CODEWITHSUNDEEP
javascriptdefineproperty
Murad Sofiyev
Murad Sofiyev

Reputation: 791

Why JavaScript Object's property is already configurable:false, writable can always be changed from true to false without error?

When I read YDKJS book in there say:

There's a nuanced exception to be aware of: even if the property is already configurable:false, writable can always be changed from true to false without error, but not back to true if already false.

"use strict";
const obj = Object.create(null);
Object.defineProperty(obj, "name", {
   value: "Murad",
   writable: true,
   configurable: false,
   enumerable: true
});

Object.defineProperty(obj, "name", {
   value: "Tofiq",
   writable: false,
   configurable: false,
   enumerable: true
});

But this is so interesting for me why JS have exception for writable?

Upvotes: 1

Views: 2058

Answers (1)

Salim Djerbouh
Salim Djerbouh

Reputation: 11034

It's not a bug, it's by design

It is deliberately designed in the Language and implemented in v8

From MDN Docs

Modifying a property

When the property already exists, Object.defineProperty() attempts to modify the property according to the values in the descriptor and the object's current configuration. If the old descriptor had its configurable attribute set to false the property is said to be “non-configurable”. It is not possible to change any attribute of a non-configurable accessor property. For data properties, it is possible to modify the value if the property is writable, and it is possible to change writable attribute from true to false. It is not possible to switch between data and accessor property types when the property is non-configurable.

A TypeError is thrown when attempts are made to change non-configurable property attributes (except value and writable, if permitted) unless the current and new values are the same.

Writable attribute

When the writable property attribute is set to false, the property is said to be “non-writable”. It cannot be reassigned.

But the error is thrown only in strict mode to enforce immutable types

Upvotes: 2

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