When to use const with objects in JavaScript?

Question

I recently read about ES6 const keyword and I can understand its importance when having something like this:

(function(){     const PI = 3.14;     PI = 3.15; //  Uncaught TypeError: Assignment to constant variable })(); 

So, nobody can change my PI variable.

The misunderstanding I have is that I don't understand in which situation the use of const with objects can make sense (other than preventing people to do myObj = newValue;).

(function(){     const obj = {a:1 ,b: 2, c:3};     //obj = {x:7 , y:8, z: 9}     //This is good     //TypeError: Assignment to constant variable.      obj.a=7; obj.b=8 ; obj.c=9;     console.log(obj); //outputs: {a: 7, b: 8, c: 9} })(); 

So when declaring an object: when should I say: Now I must declare my object with const?

Answer 1

it is a common misconception around the web, CONST doesn't creates immutable variables instead it creates immutable binding.

eg.

 const temp1 = 1;  temp1  = 2 //error thrown here. 

But

 temp1.temp = 3 // no error here. Valid JS code as per ES6 

so const creates a binding to that particular object. const assures that variable temp1 won't have any other object's Binding.

Now coming to Object. we can get immutable feature with Object by using Object.freeze

const temp3 = Object.freeze( {a:3,b:4}) temp3.a = 2 // it wont update the value of a, it still have 3 temp3.c = 6 // still valid but wont change the object 

Answer 2

According to ES6-Features.org, constants are used to make "variables which cannot be re-assigned new content".

The const keyword makes a variable itself immutable, not its assigned content. When the content is an object, this means the object itself can still be altered.

Therefore, it's possible to change the content of the object that is declared with const variable, but you cannot assign a new object to a const variable.

You are still allowed to add new attributes to your object.

const myVar = "someValue"; const myObj = {"name": "nameValue", "age": 14}  console.log(myVar); //someValue console.log(myObj.name); //nameValue  myObj.name = "newNameValue";  console.log(myObj.name); //newNameValue  myObj.someNewAttr = "newAttrValue"; console.log(myObj.someNewAttr); //newAttrValue  myObj = {"newNameAttr": "newNameValue"}; //Uncaught TypeError: Assignment to constant variable. console.log(myObj.newNameAttr);  myVar = "newValue"; //Uncaught TypeError: Assignment to constant variable. console.log(myVar);

You can also see on this fiddle: https://jsfiddle.net/am2cbb00/1/