Object.md

Object

There are two types of creating object in javascript:

1. declarative (const a = {})
2. constructed (const a = new Object())

Built-in Objects:

There are sub-types of object in javascript:

String, Number, Boolean, Function, Array, Date, RegExp, Error

Property Descriptors:

Writable:

var myObject = {};

Object.defineProperty( myObject, "a", {
	value: 2,
	writable: false, // not writable!
	configurable: true,
	enumerable: true
} );

myObject.a = 3;

myObject.a; // 2 or TypeError in strict mode

Configurable:

Object.defineProperty( myObject, "a", {
	value: 2,
	writable: true,
	configurable: false,
	enumerable: true
} );

myObject.a;				// 2
delete myObject.a;
myObject.a;				// 2

Enumerable:

Set enumerations ability, such as the for..in

Immutability:

By combining writable:false and configurable:false, you can create a constant (cannot be changed, redefined or deleted) object property

Prevent Extensions:

var myObject = {
	a: 2
};

Object.preventExtensions( myObject );

myObject.b = 3;
myObject.b; // undefined

Seal:

Object.seal(..) creates a "sealed" object, which means it takes an existing object and calls Object.preventExtensions(..) on it, but also marks all its existing properties as configurable:false.

• Add more properties
• Reconfigure properties
• Delete properties
• Modify properties values

Freeze:

Object.freeze(..) creates a frozen object, which means it takes an existing object and calls Object.seal(..) on it, but it also marks all "data accessor" properties as writable:false.

• Add more properties
• Reconfigure properties
• Delete properties
• Modify properties values