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Using objects in JavaScript. Accessing DOM in JavaScript Vyacheslav Koldovskyy Last update: 29/03/2016

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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

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Conditions: if-else Syntax: if (condition) statement1 [else statement2] Example:

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Conditional (Ternary) Operator ?: Syntax: condition ? expr1 : expr2 Example:

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Loops: for

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Loops: while and do-while

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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.

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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

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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).

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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

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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.

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Array: useful methods

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Iterating an Array

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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:

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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:

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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 :)

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Object creation You know that we can create a simple object in JavaScript. We use JSON for this.

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Object or Dictionary But this way it looks like hash table creation. What is the difference between hash table and object, then?

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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.

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Difference in use

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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.

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Constructors: example

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BOM and DOM

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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.

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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.

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Inline handling

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Using of onevent attribute

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Proper ways

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Proper ways

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Bubbling and Capturing

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Bubbling and Capturing

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Event object For every event in the browser instance of Event object will be created.

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Control of Default behavior Sometimes a default scenario of event processing includes some additional behavior: bubbling and capturing or displaying context menu.

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Sample

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Practice Task

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Advanced

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Context Let's imagine two identical objects. They are created by Cat constructor:

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Context If we call method run() for both cats, we’ll take correct results:

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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.

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Loss of context Be careful! There are situations when you can lose a context. For example:

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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.

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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.

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Scope

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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.

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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.

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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.

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Unreachable links

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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.