{configurable:false} or Object.seal() is not working correctly

Question

I am trying to learn javascript, and have seen that we can play with object's property's attributes. (i mean value, writable, enumerable, configurable).

And from what i learned, i have thought changing {configurable: false} would restrict any more configuration changes like {writable: false, enumerable: false}

I have written below to try it out, but the results i have got were nothing like the results i have expected.

Am i wrong about what {configurable:false} means or, is there something wrong with the code? TIA.

"use strict";

window.onload = function(){

  var o = {x:1};

  //Make "x" non-configurable
  Object.defineProperty(o, "x", {configurable: false});
  //Seal "o";
  Object.seal(o);

  console.log(Object.getOwnPropertyDescriptor(o, "x"));
  //outputs => Object { value: 1, writable: true, enumerable: true, configurable: false }
  console.log(Object.isSealed(o));
  //outputs => true

  Object.defineProperty(o, "x", {writable: false}); //this doesn't cause any errors.
  console.log(Object.getOwnPropertyDescriptor(o, "x"));
  //outputs => Object { value: 1, writable: false, enumerable: true, configurable: false }
}

Mouser · Accepted Answer · 2017-10-22T14:03:37.000

1

MDN - seal

Configurable attribute
The configurable attribute controls at the same time whether the property can be deleted from the object and whether its attributes (other than writable to false) can be changed

The Object.seal()
method seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Values of present properties can still be changed as long as they are writable.

You need freeze

MDN - freeze

Comparison to Object.seal() Objects sealed with Object.seal() can have their existing properties changed. Existing properties in objects frozen with Object.freeze() are made immutable.

"use strict";

window.onload = function(){

  var o = {x:1};

  //Make "x" non-configurable
  Object.defineProperty(o, "x", {configurable: false});
  //freeze "o";
  Object.freeze(o);

  console.log(Object.getOwnPropertyDescriptor(o, "x"));
  //outputs => Object { value: 1, writable: true, enumerable: true, configurable: false }
  console.log(Object.isSealed(o));
  //outputs => true

  Object.defineProperty(o, "x", {writable: true}); //Now this doesn't cause.
  console.log(Object.getOwnPropertyDescriptor(o, "x"));
  //outputs => Object { value: 1, writable: false, enumerable: true, configurable: false }
}

edited Oct 22 '17 at 14:03

answered Oct 22 '17 at 13:50

Mouser

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thanks for introducing a new function, but i don't want all the properties to become read-only. Besides, setting writable to false was for an example purpose only. Why am i able to configure a property after sealing it and marking configrauble false – QnARails Oct 22 '17 at 13:55
Because `seal` allows you to edit existing properties. – Mouser Oct 22 '17 at 13:57
Your answer says otherwise The Object.seal() method seals an object, preventing new properties from being added to it and marking all existing properties as non-configurable. Am i not seeing how configurable:false works? – QnARails Oct 22 '17 at 13:58
But still writable - `writable` is existing and can be changed, we cannot add new properties to it. Freezing it does block all mutation. – Mouser Oct 22 '17 at 14:00
then, may i ask, what good is **configurable:false** for – QnARails Oct 22 '17 at 14:02
See added explanation, it describes `configurable` – Mouser Oct 22 '17 at 14:04
The first output should be `{value: 1, writable: false, enumerable: true, configurable: false}` – grantonzhuang Oct 22 '17 at 14:09
@grantonzhuang not when you `freeze` the object. – Mouser Oct 22 '17 at 14:19

{configurable:false} or Object.seal() is not working correctly

1 Answers1