JavaScript Basics

2009 Mats Bryntse

Contents What is JavaScript JavaScript Basics Functions Objects Bad parts Prototype Scope Ajax JSON Debugging Tools Performance Events Error handling Future of JavaScript

What is JavaScript

ECMAScript Current version in browsers is ECMA-262 edition 3, 3.1 to be finalized in December

2009 http://www.ecma-international.org Not tied to web browsers

DOM – Document object model API for working with XML/HTML, 3 levels (level 1 became W3C recommendation in October 1998) http://www.w3.org/DOM/

BOM (Browser object model) navigator, location, screen, XMLHttpRequest, ActiveXObject... No backing standard

ECMAScript

DOM BOM

JavaScript

- The worlds most popular programming language..?

JavaScript overviewJavaScript is a class-free, object-oriented languageDynamic language, you can change any object at any

timePrototypal inheritance (inherit from objects)Lamda functions (or ’anonymous’ functions)5 primitive types:

number string boolean null undefined

Loosely typed language var a = 2;a === "2" // falsea = "2"; // a is now a stringa === "2" // true

FunctionsFunctions are first class objects

var Cat = function () { // This is called a constructor function this.purr = function() {};}

Create instance: use the new keywordvar myCat = new Cat(); typeof(myCat) // ”object”, not very intuitivemyCat instanceof Cat // true, instanceof gives the expected answer

// Watch out when forgetting the new operatorvar cat = Cat();window.purr // window object is now clobbered

Function arguments available through arguments

function myFunc() {return arguments.length;

}myFunc(”a”, ”b”, ”c”); // returns 3

Objects and arrays Everything that is not a primitive derives from Object

window.toString instanceof Object // = true Objects are associative arrays / hashtables

var a = { text : 'test'};a["text"] == a.text // true

Testing for object property”text” in a // true

Enumerating object propertiesfor (var o in window) { console.log(o + ':' + window[o]);}

Array basics push : adds an element length concat : join 2 arrays join(string) : string represenation of the array split by the argument slice(start, end) : returns elements between args sort ([function]) : sorts by alphabet or by function supplied as arg pop : extracts last element

Some bad partsGlobal variables

window object is shared by everyoneno warning or crash if a variable is overwritten Easy to end-up with ”function soup”, an unmaintainable

mess of global objects & functions (MS Virtual Earth)eval & with

var o = {};with (o) { prop = 2; // prop isn’t defined on object o and ends up on the global object}alert(prop); // 2

== & !=typeofsemi-colon insertion0.1 + 0.2 == 0.3 // false (IEEE 754 floating point)

PrototypeEvery function has a prototype, a shared object

var sword = function() { this.swing = function(){ // Every sword instance will have its own version of

swing console.log(”Swing”); };};

var sword = function() {};

sword.prototype.swing = function(){ // Every sword created will share this function

console.log(”Swing”);};

Use hasOwnProperty to distinguish prototype methods from own properties

Execution ScopeScope is the execution context, the this property

var obj = { number : 42, showNumber: function () { alert(this.number); }}; obj.showNumber(); // 42

document.body.onclick = obj.showNumber; // clicking the body shows ”undefined”

call and apply can bind a new scope to a functionvar anotherObj = { number : ”blablabla” };obj.showNumber.call(anotherObj); // ”blablabla”

call (scope, arg1, arg2, ...) apply(scope, [arg1, arg2, ...])

Variable scope: function scope, not block scopefor(var i = 0; i<5;i++) {}alert(i); // 5

Asynchronous JavaScript and XMLTerm coined by Jesse James Garret in 2005XHR added in IE5 through an ActiveX objectAll browsers (IE7+) supports the

XMLHttpRequest objectCross domain restrictions applyIE8 implements XDomainRequests, (does

not send cookies)

JSONJavaScript Object NotationInvented by Douglas Crockford of YahooThe preferred data transfer format of the webMore lightweight than XML{ ”property” : ”value”}

Possible value types: String Number Object Array true false null

eval the JSON to get a native JS object, or use a JSON parser. ECMAScript 3.1 will have native support for JSON.parse and JSON.stringify (already in FF3.1)

DebuggingFireBug for Firefox(Lite for IE) (1.4 adds JSON

viewer)Fiddler (if using http://localhost, use

http://localhost.) JsonViewer plugin SyntaxViewer plugin

IE: Internet Options -> Advanced -> Disable script debugging

debugger; attaches a client-side debuggerIE8 has a developer toolbar builtin, for previous

versions there is IE Developer Toolbar

Tools Validators

JsLint JavaScriptLint

Minifiers JsMin Dojo ShrinkSafe YUI Compressor

Unit testing JsUnit YUI Test Dojo Object Harness

Documentation generators JsDoc YUI Doc

Secure execution environments ADSafe (Crockford) Caja

Performance YUI Exceptional Performance Team Use Yslow plugin to FB If using namespaced objects repeatedly, assign to a local variable first

BADmyNS.subNS.obj.prop = 2;myNS.subNS.obj.name = ”Test”;myNS.subNS.obj.index = 2345;

BETTERvar m = myNS.subNS.obj;m.prop = 2;m.name ....

Even if the JavaScript engines ran at infinite speed, web pages would not run noticeably faster. The DOM operations are typically the culprit when it comes to poor performance.

Read Steve Souders blog on High performance websites

EventsEvents handling in the DOM is tricky, browser

implementations vary. Using a JS library that normalizes the event

object is very helpfulRegistering events the old fashioned way (DOM

level 0) <a href="http://www.facebook.com” onclick="return

fbs_click(this)">Facebook</a> element.onclick = function() {} Only one listener can be registered, last listener assigned wins

”Correct” way of doing this W3C : element.addEventListener(’click’, fn, [executeInCapturePhase]) IE : element.attachEvent('onclick', fn) // flawed (this points to window in

fn, useless)

Error handling Exception handling: try/catch/finally Throw an error

throw new Error(’Oops’); Use window.onerror (currently only supported by IE & FF)

window.onerror = function(message, url, line) { logError(message); return true; // Indicate the error is handled};

function logError(msg, severity) {

var img = new Image(); severity = encodeURIComponent(severity || 0); msg = encodeURIComponent(msg); img.src = "CaptureClientError.aspx?severity=" + severity + "&msg=" + msg;}

Future of JavaScriptECMAScript 3.1 to be finalized in December. Some parts

already implemented in FF3.xSupport for getters/settersObject hardening (.seal and .freeze) JSON support

Questions?

mats.bryntse@avensia.se