Why can I change a constant object in JavaScript?

Question

I know that ES6 is not standardized yet, but a lot of browsers currently support const keyword in JS.

In spec, it is written that:

The value of a constant cannot change through re-assignment, and a constant cannot be re-declared. Because of this, although it is possible to declare a constant without initializing it, it would be useless to do so.

and when I do something like this:

const xxx = 6;
xxx = 999;
xxx++;
const yyy = [];
yyy = 'string';
yyy = [15, 'a'];

I see that everything is ok: xxx is still 6 and yyy is [].

But if I do yyy.push(6); yyy.push(1); , my constant array has been changed. Right now it is [6, 1] and by the way I still can not change it with yyy = 1;.

Is this a bug, or am I missing something? I tried it in the latest chrome and FF29

Answer 1

The documentation states:

...constant cannot change through re-assignment
...constant cannot be re-declared

When you're adding to an array or object you're not re-assigning or re-declaring the constant, it's already declared and assigned, you're just adding to the "list" that the constant points to.

So this works fine:

const x = {};

x.foo = 'bar';

console.log(x); // {foo : 'bar'}

x.foo = 'bar2';

console.log(x); // {foo : 'bar2'}  

and this:

const y = [];

y.push('foo');

console.log(y); // ['foo']

y.unshift("foo2");

console.log(y); // ['foo2', 'foo']

y.pop();

console.log(y); // ['foo2']

but neither of these:

const x = {};
x = {foo: 'bar'}; // error - re-assigning

const y = ['foo'];
const y = ['bar']; // error - re-declaring

const foo = 'bar'; 
foo = 'bar2';       // error - can not re-assign
var foo = 'bar3';   // error - already declared
function foo() {};  // error - already declared

Answer 2

This happens because your constant is actually storing a reference to the array. When you join something into your array you are not modifying your constant value, but the array it points to. The same would happen if you assigned an object to a constant and tried to modify any property of it.

If you want to freeze an array or object so it can't be modified, you can use the Object.freeze method, which is already part of ECMAScript 5.

const x = Object.freeze(['a'])
x.push('b')
console.log(x) // ["a"]

Answer 3

Came through this article while searching on why I was able to update an Object even after defining it as const. So the point here is that it is not the Object directly but the attributes it contains which can be updated.

For example, my Object looks like:

const number = {
    id:5,
    name:'Bob'
};

The above answers correctly pointed out that it's the Object which is const and not its attribute. Hence, I will be able to update the id or name by doing:

number.name = 'John';

But, I will not be able to update the Object itself like:

number = {
    id:5,
    name:'John'
  };

TypeError: Assignment to constant variable.

Answer 4

This is consistent behavior with every programming language I can think of.

Consider C - arrays are just glorified pointers. A constant array only means that the value of the pointer will not change - but in fact the data contained at that address is free to.

In javascript, you are allowed to call methods of constant objects (of course - otherwise constant objects would not serve much purpose!) These methods might have the side effect of modifying the object. Since arrays in javascript are objects, this behavior applies to them as well.

All you are assured of is that the constant will always point to the same object. The properties of the object itself are free to change.

Answer 5

The keyword const is a little misleading.

It does not define a constant value. It defines a constant reference to a value.

Because of this you can NOT:

• Reassign a constant value
• Reassign a constant array
• Reassign a constant object

But you CAN:

• Change a constant array
• Change a constant object

Answer 6

I think this would give you more clarity on the issue : https://codeburst.io/explaining-value-vs-reference-in-javascript-647a975e12a0 .

Basically it boils down to the const always pointing to the same address in memory. You can change the value stored in that address but cannot change the address the const is pointing too.

The definition of const you mentioned will hold true when the const is pointing to an address that holds a primitive value . This is because you cannot assign a value to this const without changing its address (because this is how assigning primitive values work) and changing the address of a const is not allowed.

Where as if the const is pointing to non-primitive value , it is possible to edit the value of the address.

Answer 7

The const declaration creates a read-only reference to a value. It does not mean the value it holds is immutable, just that the variable identifier cannot be reassigned. For instance, in the case where the content is an object, this means the object's contents (e.g., its parameters) can be altered.

In addition, an also important note:

Global constants do not become properties of the window object ...

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/const

Answer 8

The value of a const can't be changed through reassignment, and it can't be redeclared.

const testData = { name:"Sandeep",lastName:"Mukherjee",company:"XYZ"}

First case

testData = {name:"hello"}
console.log(testData);//throws an Error:Assignment to constant variable
Here we are reassigning testData again

Second case

const testData = {name:"Sandeep",lastName:"Mukherjee",company:"ABC"}
console.log(testData); //throws an Error: Identifier 'testData' has already been declared
Here we are redeclaring testData again

When a variable is declared using const it means it points to some memory location the behaviour of const is we can manipulate the value stored in that memory location but not the memory location,when we reassign/redeclare the const variable it does not allow to change the memory location

We can change the value of a specific key

testData.company = "Google"
 console.log(testData);
 //{ name: 'Sandeep', lastName: 'Mukherjee', company: 'Google' }

We can add any new key value pair to it

 testData.homeTown = "NewYork"
 console.log(testData)
 //{name: 'Sandeep',lastName:'Mukherjee',company:'Google',homeTown: 'NewYork'}

Answer 9

Because in const you can change the values of an object, so the object does not actually store the assignment data but instead, it points to it. so there is a difference between primitives and objects in Javascript.

Answer 10

const variable stores the address (memory address such as 0xFF2DFC) that is constant.

The constant is NOT the content at the memory.

constant is memory address ONLY

Thank you for reading.

Answer 11

const MY_OBJECT = {'key': 'value'};

// Attempting to overwrite the object throws an error // Uncaught TypeError: Assignment to constant variable. MY_OBJECT = {'OTHER_KEY': 'value'};

// However, object keys are not protected, // so the following statement is executed without problem MY_OBJECT.key = 'otherValue'; // Use Object.freeze() to make object immutable

// The same applies to arrays const MY_ARRAY = []; // It's possible to push items into the array MY_ARRAY.push('A'); // ["A"] // However, assigning a new array to the variable throws an error // Uncaught TypeError: Assignment to constant variable. MY_ARRAY = ['B'];

Answer 12

In your constant is saved not the object, but link to the object.

You can't change this link, because it is constant. But object you can change.

Answer 13

First of all const declared variables cannot be redeclared or reassigned.

const foo = 5; //initializing to a primitive data type , Assigned to the value

const bar = [1,2,3]; //initializing to an object(array,object,function,...) , Assigned to the reference of the object

If you are trying to reassign a variable with const you will get Uncaught TypeError:Assignment to constant variable.

You can't do either of this!

const five=5; // initialized to a value
five=6;       // trying to reassign

const obj={};
obj = {foo:"foo",bar:"bar"}; //trying to reassign 

But you can modify object like this

const obj ={};
obj.foo="foo";
obj.bar="bar";
console.log(obj); //{foo:"bar,bar:"bar"}

const arr=[];
arr[0] = 1;
arr[1] = 2;
arr[3] = 4;
console.log(arr); //[1,2,undefined,4]

Note: We're not changing the reference, just changing the data that the reference points to.

Feel free to comment if any doubts...