function, the modification will be reflected even outside of such function.
That's because you are accessing the property via the pointer which points to the unique copy of the property. The confusion here has arises over "passing a pointer by value" and "passing an object by reference". I have found the extend method of the Underscore.
Simply speaking, for primitive types, the values get copied in the beginning of function call, for object type, the reference get copied. There's some discussion about the use of the term "pass by reference" in JavaScript here , but to answer your question:. An easy way to determine whether something is "pass by reference" is whether you can write a "swap" function. For example, in C, you can do:. Stack Overflow for Teams — Collaborate and share knowledge with a private group.
Create a free Team What is Teams? Collectives on Stack Overflow. Is JavaScript a pass-by-reference or pass-by-value language? Ask Question. Asked 12 years, 9 months ago. Active 6 days ago. Tutorials, references, and examples are constantly reviewed to avoid errors, but we cannot warrant full correctness of all content.
While using W3Schools, you agree to have read and accepted our terms of use , cookie and privacy policy. Copyright by Refsnes Data. All Rights Reserved. W3Schools is Powered by W3. That allows you to call a function before it has been declared:. The reason the preceding code works is that JavaScript engines move the declaration of foo to the beginning of the scope. They execute the code as if it looked like this:. Therefore, using a var declaration and a function expression similarly to the previous example results in an error:.
Most JavaScript engines support the nonstandard property name for function objects. Function declarations have it:. The name of anonymous function expressions is the empty string:. The name of a function is useful for debugging. Some people always give their function expressions names for that reason. Should you prefer a function declaration like the following? Or the equivalent combination of a var declaration plus a function expression? They are basically the same, but function declarations have two advantages over function expressions:.
They can supply a value for this when invoking a method and thus are mainly interesting in an object-oriented context see Calling Functions While Setting this: call , apply , and bind. This section explains two use cases for nonmethods. This method uses the elements of argArray as arguments while calling the function func ; that is, the following two expressions are equivalent:. It is not needed in a non-object-oriented setting and is thus null here. Thanks to apply , we can use Math.
This performs partial function application —a new function is created that calls func with this set to thisValue and the following arguments: first arg1 until argN , and then the actual arguments of the new function.
Here, we use bind to create a new function plus1 that is like add , but only requires the parameter y , because x is always Hence, the number of actual parameters and formal parameters can differ in two ways:. The special variable arguments exists only inside functions including methods. It is an array-like object that holds all of the actual parameters of the current function call.
The following code uses it:. It is array-like, but not an array. On one hand, it has a property length , and individual parameters can be read and written by index. On the other hand, arguments is not an array, it is only similar to one. It has none of the array methods slice , forEach , etc.
Thankfully, you can borrow array methods or convert arguments to an array, as explained in Array-Like Objects and Generic Methods. It is an object, so all object methods and operators are available. Strict mode drops several of the more unusual features of arguments :. In nonstrict mode, arguments stays up-to-date if you change a parameter:. There are three ways to find out whether a parameter is missing. First, you can check if it is undefined :. Second, you can interpret the parameter as a boolean.
Then undefined is considered false. However, there is a caveat: several other values are also considered false see Truthy and Falsy Values , so the check cannot distinguish between, say, 0 and a missing parameter:. Third, you can also check the length of arguments to enforce a minimum arity:.
If a parameter is optional, it means that you give it a default value if it is missing. Similarly to mandatory parameters, there are four alternatives.
0コメント