julian on javascript: objects julian m bucknall, cto
TRANSCRIPT
Julian on JavaScript: Objects
Julian M Bucknall, CTO
Ubiquitous objects− Everything that’s not a boolean, number,
string, null or undefined is … an object− Namely:
− Arrays are objects− Functions are objects− And objects are objects
An object is…− A hash table, or− A collection of key/value pairs
− The keys are strings− The values can be any type
− If they’re data they’re known as properties− If functions, they’re known as methods
− Passed as a reference not a value
− NOT an instance of a class
Extending objects− Objects are not fixed:
− You can add new properties/methods− You can remove a property/method
− delete(myObject.name);− You can redefine a property/method
Creating an empty object− var o = {};
− Preferred − var o = new Object();
− It’s beginner stuff, no one does it this way− var o = Object.create(Object.prototype);− Just don’t
Creating an object− Easy way: object literals
− Start with open brace, end with closing brace− Define set of key/value pairs
− Separate them with commas− Each key is a string or some identifier− Each value can be anything, including expressions− Separate key from value with colon
Object literals…− Are great since you can use them
wherever a value is needed
Object literals…− Are great for passing a set of data
(parameters) to a function as a single parameter
− Allows for default parameters− Needs an “extend” type function
Inheritance− Objects are NOT instances of some class
− There are no classes in JavaScript− Objects can inherit from each other
− Done through the “prototype”
Prototypes: readingmanager
− officeNumber− (prototype link)
employee− name− (prototype link)
Prototypes: writingmanager
− officeNumber− − (prototype link)
employee− name− (prototype link)
Object.create− Object.create() makes a new object that inherits
from (has as prototype) an existing one
Prototypal inheritance− C# has classes and classes inherit from
each other− JavaScript has objects and objects inherit
from each other through the prototype
− It does the same thing, but differently
Prototypal inheritance− Works by…
− Defining some interesting object− Then creating other objects with that as
prototype− Object.create
− Then customizing those new objects
Prototypal inheritance− The end of the prototypal chain is always Object.protoype
− So all objects have some inherited methods− toString()− hasOwnProperty()
Arrays− Are objects− Are wacky compared to C#
− Elements are key/value pairs− Key is the index as a string− length is always computed as “largest index”
plus 1− No single type for values− Don’t use “for .. in” with arrays, use for
− The prototype is Array.prototype
How do you identify an array?− typeof obj returns “object”
− Rats!− obj instanceof Array
− Pretty good, but doesn’t work if object is from another iframe
− ECMAScript 5? Array.isArray(obj)− Object.prototype.toString(obj)
returns "[object Array]"
Creating an array− var a = new Array();
− It’s beginner stuff, people− var a = [];
− Much better, no need to declare initial size− var a = [1, 2, "3", true];
− Much easier to read
Array methods− Defined on the Array prototype− Examples:
− push/pop− shift/unshift− concat/join− slice/splice− sort
Removing an element− delete myArray[index];
− Just sets the element to undefined− myArray.splice(index, 1);
− Removes the element− Reindexes the higher elements− Changes the length
