17javascript.ppt17javascript.ppt17javascript.ppt

Vitaly Shmatikov
CS 345
Introduction to JavaScript

What’s a Scripting Language?
 Language used to write programs that compute
inputs to another language processor
• One language embedded in another
– Embedded JavaScript computes HTML input to the browser
– Shell scripts compute commands executed by the shell
 Common characteristics of scripting languages
• String processing – since commands often strings
• Simple program structure, define things “on the fly”
• Flexibility preferred over efficiency, safety
– Is lack of safety a good thing? (Example: JavaScript used for
Web applications…)

Why JavaScript?
 “Active” web pages
 Web 2.0
• AJAX, huge number of Web-based applications
 Some interesting and unusual features
• First-class functions - interesting
• Objects without classes - slightly unusual
• Powerful modification capabilities - very unusual
– Add new method to object, redefine prototype, …
 Many security and correctness issues
 “The world’s most misunderstood prog.
language”

JavaScript History
 Developed by Brendan Eich at Netscape
• Scripting language for Navigator 2
 Later standardized for browser compatibility
• ECMAScript Edition 3 (aka JavaScript 1.5)
 Related to Java in name only
• “JavaScript is to Java as carpet is to car”
• Name was part of a marketing deal
 Various implementations available
• SpiderMonkey C implementation (from Mozilla)
• Rhino Java implementation (also from Mozilla)

Motivation for JavaScript
 Netscape, 1995
• > 90% browser market share
– “I hacked the JS prototype in ~1 week in May and it showed!
Mistakes were frozen early. Rest of year spent embedding in
browser” -- Brendan Eich, ICFP talk, 2006
 Design goals
• Make it easy to copy/paste snippets of code
• Tolerate “minor” errors (missing semicolons)
• Simplified onclick, onmousedown, etc., event handling
• Pick a few hard-working, powerful primitives
– First-class functions, objects everywhere, prototype-based
• Leave all else out!

Common Uses of JavaScript
 Form validation
 Page embellishments and special effects
 Navigation systems
 Basic math calculations
 Dynamic content manipulation
 Sample applications
• Dashboard widgets in Mac OS X, Google Maps,
Philips universal remotes, Writely word processor,
hundreds of others…

Example 1: Add Two Numbers
<html>
…
<p> … </p>
<script>
var num1, num2, sum
num1 = prompt("Enter first number")
num2 = prompt("Enter second number")
sum = parseInt(num1) + parseInt(num2)
alert("Sum = " + sum)
</script>
…
</html>

Example 2: Browser Events
<script type="text/JavaScript">
function whichButton(event) {
if (event.button==1) {
alert("You clicked the left mouse button!") }
else {
alert("You clicked the right mouse button!")
}}
</script>
…
<body onmousedown="whichButton(event)">
…
</body>
Mouse event causes
page-defined
function to be called
Other events: onLoad, onMouseMove, onKeyPress, onUnLoad

Example 3: Page Manipulation
 Some possibilities
• createElement(elementName)
• createTextNode(text)
• appendChild(newChild)
• removeChild(node)
 Example: add a new list item
var list = document.getElementById('t1')
var newitem = document.createElement('li')
var newtext = document.createTextNode(text)
list.appendChild(newitem)
newitem.appendChild(newtext)
This uses the browser
Document Object Model
(DOM). We will focus on
JavaScript as a language,
not its use in the
browser

Document Object Model (DOM)
 HTML page is structured data
 DOM provides representation of this hierarchy
 Examples
• Properties: document.alinkColor, document.URL,
document.forms[ ], document.links[ ],
document.anchors[ ], …
• Methods: document.write(document.referrer)
– These change the content of the page!
 Also Browser Object Model (BOM)
• Window, Document, Frames[], History, Location,
Navigator (type and version of browser)
slide 10

Browser and Document
Structure
W3C standard differs from
models supported in existing
browsers slide 11

Reading Properties with
JavaScript
Sample script
• Example 1 returns "ul"
• Example 2 returns "null"
• Example 3 returns "li"
• Example 4 returns "text"
– A text node below the "li" which holds the actual text data as its value
• Example 5 returns " Item 1 "
1. document.getElementById('t1').nodeName
2. document.getElementById('t1').nodeValue
3. document.getElementById('t1').firstChild.nodeName
4. document.getElementById('t1').firstChild.firstChild.nodeName
5. document.getElementById('t1').firstChild.firstChild.nodeValue
<ul id="t1">
<li> Item 1 </li>
</ul>
Sample HTML

Language Basics
 JavaScript is case sensitive
• onClick, ONCLICK, … are HTML, thus not case-sensitive
 Statements terminated by returns or semi-colons
• x = x+1; same as x = x+1
 “Blocks” of statements enclosed in { …}
 Variables
• Define using the var statement
• Define implicitly by its first use, which must be an
assignment
– Implicit defn has global scope, even if occurs in nested scope!

JavaScript Blocks
 Use { } for grouping; not a separate scope
js> var x=3;
js> x
3
js> {var x=4; x}
4
js> x
4
 Not blocks in the sense of other languages

JavaScript Primitive Datatypes
 Boolean: true and false
 Number: 64-bit floating point
• Similar to Java double and Double
• No integer type
• Special values NaN (not a number) and Infinity
 String: sequence of zero or more Unicode
chars
• No separate character type (just strings of length 1)
• Literal strings using ' or " characters (must match)
 Special objects: null and undefined

Objects
 An object is a collection of named properties
 Think of it as an associative array or hash table
• Set of name:value pairs
– objBob = {name: “Bob", grade: 'A', level: 3};
• Play a role similar to lists in Lisp / Scheme
 New members can be added at any time
– objBob.fullname = 'Robert';
 Can have methods
 Can refer to this

Functions
 Functions are objects with method called “( )”
• A property of an object may be a function (=method)
– function max(x,y) { if (x>y) return x; else return y;};
– max.description = “return the maximum of two arguments”;
• Local declarations may appear in function body
 Call can supply any number of arguments
• functionname.length : # of arguments in definition
• functionname.arguments.length : # arguments in call
• Basic types are passed by value, objects by reference
 “Anonymous” functions
• (function (x,y) {return x+y}) (2,3);

Examples of Functions
 Curried functions
– function CurriedAdd(x) { return function(y){ return x+y} };
– g = CurriedAdd(2);
– g(3)
 Variable number of arguments
– function sumAll() {
var total=0;
for (var i=0; i< sumAll.arguments.length; i++)
total+=sumAll.arguments[i];
return(total); }
– sumAll(3,5,3,5,3,2,6)

Anonymous Functions
 Anonymous functions very useful for callbacks
• setTimeout(function() { alert("done"); }, 10000)
• Evaluation of alert("done") delayed until function call
 Simulate blocks by function definition and call
• var u = { a:1, b:2 }
• var v = { a:3, b:4 }
• (function (x,y) {
var tempA = x.a; var tempB =x.b; // local variables
x.a=y.a; x.b=y.b;
y.a=tempA; y.b=tempB
}) (u,v) // Works because objs are passed by ref

Basic Object Features
 Use a function to construct an object
• function car(make, model, year) {
this.make = make;
this.model = model;
this.year = year; }
 Objects have prototypes, can be changed
• var c = new car(“Ford”,”Taurus”,1988);
• car.prototype.print = function () {
return this.year + “ “ + this.make + “ “ + this.model;}
• c.print();

JavaScript in Web Pages
 Embedded in HTML page as <script> element
• JavaScript written directly inside <script> element
– <script> alert("Hello World!") </script>
• Linked file as src attribute of the <script> element
<script type="text/JavaScript" src=“functions.js"></script>
 Event handler attribute
<a href="http://www.yahoo.com "
onmouseover="alert('hi');">
 Pseudo-URL referenced by a link
<a href=“JavaScript: alert(‘You clicked’);”>Click me</a>
We are looking at JavaScript as a language; ignore BOM, DOM, AJAX

Language Features in This
Class
 Stack memory management
• Parameters, local variables in activation records
 Garbage collection
 Closures
• Function together with environment (global variables)
 Exceptions
 Object features
• Dynamic lookup, encapsulation, subtyping,
inheritance
 Concurrency

Stack Memory Management
 Local variables in activation record of function
function f(x) {
var y = 3;
function g(z) { return y+z;};
return g(x);
}
var x= 1; var y =2;
f(x) + y;

Garbage Collection
 Automatic reclamation of unused memory
 Navigator 2: per-page memory management
• Reclaim memory when browser changes page
 Navigator 3: reference counting
• Each memory region has associated count
• Count modified when pointers are changed
• Reclaim memory when count reaches zero
 Navigator 4: mark-and-sweep, or equivalent
• Garbage collector marks reachable memory
• Sweep and reclaim unreachable memory

Closures
 Return a function from function call
• function f(x) {
var y = x;
return function (z){y += z; return y;}
}
• var h = f(5);
• h(3);
 Can use this idea to define objects with
“private” fields (subtle)
• See
http://www.crockford.com/JavaScript/private.html

Exceptions
 Throw an expression of any type
throw "Error2";
throw 42;
throw {toString: function() { return "I'm an object!"; } };
 Catch
try {
} catch (e if e == “FirstException") { // do something
} catch (e if e == “SecondException") { // do something else
} catch (e){ // executed if no match above
}
Reference: http://developer.mozilla.org/en/docs/
Core_JavaScript_1.5_Guide :Exception_Handling_Statements

Object features
 Dynamic lookup
• Method depends on run-time value of object
 Encapsulation
• Object contains private data, public operations
 Subtyping
• Object of one type can be used in place of another
 Inheritance
• Use implementation of one kind of object to
implement another kind of object

Concurrency
 JavaScript itself is single-threaded
• How can we tell if a language provides concurrency?
 AJAX provides a form of concurrency
• Create XMLHttpRequest object, set callback function
• Call request method, which continues asynchronously
• Reply from remote site executes callback function
– Event waits in event queue…
• Closures important for proper execution of callbacks
 Another form of concurrency
• Use SetTimeout to do cooperative multi-tasking

JavaScript eval
 Evaluate string as code (seen this before?)
• The eval function evaluates a string of JavaScript code,
in scope of the calling code
– var code = "var a = 1";
– eval(code); // a is now '1‘
– var obj = new Object();
– obj.eval(code); // obj.a is now 1
• Common use: efficiently deserialize a complicated data
structure received over network via XMLHttpRequest
 What does it cost to have eval in the language?
• Can you do this in C? What would it take to implement?

Unusual Features of JavaScript
 Eval, run-time type checking functions
 Support for pattern matching (reg.
expressions)
 Can add methods to an object
 Can delete methods of an object
• myobj.a = 5; myobj.b = 12; delete myobj.a;
 Iterate over methods of an object
• for (variable in object) { statements }
 With statement (“considered harmful” – why?)
• with (object) { statements }

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