Transcript 4.4 JavaScript

4.4 JavaScript (JS)
Deitel Ch. 7, 8, 9, 10
2
JavaScript & Java: Similarities
• JS (JavaScript) is case-sensitive
• Operators
–
–
–
–
arithmetic: unary +, unary -, +, -, *, /, %
bit-wise: ~ (unary), &, |, <<, >>, >>>
logical: ! (unary), &&, ||
strings: + (concatenation)
– compound assignment:
• ++, --,
• op= where op is one of the binary operators above, e.g., +=, -=, |=, &&=,
– relational: ==, !=, <, >, <=, >=
– conditional: op1 ? op2 : op3 (if op1 then op2 else op3)
3
JS & Java: Similarities (cont.)
• Assignment with =
• Syntax and semantic of control structures
–
–
–
–
–
–
if
if-else
shift-case
while
do-while
for
• except for-in, that's different
• Block {…}
4
JS & Java: Similarities (cont.+)
• Syntax of identifers
– e.g., index_1, theCamelNotation
• Syntax of numeric and string constants:
– e.g., -40, 3.14, 0.1E-7, 1.2e2, NaN, "a string",
0xFF00FF, 0x0a1b2c
• Special characters: /n, /t, /f , /r, /", /', //, etc.
• Special values: null, true, false
• Subprogram calls: sub(p1,p2,…)
• Array initialization
– e.g.: var cars = ["Mini", "Miata"];
• Array indices begin with 0, end with length-1
• Array access: cars[0]
5
JS & Java: Similarities (cont.++)
•
•
•
•
break statement
continue statement
return statement
Subprograms can throw exceptions
– exceptions are objects, can carry information
– difference: subprograms don't declare what they throw
• try-catch statement
• Object instantiation: new Name(…)
• All objects inherit method toString(…)
6
JS & Java: Differences
•
•
•
•
•
•
•
•
Type-less variables vs. strongly typed
Interpreted vs. hybrid
Functional features vs. purely imperative
Syntax: declarations, subprograms, classes, …
Additional statements
Different scope rules
Only few standard objects vs. rich set of supporting classes (JDK)
Very different Object-Oriented Facilities
–
–
–
–
object-prototype vs. class concept
no encapsulation
different inheritance
different syntax
7
Variables
• Variables can be declared with var
– mandatory for locals
– optional, but recommended for globals
• Global variables are assigned outside of functions
• Typing is implicit
– values have types, not variables
– a variable can hold a value of any type
– the expression decides the type
• Type conversion errors are detected at run-time!
8
Types
• Types:
– "primitive":
• undefined, null, boolean, number, string
– function, object
• Operator typeof
– returns "undefined", "boolean", "number",
"string", "function" or "object"
– null or arrays are also "object"s
9
Values
• Strings can be enclosed in '
– 'the "other" way'
• Special value: undefined
– var a = [1, , , 2]; // a[1],a[2] are undefined
– undefined can be tested
• if (value) {use(value)} else {alert("undefined value");}
• In a condition
– 0, "", null and undefined are interpreted as false
– everything else is interpreted as true
– Watch out: JS doesn't protect you against misuse
of values in a condition!
10
Arrays
• Array is an explicit object Array
– e.g.: var cards = new Array();
– e.g.: var cards = new Array(12);//0..11 undefined
– e.g.: var nos = new Array(1,2,3);//=[1,2,3]
• Predefined property length
• Assigning an element at index that didn't exist is ok
– it extends the array
• An associative array is not an Array object
– indices can be strings
– array notation used for accessing an object's properties
• see later
11
Array methods
• push(item) appends the item at index length
• i.e., increments length
• pop() removes and returns the last item
– decrements length
• shift() removes and returns the first item
– decrements length and indices of all other items
• slice(from,[to]) returns the subarray from..to-1
• concat(array) appends all items of array
• splice(from,n,[it1,it2,…])
– replaces n items starting at from by items it1,it2,…
• reverse(), sort(), toString()
12
Statements
• Statements don't need to be separated by;
– but they can – it's recommended
– ; is needed for several statements on one line - don't do that!
• for-in statements assigns consecutive indices
– e.g.: for (var i in anArray) {alert(anArray[i]);}
• with statement
– allows using properties of an object without object prefix
– e.g.:
with (document) {
write("<h1>Greetings</h1>"); //= document.write()
write("<p>Hi there!</p>");
}
13
Subprograms
• Syntax
– function name (…) {…}
• Anonymous functions
– name = function (…) {…}
– called the same way: name(…)
• Function body is stored as text
– it can be retrieved and modified!
• Functions are data
– they can be assigned to variables, stored in an array, etc.
• A function is an object, can be defined that way
– var sum = new Function("a","b","return a+b");
• A function can also define a "class" (see later)
14
Function Parameters
• It's ok to call a function with fewer parameters
– you can check for undefined values
– also, arguments property holds an array of all
parameter values
• Parameter passing
– by value
• numbers, strings, true, false, null, undefined
– by reference
• objects
• Return values are passed the same way
15
Built-in Functions: Dialogs
• alert(message)
– displays a modal dialog with message and OK button
– often used while debugging
• e.g., alert("variable=" + variable);
– used to display multiple lines
• e.g., alert("line 1\nline 2\nline3");
• confirm(message)
– displays a modal dialog with message and with buttons OK and Cancel
– returns true if the user clicked OK
– returns false if the she clicked Cancel
• prompt(message)
– displays a modal dialog with message, input field and OK button
– returns the text that the user typed into the input field
16
Built-in Functions (cont.)
• parseInt(string)
– attempts to convert the string into an integer number and return it
– returns the integral part of a real number or NaN if string isn't a number
• parseFloat(string)
– attempts to convert the string into an floating point number and return it
– returns NaN if string isn't a number
• eval(expression)
– interprets the expression as JavaScript code
17
JavaScript Pitfalls
• JavaScript is "silent"
– run-time errors stop script's execution
– syntax errors may not matter if the code is not
executed
don't do that!
• JS allows assignments within expressions
– like Java
– e.g. var x=++y – x=y--; //heck, what?
– never use that!
18
Browser Pitfalls
• IE just reports "runtime error"
• Firefox error line numbers are off
– check a line before or after
• JS in IE is not the same as JS in Firefox is not the same as
JS in Opera is not the same as …
– in other words:
var browsers = [Firefox,IE,Opera,Safari];
var text="You have to test and test because\n";
for (var i = 0; i < browsers.length; i++) {
text+="JS in "+browsers[i]+" isn't the same as in\n";
}
alert (text+"...");
19
JS & Java
• Facit:
There are a lot of similarities
But despite these similarities
JS is very different from Java
20
More to Come
• Object-Oriented JavaScript
• JavaScript with DOM
• Events with JavaScript