Objects

Javascript simple types:

• numbers (has object-like methods but they are immutable)
• strings (has object-like methods but they are immutable)
• booleans (has object-like methods but they are immutable)
• null
• undefined

All other values are objects including arrays and functions.

Objects are class free, can contain other objects and can inherit properties from their prototypes (which can reduce object initialisatioin time and memory consumption).

Object Literals

• An object literal is zero or more comma-separated name/value pairs surrounded by curly braces {}

var empty_object = {};

var today = {
    day: "Wednesday",
    month: "April",
    year: 2014,

    weather: { //objects can contain nested objects like this one
        morning: "sunny",
        afternoon: "cloudy"
    }
}

Retrieval

• Can be done with either dot notation today.weather.morning or with square brackets today['month']
• Or operand (||) can be used to fill in default values for nonexistent data to prevent and undefined error: var weath = today.weather.evening || "unknown"

Update

• Assigning a property value to an object overwrites any existing property values with that property name

Reference

• Objects refer to each other, they don't hold duplicate copies of data

Prototype

• Every object has a prototype object from which it inherits properties
• Object.prototype comes standard with Javascript and is almost like a 'root parent'
• The Object.create method is now available in ES5 (but the method is in the book if required for older versions)
• If an object does not have a property you ask it for, it will keep looking up the prototype chain until it finds it
  • If the property does note exist anywhere in the chain, it will return undefined
• A new property is immediately visible to all of the objects below it in the chain once created

Reflection

• Determining what properties an object has
• Using typeof includes all properties in the prototype chain including functions
• To avoid inherited properties, use hasOwnProperty(type); which returns true if that property exists only in that object itself (not the chain)

  today.hasOwnProperty('number')  //will return true
  today.hasOwnProperty('constructor')   //will return false
  

Enumeration

• Best way to enumerate all the properties you want is a for loop:

  var i;
  var properties = [
    'day', 'month', 'year'
    ];
  for (i = 0; i < properties.length; i++) {
    document.writeIn(properties[i] + ':' + today[properties[i]]);
  }
  

• This ensures you get the properties you want (i.e. not up the prototype chain) and in the order you want, as opposed to a for in loop which achieves neither of these

Delete

• Removes property from object, but also reveals property from further up the prototype chain if it exists
• Format: delete today.month

Global Abatement

• One way to mitigate the risks of global variables is to create a single global variable which then contains your whole application

var MYAPP = {}

MYAPP.today = {
    day: "Wednesday",
    month: "April",
    year: 2014,

    weather: { //objects can contain nested objects like this one
        morning: "sunny",
        afternoon: "cloudy"
    }
}
//Making sure all other variables (like today) are contained within this one global variable (MYAPP) means none of them have global scope and therefore the risk of naming conflicts, etc in your application is reduced

• Closures are also a way of mitigating the risks of global variables
• Note: Most Javascript MVCs these days (2014) will take care of wrapping your app for you