Why You Can Change Properties of a const Object in JavaScript. JavaScript const Keyword. const Object Property Updates Explained with Example. const Does not Mean Constant JavaScript Object Mutability Explained Clearly

Lets explain the Code:

const obj = { a: 1 };
obj.a = 2;
console.log(obj);

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Step 1: Declare the Object

const obj = { a: 1 };

• const Keyword:

• In JavaScript, const means the reference to the variable cannot be changed.
• But it does not make the object immutable.
• You can still change the properties inside the object.
• What is Happening Here?
• You are declaring a constant variable named obj.
• You are assigning it an object with one property:

{

  a: 1

}

• This creates a reference in memory pointing to that object.

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Step 2: Modify the Property

obj.a = 2;

• You are not changing the reference to the object.
• You are just modifying the value of property a inside the same object.
• This is allowed even though the object was declared with const.

Important:

• If you try to reassign the whole object like this:

obj = { a: 10 }; // ❌ This will throw an error.

This is not allowed because you cannot change the reference of a const variable.

• But updating existing properties:

obj.a = 2; // ✅ This is allowed.

This is completely valid.

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Step 3: Log the Object

console.log(obj);

• This will print the current state of the object to the console.
• Output:

{ a: 2 }

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Why This Works:

• In JavaScript, const only freezes the binding (the link between the variable and the object), not the object’s contents.
• Objects in JavaScript are mutable by default.
• If you want to make the object truly immutable, you would have to use:

Object.freeze(obj);

But even that only works at the top level (it’s shallow).

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Quick Summary:

Step	Action	Result
1	Declare const obj = {a: 1}	Creates an object, reference is constant
2	Modify obj.a = 2	Changes the property value inside the same object
3	Log the object	Prints { a: 2 }

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Key Takeaways:

• const prevents reassignment, not mutation.
• Objects declared with const can still have their properties changed.
• If you want an object that cannot be changed, you must use Object.freeze.

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✔ Final Answer:

D) { a: 2 }

This question appeared in the Deloitte 2nd round of interview, testing the candidate’s knowledge of const keyword with object in JavaScript.