when do you use Object.defineProperty()

Question

I'm wondering when I should use

Object.defineProperty 

to create new properties for an object. I'm aware that I'm able to set things like

enumerable: false 

but when do you need this really? If you just set a property like

myObject.myprop = 5; 

its descriptors are all set to true, right? I'm actually more curious when you guys use that rather verbose call to .defineProperty() and for what reasons.

Answer 1

Object.defineProperty is mainly used to set properties with specific property descriptors (e.g. read-only (constants), enumerability (to not show a property in a for (.. in ..) loop, getters, setters).

"use strict"; var myObj = {}; // Create object // Set property (+descriptor) Object.defineProperty(myObj, 'myprop', {     value: 5,     writable: false }); console.log(myObj.myprop);// 5 myObj.myprop = 1;         // In strict mode: TypeError: myObj.myprop is read-only 

Example

This method extends the Object prototype with a property. Only the getter is defined, and the enumerability is set to false.

Object.defineProperty(Object.prototype, '__CLASS__', {     get: function() {         return Object.prototype.toString.call(this);     },     enumerable: false // = Default }); Object.keys({});           // [] console.log([].__CLASS__); // "[object Array]" 

Answer 2

Features like 'enumerable' are rarely used in my experience. The major use case is computed properties:

var myObj = {};  myObj.width = 20; myObj.height = 20;  Object.defineProperty(myObj, 'area', {     get: function() {         return this.width*this.height;     } }); console.log(myObj.area);