Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

Object.create method in javascript

Being a beginner in javascript, i tried to understand Object.create() method from here

https://developer-new.mozilla.org/en-US/docs/JavaScript/Reference/Global_Objects/Object/create

In the example code, line 18. A accessor property is created with writable set to true. I also read that writable is only for data descriptors.

Tried running,

var o = Object.create(Object.prototype, {
  // foo is a regular "value property"
  foo: { 
    writable:true, configurable:true, value: "hello" 
  },
  // bar is a getter-and-setter (accessor) property
  bar: {
    writable: true,
    configurable: false,
    get: function() { return 10 },
    set: function(value) { console.log("Setting `o.bar` to", value) }
  }
  });
console.log(o); 

I get invalid property error.

like image 873
John Avatar asked Jul 27 '12 04:07

John


People also ask

What is object create () in JavaScript?

The Object.create() method creates a new object, using an existing object as the prototype of the newly created object.

Can we create objects using function?

Once an object is created, you can assign property values and invoke methods just like any other object. Function objects can also be created as part of an object literal. Below we create an object named circle with a property named area which is a function object.


2 Answers

The issue is that writable and set/get are mutually exclusive. The code generates this helpful error in Chrome:

Invalid property. A property cannot both have accessors and be writable...

This makes some logical sense: if you have set/get accessors on a property, that property is never going to be written to and/or read from, because any attempts to read/write it will be intercepted by the accessor functions. If you define a property as writable and give it accessor functions, you are simultaneously saying:

  1. "The value of this property can be directly altered," and
  2. "Block all attempts to read and/or write to this property; instead, use these functions."

The error is simply stopping you from specifying a contradiction. I assume from the fact that you wrote a getter and setter, you don't really want the property to be writable. Just remove that line, and your code runs perfectly.

like image 127
apsillers Avatar answered Sep 20 '22 04:09

apsillers


Late answer, not looking for votes, but hoping this will be helpful.

There are two kinds of properties. Each property is EITHER:

  1. a data property which has these four attributes:

    • value
    • writable
    • enumerable
    • configurable
  2. OR an accessor property which has these four attributes:

    • get
    • set
    • enumerable
    • configurable

Therefore there is no property that can have both get and writable. That's just the way JavaScript is! Please see section 8.6 of the ECMAScript Standard for the gory details.

like image 39
Ray Toal Avatar answered Sep 18 '22 04:09

Ray Toal