By reference or by value

JavaScriptJunior~1 min read

1. What "by value" means

When the variable stores the value itself, not a reference to it. Copying creates a new independent copy of the data.

Primitives (simple data types) are passed by value.

These are:

number
string
boolean
null
undefined
symbol
bigint

Example:

javascript

let a = 5;
let b = a; // the value 5 is copied

b = 10;

console.log(a); // 5  ← unchanged
console.log(b); // 10

a and b are now two independent values. Changing b does not affect a.

2. What "by reference" means

When the variable does not store the object itself, but stores a reference (pointer) to the place in memory where that object lives.

All objects and complex data structures are passed by reference:

object
array
function
Date, Map, Set, and so on

Example with an object:

javascript

const user1 = { name: 'Oleh' };
const user2 = user1; // the REFERENCE is copied, not the object itself

user2.name = 'Maria';

console.log(user1.name); // "Maria"
console.log(user2.name); // "Maria"

Both point to the same object in memory. So changing user2 also changes user1.

Visually

javascript

┌──────────────┐
│ user1        │ ─┐
└──────────────┘  │
                  ▼
             { name: "Oleh" }
                  ▲
┌──────────────┐  │
│ user2        │ ─┘
└──────────────┘

3. Example with an array

javascript

const arr1 = [1, 2, 3];
const arr2 = arr1;

arr2.push(4);

console.log(arr1); // [1, 2, 3, 4]
console.log(arr2); // [1, 2, 3, 4]

Both arrays are the same object in memory.

4. How to make a copy of an object (to avoid a reference)

If you want to create a new independent object, you need to explicitly copy the data.

Shallow copy:

javascript

const user1 = { name: 'Oleh', age: 25 };
const user2 = { ...user1 }; // spread operator

user2.name = 'Maria';

console.log(user1.name); // "Oleh"
console.log(user2.name); // "Maria"

Deep copy (for nested objects)

javascript

const user1 = { name: 'Oleh', address: { city: 'Kyiv' } };
const user2 = JSON.parse(JSON.stringify(user1)); // deep cloning

user2.address.city = 'Lviv';

console.log(user1.address.city); // "Kyiv"
console.log(user2.address.city); // "Lviv"

5. An analogy

Type	Where it is stored	On copying
Primitive (by value)	in the stack	a new copy is created
Object (by reference)	in the heap	the reference to the same spot is copied

Summary

Data type	Passing	Example
`string`, `number`, `boolean`, `null`, `undefined`, `symbol`, `bigint`	by value	`let a = 5; let b = a;`
`object`, `array`, `function`, `Date`, `Map`, `Set`	by reference	`let obj2 = obj1;`

In short

By value → a copy of the data is created.
By reference → a pointer to the same object is copied.
So changing one object affects the other, if they share a reference.

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