Overview of JavaScript

1. Overview of JavaScript • Developed by Netscape, as LiveScript • Became a joint venture of Netscape and Sun in 1995, renamed JavaScript • Now standard of the European Computer Manufacturers Association ECMA-262 (also ISO 16262)

2. JavaScript • Three categories • Core, Client-side, & Server-side • Collections of JavaScript code as called scripts • Java and JavaScript only related through syntax • JavaScript is dynamically typed • JavaScript's support for objects is very different • JavaScript can be embeded in many things, but its primary use is in HTML

3. JavaScript • DocumentObject Model (DOM) makes it possible to support dynamic HTML with JavaScript • Promotes user interaction through forms • User interactions with HTML documents in JavaScript use the event-driven model of computation • User interaction with form elements can be used to trigger scripts

4. OO & JavaScript • JavaScript is not OO programming language • Does not support class-based inheritance • Cannot support polymorphism • Prototype-based inheritance • Which is much different

5. JavaScript Objects • Objects are collections of properties (like members of the class in Java) • Data properties • Method properties • Primitives for simple types • JavaScript objects are accessed through references • Objects appear as lists of property-value pairs • Properties can be added or deleted dynamically

6. JavaScript General Syntax • Embeded in HTML documents • Directly as the content of <script> tag <script language =”JavaScript” > ... </script> • Indirectly as a file specified in the src attribute of script <script language =”JavaScript” src=myscript.js”> ... </script>

7. JavaScript General Syntax • Place scripts inside HTML comments to hide them from bowsers that do not include JavaScript interpreters • JavaScriot does not need to be terminated by ; • But it is a good practice to do so

8. Primitives, Operations & Expressions • Primitive values, one of five: • Number, String, Boolean, Undefined, or Null • Number, String, and Boolean have wrapper objects: Number, String, & Boolean • Primitive values and objects of Number & String are coerced back and forth • Primitive values are treated essentially as if they were objects

9. Primitives, Operations & Expressions • Numeric literals are just as in Java • Numeric values are stored in double-precision floating point • String literals are delimited by ' or “ • Can include scape sequences (\t) • Embedded variable names are not interpolated • String literals are primitive values

10. Primitives, Operations & Expressions • Boolean values are trueand false • The only value of Null is null • The only Undefined value is undefined • JavaScript is dynamically typed • Any variable can be used for any thing • Primitive value or reference to any object • The interpreter determines type of each occurrence of a variable

11. Primitives, Operations & Expressions • Variables declared explicitly or implicitly var = sum = 0, today = “Monday”, flag = false ; • Numeric operators: ++, - -, +, -, *, /, % • All in double precision • The Math object • Floor, round, max, min, trig functions, etc. • Assignment opperator – Just like Java

12. Primitives, Operations & Expressions • The Number object with method toString • Some useful properties: MAX_VALUE, MIN_VALUE, NaN, POSITIVE_INFINITY, NEGATIVE_INFINITY, PI • Number .MAX_VALUE • Arithmetic operation creates overflow returns NaN • NaN is not == to any other number, not even itself • Test for it with isNaN(x)

13. Primitives, Operations & Expressions • String catenation operator: + • Coercions: • Catenation coerces numbers to strings • Numeric operators (not +) coerce strings to numbers • Conversions from strings to numbers that do not work return NaN

14. Primitives, Operations & Expressions • String properties & methods: length e.g., var len = str1.length; (a property) charAt(position) e.g., str.charAt(3) indexOf(string) e.g., str.indexOf('B') substring(from, to) e.g., str.substring(1, 3) toLowerCase() e.g., str.toLowerCase()

15. Primitives, Operations & Expressions • Conversion functions (not called through string objects, because these are not methods) • parseInt(string) and parseFloat(string) • String must begin with a digit or a sign and contain a legal number, otherwise NaN is returned • The typeof operator • Returns “number, “string”, or “boolean” for primitives • Returns “object” for objects and null

16. Screen Output • The Document object is the JavaScript model for the HTML document • The Window object in the model for the browser display window • Two properties: document, and window • Referring to Document and Window objects

17. Screen Output • The Document object has a method, write, • Dynamically creates content • Parameter is a string that is sent to the browser • it can be any thing that can appear in HTML including tags • Parameter often catenated from parts some of which are variables document.write(“It is OK” + “to learn about” + “<br />”) ;

18. Screen Output • The Window object has three methods • alert, confirm, and prompt • The default object is the current window • Object need not be included in calls to any of these • Alert( “ Hello World! \n”) • Parameter plain text, not HTML • Opens a dialog box which displays the parameter string and an OK button • Waits for used to press the OK button

19. Screen Output • Confirm (“What would you like?”) • Opens a dialog box, displays the parameter and two buttons, OK and cancel • Returns a Boolean • prompt(“What is your pleasure?”,””) • Opens a dialog box, displays the parameter, along with a text box and two buttons, OK and cancel • The second parameter is for default value if the user presses OK without typing any response

20. Control Expressions • Primitive Values • If it is string, it is true unless it is empty or “0” • If it is a number, it is true unless it is zero • Relational Expressions: ==, !=, <, >, >=, >= • Operands coerced if necessary • String to number / Boolean to number • Unusual: ===, !== • Same as == and !=, except that no coercions are allowed. Operands must be identical

21. Control Expressions • Compound expressions &&, ||, and ! • Primitive values, true and false, not to be confused with Boolean Object properties • Boolean object in a expression is false only when it is null or undefined • Boolean object has a method, toString, to allow them to be printed out.

22. Selection Statements • If-then-else --Just as in Java • Switch switch (expression) { case value_1: // value_1 statements case value_2: // value_2 statements … [default: // default statements] }

23. Control Statements • Loop Statements • while(control_expression) Statement or compound • For( int ; control ; increment) Statement or compound • Int can have declaration, but scope is the whole script • do Statement or compound while( control_expression)

24. Object Creation/Modification • Objects created with new • Most basic object uses the object constructor • Var myobject = new object(); • New object has no properties - a blank object • Properties can be added to object at any time var mycar = new object(); mycar.make = "volvo"; mycar.model = "245";

25. Object Creation/Modification • Objects can be nested. A property can be itself another object created with new. Var property1 = mycar["model"]; • Attempting to access non-existing properties yields undefined • Properties can be deleted with delete: delete mycar.model;

26. Object Creation/Modification • The for-in loop-statement is perfect for listing the properties of an object: for( identifier in object) statement or compound Ex: for( var prop in mycar) document.write(mycar[prop] +"<br />");

27. Arrays • Objects having some special functionality • Array elements can be primitive values or references to other objects. • Array objects can be created in two ways: • With new: var mylist = new Array(24,"butter", false);//length is 3. var mylist2 = new Array(24) /* length is 25 */ • Assigning an array literal: var mylist3 = [24, "eggs", false];

28. Arrays • Length is dynamic -length property stores it. • Length is highest subscript to which an element has been assigned, plus 1 mylist[122] = "wee-small"; // length is 123 • length property is writeable mylist.length =150; //make it larger mylist.length = 2; //make it shorter • Only assigned elements take up memory space http://www.ens.utulsa.edu/~class_diaz/cs2043/insert_names.html

29. Array Methods var list = new Array("John","Paul","George","Ringo"); • join - var listStr = list.join(": "); listStr contains "John: Paul: George: Ringo" • reverse • sort -coerces elements to strings and puts them in alph order • concat - newList = list.concat(47,26);

30. Array Methods • slice listPart = list.slice(1,3); // listPart is ["Paul","Ringo"] listPart = list.slice(2); // listPart is ["George","Ringo"] • toString -- exactly as join(", ") • push, pop, unshift, shift http://www.ens.utulsa.edu/~class_diaz/cs2043/nested_arrays.htm

31. JavaScript Functions function function_name( formal_arguments){ ... // function_body } • A return statement turns control to caller. • A function returns undefined when: • End of the function is reached, or • Specific return statement executed does not have expression • There are no return statements in the function

32. Functions • Functions are objects! • Variables that reference them can be treated as any other object reference: • Can be passed as paramenters, • Can be assigned variables, • Can be elements of an array ref_fun = fun; // where fun is the name of a function /* ref_fun is a reference to fun*/ ref_fun(); /* A call to fun */

33. Functions • All variables declared, implicitly or explicitly, outside functions have global scope. • Variables explicitly declared in a function have local scope. • Local variables have precedence over global variables with the same name. • Functions can be nested - we will not discuss it.

34. Function Parameters • JavaScript functions parameters are normally passed-by-value. • There is not type checking of parameters. • There is no checking on the number of parameters: excess actual parameters are ignored excess formal parameters are set to undefined.

35. Function Parameters • All parameters passed through a property array, arguments. • A function can determine the number of actual parameters by accessing arguments.length • Because the arguments array is accessible directly, all actual parameters in the call are available, including the actual parameters that do not correspond to any formal parameters http://www.ens.utulsa.edu/~class_diaz/cs2043/parameters.htm

36. Function Parameters • When a reference is passed, the function has access to objects in the calling program. Hence it can also pass-by-reference objects. • There is no clean way to send a primitive by reference Function by10(a){a[0]*=10;} ... Var listx = new Array(1); Listx[0] = x; By10[listx); X = list[0]; • Alternative, function returns the new value.

37. The sort method • Sort methodcoerces elements to strings and puts them in alph order. • To sort something other than strings to alpha ord, write a function that performs the comparison and send it to the sort method. • The comparison must return a negative number if the two numbers are in order, zero if they are equal, positive number if they are not. Function num_order(a, b) {return a-b;} num_list.sort(num_order);

38. function median( list){ • list.sort(function(a,b){return a-b;}); • var list_len = list.length; • if (( list_len %2 == 1 ) • return list[Math.floor(list_len / 2)]; • else • return Math.round( (list[list_len /2 -1]+ • list[list_len /2 ] ) / 2 ); • } // End of function median. http://www.ens.utulsa.edu/~class_diaz/cs2043/medians.html

39. Constructors • Methods that create and initialize properties of newly created objects • Constructors are called by the new operator which precedes them in the new expression var array = new Array(100); • Constructor reterences the object with this, a predefined reference variable used to construct and initialize the properties of the object.

40. Constructors • Example function car(new_make, new_model, new_year){ this.make = new_make; this.model = new_model; this.year = new_year; } my_car = new car("Volvo", "S90", "2002");

41. Constructors • Methods for the objects are initialized just as if it were a data property: function diplay_car(){ document.write("Car make: ", this.make, "<br />"); document.write("Car model: ", this.model, "<br />"); document.write("Car year: ", this.year, "<br />"); } • Add to the constructor: this.display = display_car; • Now call method: my_car.display();

42. Pattern MatchingRegular Expressions • Pattern matching based on the RegExp object are not covered in this class. • Pattern matching based on methods of the String object are covered. • Methacharacters have special meaning in some contexts in patterns: \ | ( ) [ ] { } ^ $ * + ? . • Normal characters are not methacharacters.

43. Pattern Matching • search(pattern) • Returns position in the string object where the pattern matched var str = "Hurricane"; var position = str.search(/can/); /* postion is now 5 */ • If there is no match it returns-1 • The period matches any character except new line. /snow./ matches "snowy", "snowd", etc.

44. Pattern Matching • Placing desired characters in brackets allows class of characters [abc] matches for 'a', 'b', or 'c' [a-h] matches any lower case letter from 'a' to 'h' [^aeiou] matches any consonant • Repeated character or character-class pattern /xy{4}z/ matches for xyyyyz

45. Pattern Matching • Predefined Character Classes \d[0-9] A digit \D[^0-9] Not a digit \w[A-Za-z_0-9] An alphanumeric character \W[^A-Za-z_0-9] Not an alphanumeric \s[ \r\t\n\f] A whitespace character \S [^ \r\t\n\f] Not a whitespace character \b matches boundary between \w and \W \B matches a non-word boundary, opposite of \b

46. Pattern Matching • Symbolic quantifiers * zero or more repetitions + one or more repetitions ? one or none repetitions /x*y+z?/ any number of xs, one or more y, may be a z /\d+\.\d*/ a string of one or more digits followed by a decimal period and may be more digits.

47. Pattern Matching • Anchors • Beginning of string (^): /^pearl/ matches patterns beginning with "pearl ..." • Ending of string ($): /gold$/ matches patterns ending in " ... gold" • Note anchor characters only have such meaning in these locations: /A^is$/ matches for /A^is$/ only

48. Pattern Matching • Modifiers attached to pattters to change how they are used: /ab/i matches for either upper or lower case 'ab' /sp/x allows for white space to appear in the pattern

49. Pattern Matching • replace (pattern, string) • Replace substrings in the string object that match pattern. Var str = "It rains mostly in the plains"; str.replace(/in/g, "nk"); /* str becomes "It ranks mostly nk the planks" */ $1, $2, $3 predifined variables set to "in" g modifier makes replacement global, every match replaced.

50. Pattern Matching • match(pattern) Returns array of results of pattern-maching operation • With the g modifier it returns an array of substrigs that matched • Whithout the g modifier, first element of returned array has the matched substring, the other elements have the values of $1, ... var str "my 3 kings beat your 2 aces"; var matches = str.match(/[ab]/g); /* matches is set up to ["b", "a", "a"]