Презентация Using objects in JavaScript. Accessing DOM in JavaScript онлайн
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Слайды и текст к этой презентации:
№1 слайд
Содержание слайда: Using objects in JavaScript. Accessing DOM in JavaScript
Vyacheslav Koldovskyy
Last update: 29/03/2016
№2 слайд
Содержание слайда: Agenda
Program flow control
Collections
Custom objects
Constructors
Context and "this"
Operator "new"
Browser Object Model (BOM) and Document Object Model (DOM)
Events
Memory and Sandbox
Closures
№3 слайд
Содержание слайда: Conditions: if-else
Syntax:
if (condition)
statement1
[else
statement2]
Example:
№4 слайд
Содержание слайда: Conditional (Ternary) Operator ?:
Syntax:
condition ? expr1 : expr2
Example:
№5 слайд
Содержание слайда: Loops: for
№6 слайд
Содержание слайда: Loops: while and do-while
№7 слайд
Содержание слайда: Loops: keywords break and continue
There are two keywords for loops control :
break – aborts loop and moves control to next statement after the loop;
continue – aborts current iteration and immediately starts next iteration.
Try not to use this keywords. A good loop have one entering point, one condition and one exit.
№8 слайд
Содержание слайда: Switch
Switch statement allows to select one of many blocks of code to be executed. If all options don’t fit, default statements will be processed
№9 слайд
Содержание слайда: Collections
Collection is a set of variables grouped under common name.
Usually elements of collections are grouped according to some logical or physical characteristic.
Collections help to avoids situations when we have to declare multiple variables with similar names::
var a1, a2, a3, a4…
There are two types of collections that are typical for JS: arrays and dictionaries (hash tables).
№10 слайд
Содержание слайда: Array: processing
Usage of arrays:
var array = [] // declaration of empty array
var array = [5, 8, 16] // declaration of predefined array
array[0] = 4; // writing value with index 0
tmp = array[2]; // reading value by index (in tmp - 16)
array.length // getting length of array
№11 слайд
Содержание слайда: Array: features
Arrays in JavaScript differ from arrays in classical languages.
Arrays in JS are instances of Object.
So Array in JS can be easily resized, can contain data of different types and have string as an index.
Length of array is contained in length property, its value is equal to index of last element increased by one.
№12 слайд
Содержание слайда: Array: useful methods
№13 слайд
Содержание слайда: Iterating an Array
№14 слайд
Содержание слайда: Dictionary
Dictionaries allow us to have set of data in form of "key-value" pairs
We can create hash and initialize it at the same time. For this we should write values separated by a comma like in array. But for all values we have to set key:
№15 слайд
Содержание слайда: Using Dictionary
Usage of dictionaries tables is very similar to arrays:
dict['good'] = 4; // writing value in element with key “good”
tmp = dict['excellent']; // reading value by key “excellent”
The difference is in usage of for-in statement:
№16 слайд
Содержание слайда: Array vs Dictionary
Use Array for collections with digital indexes.
Use Hash if you want use string keys.
Don't look for property length in Hash.
Don't look for forEach and other Array methods in Hash.
Always explicitly declare Array otherwise you get a Hash.
Don't use for with hash, use for-in instead.
At finally : use collection – be cool :)
№17 слайд
Содержание слайда: Object creation
You know that we can create a simple object in JavaScript. We use JSON for this.
№18 слайд
Содержание слайда: Object or Dictionary
But this way it looks like hash table creation. What is the difference between hash table and object, then?
№19 слайд
Содержание слайда: Object or Dictionary
Typically we use hash table if we want to represent some collection, and we use an object to describe some system or entity.
№20 слайд
Содержание слайда: Difference in use
№21 слайд
Содержание слайда: Constructors
Sometimes we need to create more than one single object. It is not a good idea to use the literal way for this. It will be better create a scenario for objects reproducing.
Constructor is a function that implements this scenario in JavaScript.
Constructor consists of declaration attributes and methods that should be added into each new object with presented structure.
№22 слайд
Содержание слайда: Constructors: example
№23 слайд
Содержание слайда: BOM and DOM
№24 слайд
Содержание слайда: Description
How JavaScript communicates with the world?
In outline this mechanism works by next scenario: user does something and this action is an event for browser. JavaScript observes pages in the browser. And if event has occurred, script will be activated.
№25 слайд
Содержание слайда: Event handling
But JavaScript doesn't observe events by default. You should specify to your code what events are interesting for it.
There are 3 basic ways to subscribe to an event:
- inline in HTML
- using of onevent attribute
using special methods
First and second ways are deprecated for present days. Let's take a look at event handling in more details.
№26 слайд
Содержание слайда: Inline handling
№27 слайд
Содержание слайда: Using of onevent attribute
№28 слайд
Содержание слайда: Proper ways
№29 слайд
Содержание слайда: Proper ways
№30 слайд
Содержание слайда: Bubbling and Capturing
№31 слайд
Содержание слайда: Bubbling and Capturing
№32 слайд
Содержание слайда: Event object
For every event in the browser instance of Event object will be created.
№33 слайд
Содержание слайда: Control of Default behavior
Sometimes a default scenario of event processing includes some additional behavior: bubbling and capturing or displaying context menu.
№34 слайд
Содержание слайда: Sample
№35 слайд
Содержание слайда: Practice Task
№36 слайд
Содержание слайда: Advanced
№37 слайд
Содержание слайда:
№38 слайд
Содержание слайда: Context
Let's imagine two identical objects.
They are created by Cat constructor:
№39 слайд
Содержание слайда: Context
If we call method run() for both cats, we’ll take correct results:
№40 слайд
Содержание слайда: Context
It works because we use the next form of access to attribute name: this.name.
this contains inside a reference to object on whose behalf was called method run.
Such a reference is called a context.
The context determined automatically after the method calling and can't be changed by code.
№41 слайд
Содержание слайда: Loss of context
Be careful! There are situations when you can lose a context. For example:
№42 слайд
Содержание слайда:
№43 слайд
Содержание слайда: Basic info
Free space in browser sandbox is allocated for each variable in JavaScript.
Sandbox is a special part of memory that will be managed by browser: JavaScript takes simplified and secure access to "memory“, browser translates JS commands and does all low-level work.
As a result memory, PC and user data has protection from downloaded JavaScript malware.
№44 слайд
Содержание слайда: Scope
The scope is a special JavaScript object which was created by browser in the sandbox and used for storing variables.
Each function in JavaScript has its own personal scope. Scope is formed when a function is called and destroyed after the function finishes.
This behavior helps to manage local variables mechanism.
Object window is a top-level scope for all default and global variables.
№45 слайд
Содержание слайда: Scope
№46 слайд
Содержание слайда: Value-types and Reference-types
Unfortunately some objects are too large for scope. For example string or function. There is simple division into value-types and reference-types for this reason.
Value-types are stored in scope completely and for reference-types only reference to their location is put in scope. They themselves are located in place called "memory heap".
String and all Objects are reference-types. Other data types are stored in scope.
№47 слайд
Содержание слайда: Memory cleaning
The basic idea of memory cleaning: when function is finished, scope should be destroyed and as a result all local variables should be destroyed.
This will work for value-types.
As for reference-types: deleting the scope destroys only reference. The object in heap itself will be destroyed only when it becomes unreachable.
№48 слайд
Содержание слайда: Unreachable links
An object is considered unreachable if it is not referenced from the client area of code.
Garbage collector is responsible for the cleanup of unreachable objects.
It's a special utility that will launch automatically if there isn’t enough space in the sandbox.
If an object has at least one reference it is still reachable and will survive after memory cleaning.
№49 слайд
Содержание слайда: Unreachable links
№50 слайд
Содержание слайда:
№51 слайд
Содержание слайда: Closure
If scope is an object and it is not deleted it is still reachable, isn't it?
Absolutely! This mechanism is called closure.
If you save at least one reference to scope, all its content will survive after function finishing.
№52 слайд
Содержание слайда: Example
№53 слайд
Содержание слайда: Contacts
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