Javascript define a constant object property

Question

I wanted to have an object field that is immutable. I've tried:

const a = 1;
var o = {
   x: 'This is mutable',
   y: a // This should not be mutable.
};
o.y = 'Changed';
console.log(o);

Field y gets re-assigned.
Is there any way to make this work?

You probably want to check stackoverflow.com/questions/9772307/… — versvs
– versvs, Commented Feb 22, 2018 at 10:32
Have you looked at Object.freeze()? You'd have to define your object beforehand, and then freeze it which is slightly different from what you have. Or Object.seal()? — Andy
– Andy, Commented Feb 22, 2018 at 10:33
@Andy the thing is that freeze() "locks" all the object properties — F.Almeida
– F.Almeida, Commented Feb 22, 2018 at 10:35
Question title "Is it possible to have a const object property?" kinda contradicts the question itself. Do you want to freeze object property or set a property that is not configurable (Uncaught SyntaxError: Identifier 'x' has already been declared at :1:1) — Yury Tarabanko
– Yury Tarabanko, Commented Feb 22, 2018 at 10:36
Not sure I understand the error message, where in that code snippet has x been declared?. When do you get that message? — Keith
– Keith, Commented Feb 22, 2018 at 10:36

hashedram · Accepted Answer · 2018-02-22 10:46:47Z

3

Use Object.defineProperty() and set configurable as false.

var o = {
    x: 'This is mutable'
};
Object.defineProperty(o, "y", { configurable: false, writable: false });

console.log(o);

o.y will behave as if Object.freeze were called on it separately.

answered Feb 22, 2018 at 10:46

hashedram

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Etheryte · Accepted Answer · 2018-02-22 21:18:56Z

2

If you want to have a simple object, you can use Object.defineProperty():

const tea = {};
Object.defineProperty(tea, 'bar', {
  value: 'unchanged',
  writable: false,
  configurable: false
});

console.log(tea.bar); // 'unchanged'
tea.bar = 'new value';
console.log(tea.bar); // 'unchanged'
delete tea.bar;
console.log(tea.bar); // 'unchanged'

Alternatively, if you need a class, you can use a getter along with a no-op setter:

class Foo {
  get bar() {
    return 'unchanged';
  }
  set bar(_) {}
};
const tea = new Foo();

console.log(tea.bar); // 'unchanged'
tea.bar = 'new value';
console.log(tea.bar); // 'unchanged'
delete tea.bar;
console.log(tea.bar); // 'unchanged'

edited Feb 22, 2018 at 21:18

answered Feb 22, 2018 at 10:39

Etheryte

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6 Comments

str Over a year ago

delete foo.bar; will actually delete it, though.

Aluan Haddad Over a year ago

You should add "use strict"; otherwise the set just silently fails.

Etheryte Over a year ago

@str Thanks for pointing that out, using a class fixes that issue.

Etheryte Over a year ago

@AluanHaddad Since a strong majority of new code today is either straight ES6 or transpiled, I think manually adding "use strict"; to SO answers is obsolete.

Aluan Haddad Over a year ago

@Nit but there are so many stack snippets and people running code in their browser console. But especially for this question I think it's significant. That it throws is pretty important.

|

vishwajeet singh chauhan · Accepted Answer · 2018-02-22 10:44:04Z

2

var obj = {a:1,b:2};

Object.defineProperty(obj,'b',{
    writable:false
});

obj.b = 3;

console.log(obj.b) //output 2
obj.a = 8; 
console.log(obj.a); //output 8

answered Feb 22, 2018 at 10:44

vishwajeet singh chauhan

562 bronze badges

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