Jessica's Javascript Notes

Javascript Libraries

Why use libraries?

• dom access/traversal/manipulation
  • improves efficiency at retrieval and traversal and so on
  • smooths over inconsistencies between browsers
• application conviences
  • language extensions - do things the language wasn't necessarily designed for
    • event handling - create a unified interface for attaching events, maintaining object scope, stopping events, etc.
  • language bridges - fill in missing features an older brower didn't support
  • ajax - provide a framework for handling successful calls and potential problems such as timeouts
  • templating - makes it quicker to display data on the page by not requiring as much data to be sent because you'll use a template to format it.
  • improved string handling - ways to filter, capitalize/change case of strings, etc.
  • collections - JS's array functionality can be limiting, library may give features to make collections more robust (eg: simple iteration, scanning and removing elements, selection of subset, etc)
  • JSON/XML - Libraries can make handling detection of whether a XHR request is JSON or XML automatic, preventing invalid or dangerous information being served up
• widgets
  • Pre-built components (eg: file browsers, tabbed interfaces, custom dialogs)
  • solve common design issues
  • takes pain out of dealing with complicated edge case

Common libraries:

• Dojo - focused on widgets and interface elements (to make site more usable/responsive/functional but that will degrade well) to make desktop-like web-applications
• Prototype - focuses heavily on working with DOM and application conveniences (eg: string handling, enumerations). Approaches problems in a style similar to Ruby
  • Scritp.aculo.us - animation and widget library built on top of Prototype
• jQuery - library helps with event-handling, to give desktop-like functionality to a web-application, good support for method chaining.
• YUI (Yahoo User Interface Library) - focus is on pre-built functionalities that are well designed and robust, mix of DOM tools, animation tools, and widgets (eg: calendars)

Ajax

• Stands for Asynchronous JavaScript and XML, but term is also used more generically as an umbrella term for a number of technologies.
• way to communicate with the server, returning a chunk of XML, such that you can update the page without having to do a page refresh (allowing audience to continue to interact with the page all teh while)
• XMLHttpRequest object is oftern referred to as the XHR object
• Ajax itself = straightforward, handling contingencies may not be.
• JSON = JavaScript Object notation = uses a subset of JS to safely define and transport data.
  • parsers for JSON available for many servers side languages
  • has been growing in popularity over XML for client/server data transfer
    • smaller in size (less markup required)
    • quicker to parse (native JS)

Planning for Failures

• Design should account for common failures:
  • How long to wait and what to do if a request times out
  • What to do if the data you get back isn't what's expected
  • How to handle multiple requests (esp. if the results come back out of order)
• Can storyboard/chart or flow-chart what happens on various interactions as a planning technique

Javascript Debugging Techniques

Javascript Basics

• Functions:
  • Named: function foo() { ... }
  • Anonymous: function (){ ... } - usually used on the right hand side of an equals sign (eg: var myvar = function() {...})
• Objects
  • var bar = new foo();
  • two ways to access properies:
    • dot notation - "my string".toLowerCase();
    • bracket notation - "my string"["toLowerCase"](); (this way isi considered harder to read and thus less favored, unless you are using a variable to execute a function on an object)
• Prototypes
  • when you attach new properties and methods to the prototype they'll become available to all other objets of that type
• Pass by reference/value for function calls
  • variables = by value
  • objects = by reference

DOM

Document Object: Common functions to get elements:

• getElementById() - returns single element from page
• getElementsByTagName() - returns all elements w/ a specific tag name (watch case though if using xhtml)
• childNodes - property to retrieve direct descendents of a node

There is NOT a getElementsByClassName() method in spec though. have to write your own function to do this (or use a library).

eg:

function getElementsByClassName(node, classname) {
  var a = [];
	var re = new RegExp('(^| )'+classname+'( |$)');
	var els = node.getElementsByTagName("*");
	for (var i=0,j=els.length; i<j; i++) {
	  if (re.test(els[i].classname))a.push(els[i]); } 
	return a;
}

Navigating the DOM:

Use these methods:

• childNodes
• parentNode
• nextSibling/previousSibling
• firstChild/lastChild
• Beware! Some browsers may include text nodes for whitespace between tags (or not) depending on the browser, so the number of DOM elements may not be the number of elements you expect.

getAttribute(name) and setAttribute(name, value) = how to do things like change a link's HREF attribute.

change an element's style with myElement.style.property = 'whatever value' where property is the css property you want to change for an element called myElement

Change/add a CSS class: element.className = whatever new class name (may be simpler than changing the style element)

Adding content to DOM:

• appendChild()
• insertBefore()
• replaceChild()

Code formatting best practices:

Should use camel-case for variables, methods, object names