Q349- What is call by value and call by reference?

In JavaScript, the way arguments are passed depends on data type.
Technically, JavaScript is call by value, but for objects it behaves like call by reference (more accurately: call by sharing).

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🔹 Call by Value

A copy of the value is passed to the function.
Changes inside the function do NOT affect the original variable.

Example (Primitive types)

let a = 10;

function change(x) {
  x = 20;
}

change(a);
console.log(a); // 10

🧠 Explanation

• a (10) is copied into x
• Modifying x doesn’t change a

✔ Applies to: number, string, boolean, null, undefined, symbol, bigint

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🔹 Call by Reference (Conceptual)

A reference to the same memory is passed.
Changes inside the function affect the original object.

Example (Objects & Arrays)

let obj = { name: "Teekam" };

function update(o) {
  o.name = "React";
}

update(obj);
console.log(obj.name); // React

🧠 Explanation

• Both o and obj point to the same object
• Mutation affects original data

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⚠️ Important Interview Truth

JavaScript is NOT truly call by reference.

It is call by value, where:

• For primitives → value is copied
• For objects → reference value is copied

This is called call by sharing.

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🔍 Proof (Key Trick Question)

let obj = { x: 1 };

function change(o) {
  o = { x: 2 };
}

change(obj);
console.log(obj.x); // 1

Why?

• Reference is passed by value
• Reassigning o does NOT affect obj

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🔥 Summary Table

Type	Behavior
Primitive	Call by value
Object	Reference passed by value
Object mutation	Affects original
Object reassignment	Does NOT affect original

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🎯 Short Interview Answer

JavaScript uses call by value. For primitives, the actual value is copied. For objects, a reference value is copied, so mutating the object affects the original, but reassigning the object does not.

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⭐ One-line summary

JavaScript is call by value — objects just pass a reference value, not true references.