Javascript: object['name'] issue

Question

I had an object of the form obj[prop] and in the case where prop === 'name', using the syntax obj[prop] = 'xyz' was failing to set the property. Insteadm I had to delete obj[prop] first and then set it.

The code is far too much to put down here so this is a general question. Does anybody have an idea what might cause this problem? This was ONLY happening when prop === 'name'. Every other property I tried had no issues.

if u are using a obj, not an array type, you can make use of the function obj,hasOwnProperty(propertyname) this will return a true or false — Oli Soproni B.
– Oli Soproni B., Commented Feb 18, 2016 at 8:41
What is obj? Where is it created? is it possible that name is defined with a getter/setter? To check this, place a breakpoint at the obj[prop] = 'xyz'; line, and trace into the code; it may take you inside a setter for this property. — user663031
– user663031, Commented Feb 18, 2016 at 9:02
I couldnt find any changes to the definition. Im going to guess that using JSON parse is causing some funkiness somewhere. Just going to use delete before setting the property. — Magic Lasso
– Magic Lasso, Commented Feb 18, 2016 at 9:57

Amadan · Accepted Answer · 2016-02-18 08:49:05Z

2

I know of only one such circumstance: name being defined as not writable. It also has to be configurable, otherwise you wouldn't have been able to delete it.

var prince = {}
Object.defineProperty(prince, 'name', {
  writable: false,
  configurable: true,
  value: "Prince"
})
prince.name = 'The Artist Formerly Known as Prince';
console.log(prince.name);
// => Prince

answered Feb 18, 2016 at 8:49

Amadan

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user663031 Over a year ago

Wouldn't it also be possible that name is defined with a getter/setter?

Fab313 · Accepted Answer · 2016-02-18 08:58:27Z

1

You should check if the property "name" is writable :

Object.getOwnPropertyDescriptor(obj, 'name');
> Object {value: "xyz", writable: true, enumerable: true, configurable: true}

More information about property descriptors here: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/getOwnPropertyDescriptor#Description

answered Feb 18, 2016 at 8:58

Fab313

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4 Comments

Magic Lasso Over a year ago

Thankyou I was just looking for the dump for the property!

Magic Lasso Over a year ago

Everything is the basic setup. I cannot recreate the issue either in jsFiddle. Might just have to bite the bullet and ignore the error :(

Fab313 Over a year ago

This is strange... You can also look into the "freeze/isFrozen", "seal/isSealed" and "preventExtension/isExtensible" methods of the Object constructor. They can make an object immutable and things like that. I don't know these methods that well, so here is the documentation: developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

Amadan Over a year ago

If the object was frozen or sealed, he wouldn't be able to delete the property. preventExtensions would still let him change the property.

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