Question

119

Why can I change a constant object in javascript

rated 0 times [ 123] [ 4] / answers: 1 / hits: 144378 / 10 Years ago, fri, may 2, 2014, 12:00:00

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

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answered 10 Years ago manuel · Accepted Answer

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