JavaScript in Browser презентация

Agenda JS in Browser Events Memory Closure

JavaScript in Browser

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.

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.

Inline handling

Using of onevent attribute

Proper ways

Proper ways

Bubbling and Capturing

Bubbling and Capturing

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

Event object Event object is supported in IE, too, but it’s located in object window and its name is event:

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

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.

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. window object is a top-level scope for all default and global variables.

Scope

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.

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.

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.

Unreachable links

Closure FYI: 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.

Example