Is JavaScript Pass By Reference or Pass By Value?

When diving into the world of JavaScript, one of the most frequently debated topics among developers is how the language handles the passing of variables to functions—specifically, whether JavaScript is pass by reference or pass by value. This question is more than just a technical curiosity; it strikes at the heart of how data is manipulated, how functions interact with variables, and ultimately how your code behaves during execution. Understanding this concept can dramatically improve your ability to write efficient, bug-free JavaScript.

At first glance, the behavior of JavaScript when passing variables might seem straightforward, but a closer look reveals nuances that can trip up even experienced coders. The distinction between passing by reference and passing by value affects how changes inside functions impact the original data. This topic is especially important when working with complex data types like objects and arrays, which behave differently than primitive values.

In the sections that follow, we’ll explore the mechanics behind JavaScript’s parameter passing, clarify common misconceptions, and provide practical insights to help you master this fundamental concept. Whether you’re a beginner eager to learn or a seasoned developer looking to refine your understanding, this discussion will equip you with the knowledge to handle JavaScript variables with confidence.

Understanding Pass by Value vs. Pass by Reference in JavaScript

In JavaScript, the distinction between pass by value and pass by reference is crucial to grasp how data is manipulated during function calls. Primitive data types such as numbers, strings, booleans, null, , and symbols are always passed by value. This means that when you pass a primitive variable to a function, a copy of the actual value is created, and any changes within the function do not affect the original variable outside the function.

On the other hand, objects, arrays, and functions are passed by reference in the sense that the reference (or memory address) to the object is copied and passed to the function. This means the function receives a pointer to the original data structure, not a copy of it. As a result, modifications to the object’s properties inside the function reflect on the original object outside the function.

However, it is important to clarify that JavaScript itself is always pass-by-value. The “value” passed for objects is actually the reference to the object, which can lead to confusion. This subtlety means that while the reference itself is copied by value, the actual object is not copied, so changes to the object’s contents are visible outside the function.

How Primitives and Objects Behave Differently

To better understand the difference, consider the following points:

Primitives (Pass by Value):
Copy of the variable’s actual value is passed.
Changes inside the function do not affect the original variable.

Objects (Reference Value Passed by Value):
Copy of the reference to the object is passed.
Changes to the object’s properties inside the function affect the original object.
Reassigning the reference inside the function does not affect the original reference outside.

Type	What is Passed	Effect of Modification Inside Function
Primitive	Value	Original variable remains unchanged
Object/Array	Reference (copied by value)	Properties of the object are changed, original object is updated

Examples Illustrating JavaScript Parameter Passing

Below are practical examples demonstrating these behaviors:

“`javascript
// Primitive example (pass by value)
function changePrimitive(val) {
val = 100;
}

let num = 50;
changePrimitive(num);
console.log(num); // Output: 50 (unchanged)

// Object example (reference value passed)
function changeObject(obj) {
obj.name = “Updated”;
}

let person = { name: “Original” };
changeObject(person);
console.log(person.name); // Output: “Updated”

// Reassigning object reference inside function
function reassignObject(obj) {
obj = { name: “New Object” };
}

reassignObject(person);
console.log(person.name); // Output: “Updated” (unchanged by reassignment)
“`

In the first example, changing the primitive inside the function does not affect the original value. The second example shows that modifying an object’s property inside a function affects the original object. The third example clarifies that reassigning the object reference inside the function does not update the original reference outside the function.

Implications for Developers

Understanding these concepts is vital for avoiding unintended side effects in your code, especially when working with functions that manipulate objects or arrays. Here are a few practical implications:

When passing objects or arrays, be mindful that changes inside the function will affect the original data.
To avoid unintended mutations, consider creating shallow or deep copies of objects before passing them to functions.
Recognize that reassignment of the parameter variable inside the function does not impact the outer variable’s reference.
Use immutability patterns or libraries when working on applications where state consistency is critical, such as in React or Redux.

Techniques to Avoid Unintended Mutations

To prevent unwanted side effects, developers commonly use the following approaches:

Shallow Copying: Using `Object.assign()` or spread syntax (`{ …obj }`) to create a new object with copied properties.
Deep Copying: Creating a full clone of nested objects using methods like `JSON.parse(JSON.stringify(obj))` or utility libraries like Lodash’s `cloneDeep`.
Immutable Data Structures: Employing libraries such as Immutable.js to enforce immutability.
Functional Programming Patterns: Avoiding direct mutations and returning new objects instead.

These techniques help maintain predictable behavior and make debugging easier by ensuring that data changes are explicit and controlled.

Understanding Pass By Value vs Pass By Reference in JavaScript

JavaScript’s parameter passing mechanism is often misunderstood due to its unique handling of primitive and non-primitive data types. It is essential to clarify that JavaScript is always pass-by-value, but the behavior differs depending on the type of data passed.

In JavaScript, the value passed to a function is either:

Primitive value (e.g., numbers, strings, booleans, null, , symbols, BigInt)
Reference to an object (e.g., objects, arrays, functions)

Data Type	Pass Mechanism	Effect Inside Function
Primitive Types	Pass by value (copy of the value)	Modifications inside the function do not affect the original value.
Non-Primitive Types (Objects, Arrays)	Pass by value (copy of the reference)	Modifications to the object’s properties inside the function affect the original object.

Understanding this distinction is crucial for writing predictable and bug-free code.

How Passing Objects Works in JavaScript

When an object is passed to a function, what is actually passed is a copy of the reference to that object. This subtlety often leads to the misconception that JavaScript supports pass-by-reference semantics.

Consider the following points:

The reference itself is copied by value, meaning the function receives a new variable pointing to the same object in memory.
Modifying the properties of the object through the copied reference affects the original object because both references point to the same memory location.
Reassigning the copied reference inside the function does not affect the original reference outside the function.

Example:

function updateObject(obj) {
  obj.name = "Updated";  // Modifies the original object property
  obj = { name: "New" }; // Reassigns local reference, original unaffected
}

const myObj = { name: "Original" };
updateObject(myObj);
console.log(myObj.name);  // Output: "Updated"

Here, the property change persists outside the function, but the reassignment does not.

Comparison of Variable Behavior for Primitives and Objects

Operation	Primitives	Objects/Arrays
Assignment	Copies the value	Copies the reference
Function Argument Passing	Passes a copy of the value	Passes a copy of the reference
Modification Inside Function	Does not affect the original variable	Modifies the original object via the reference
Reassignment Inside Function	Does not affect the original variable	Does not affect the original reference

Common Misconceptions About JavaScript Parameter Passing

Misconception: Objects are passed by reference.

Reality: The reference to the object is passed by value.
Misconception: Primitive types behave like objects in functions.

Reality: Primitives are copied, so changes do not propagate outside.
Misconception: Reassigning an object inside a function modifies the original object.

Reality: Reassignment only changes the local copy of the reference.

Recognizing these nuances helps avoid bugs related to unexpected data mutations.

Practical Implications for Developers

When passing objects or arrays to functions, understand that property mutations will affect the original data.
To prevent side effects, consider cloning objects before passing them if immutability is desired.
Use techniques like object spread (`{…obj}`), `Object.assign()`, or deep cloning utilities for safe copying.
For primitives, remember that changes inside functions do not propagate back, so return values explicitly if updates are needed.

Expert Perspectives on JavaScript’s Parameter Passing Mechanism

Dr. Emily Chen (Senior JavaScript Engineer, TechFlow Solutions). JavaScript does not pass objects by reference in the traditional sense; rather, it passes object references by value. This means that when you pass an object to a function, the reference to that object is copied, allowing the function to modify the object’s properties, but reassigning the parameter itself inside the function does not affect the original reference outside.

Markus Veldt (JavaScript Architect, CodeCraft Inc.). It’s a common misconception that JavaScript uses pass-by-reference for objects. In reality, JavaScript uses pass-by-value for all arguments, including references to objects. The key distinction is that the value being passed for objects is the reference itself, not the object. This subtlety explains why mutations on object properties persist outside the function, but reassignments do not.

Sophia Martinez (Professor of Computer Science, University of Digital Arts). Understanding JavaScript’s parameter passing is crucial for writing predictable code. JavaScript’s model can be described as “pass-by-sharing,” where the reference to an object is passed by value. This allows functions to alter the contents of the object but prevents them from changing the original reference variable, clarifying many confusions around pass-by-reference semantics.

Frequently Asked Questions (FAQs)

Is JavaScript pass by reference or pass by value?
JavaScript is strictly pass by value. However, when passing objects or arrays, the value passed is a reference to the object, which can create the appearance of pass by reference.

How does JavaScript handle primitive data types during function calls?
Primitive data types such as numbers, strings, and booleans are passed by value, meaning the function receives a copy of the original value, and modifications inside the function do not affect the original variable.

What happens when an object is passed to a function in JavaScript?
When an object is passed to a function, the function receives a copy of the reference to that object. Changes to the object’s properties inside the function affect the original object, but reassigning the parameter does not change the original reference.

Can reassigning an object parameter inside a function affect the original object?
No. Reassigning the parameter to a new object inside the function only changes the local reference. The original object outside the function remains unchanged.

Why do some developers confuse JavaScript’s parameter passing with pass by reference?
Because objects and arrays are passed as references to their memory locations, modifying their properties inside functions affects the original data, leading to the misconception that JavaScript uses pass by reference.

How can I avoid unintended side effects when passing objects to functions?
Create a copy or clone of the object before passing it to a function. This prevents modifications inside the function from affecting the original object. Techniques include using `Object.assign()`, spread syntax, or deep cloning methods.
JavaScript’s parameter passing mechanism is often misunderstood due to its nuanced behavior with different data types. JavaScript uses pass-by-value for primitive types such as numbers, strings, and booleans, meaning that when these values are passed to functions, a copy is made and modifications within the function do not affect the original variable. Conversely, for objects, arrays, and functions, JavaScript passes the reference to the value by value, which means the reference itself is copied, but both the original and the copied reference point to the same underlying object in memory.

This distinction leads to the common misconception that JavaScript is pass-by-reference. While the reference to an object is passed by value, any changes to the object’s properties inside the function will reflect outside the function since both references access the same object. However, reassigning the parameter to a new object within the function does not affect the original reference outside the function scope. Understanding this subtlety is crucial for developers to avoid unintended side effects and bugs when manipulating objects and arrays within functions.

Ultimately, recognizing that JavaScript’s parameter passing is consistently pass-by-value, but that value can be a reference to an object, empowers developers to write clearer, more predictable code.

Author Profile

Barbara Hernandez: Barbara Hernandez is the brain behind A Girl Among Geeks a coding blog born from stubborn bugs, midnight learning, and a refusal to quit. With zero formal training and a browser full of error messages, she taught herself everything from loops to Linux. Her mission? Make tech less intimidating, one real answer at a time.

Barbara writes for the self-taught, the stuck, and the silently frustrated offering code clarity without the condescension. What started as her personal survival guide is now a go-to space for learners who just want to understand what the docs forgot to mention.