2_Computer science grade 10Using Scripts_presentation2_1 variant

Using Scripts

10.4.2.3 use ready-made scripts in web-development

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Client Side Scripting

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Why use client-side programming?

PHP already allows us to create dynamic web pages. Why also use client-side scripting?
client-side scripting (JavaScript) benefits:
usability: can modify a page without having to post back to the server (faster UI)
efficiency: can make small, quick changes to page without waiting for server
event-driven: can respond to user actions like clicks and key presses

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Why use client-side programming?

server-side programming (PHP) benefits:
security: has access to server's private data; client can't see source code
compatibility: not subject to browser compatibility issues
power: can write files, open connections to servers, connect to databases, ...

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What is Javascript?

a lightweight programming language ("scripting language")
used to make web pages interactive
insert dynamic text into HTML (ex: user name)
react to events (ex: page load user click)
get information about a user's computer (ex: browser type)
perform calculations on user's computer (ex: form validation)
a web standard (but not supported identically by all browsers)
NOT related to Java other than by name and some syntactic similarities

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Linking to a JavaScript file: script

script tag should be placed in HTML page's head
script code is stored in a separate .js file
JS code can be placed directly in the HTML file's body or head (like CSS)
but this is bad style (should separate content, presentation, and behavior

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HTML

Event-driven programming

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split breaks apart a string into an array using a delimiter
can also be used with regular expressions (seen later)
join merges an array into a single string, placing a delimiter between them

A JavaScript statement: alert

a JS command that pops up a dialog box with a message

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alert("IE6 detected. Suck-mode enabled."); JS

Event-driven programming

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you are used to programs start with a main method (or implicit main like in PHP)
JavaScript programs instead wait for user actions called events and respond to them
event-driven programming: writing programs driven by user events
Let's write a page with a clickable button that pops up a "Hello, World" window...

Buttons

button's text appears inside tag; can also contain images
To make a responsive button or other UI control:
choose the control (e.g. button) and event (e.g. mouse 1. click) of interest
write a JavaScript function to run when the event occurs
attach the function to the event on the control

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Click me! HTML

JavaScript functions

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function name() {
statement ;
statement ;
...
statement ;
} JS

the above could be the contents of example.js linked to our HTML page
statements placed into functions can be evaluated in response to user events

function myFunction() {
alert("Hello!");
alert("How are you?");
} JS

Event handlers

JavaScript functions can be set as event handlers
when you interact with the element, the function will execute
onclick is just one of many event HTML attributes we'll use
but popping up an alert window is disruptive and annoying
A better user experience would be to have the message appear on the page...

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... HTML

Click me! HTML

Document Object Model (DOM)

most JS code manipulates elements on an HTML page
we can examine elements' state
e.g. see whether a box is checked
we can change state
e.g. insert some new text into a div
we can change styles
e.g. make a paragraph red

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DOM element objects

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Accessing elements: document.getElementById

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var name = document.getElementById("id");
JS

Click me!
id="output">replace me
id="textbox" type="text" /> HTML

function changeText() {
var span = document.getElementById("output");
var textBox = document.getElementById("textbox");

textbox.style.color = "red";

} JS

Accessing elements: document.getElementById

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document.getElementById returns the DOM object for an element with a given id
can change the text inside most elements by setting the innerHTML property
can change the text in form controls by setting the value property

Changing element style: element.style

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Preetify

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function changeText() {
//grab or initialize text here

// font styles added by JS:
text.style.fontSize = "13pt";
text.style.fontFamily = "Comic Sans MS";
text.style.color = "red"; // or pink?
} JS

Variables

variables are declared with the var keyword (case sensitive)
types are not specified, but JS does have types ("loosely typed")
Number, Boolean, String, Array, Object, Function, Null, Undefined
can find out a variable's type by calling typeof

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var name = expression; JS

var clientName = "Connie Client";
var age = 32;
var weight = 127.4; JS

Number type

integers and real numbers are the same type (no int vs. double)
same operators: + - * / % ++ -- = += -= *= /= %=
similar precedence to Java
many operators auto-convert types: "2" * 3 is 6

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var enrollment = 99;
var medianGrade = 2.8;
var credits = 5 + 4 + (2 * 3); JS

Comments (same as Java)

identical to Java's comment syntax
recall: 4 comment syntaxes
HTML:
CSS/JS/PHP: /* comment */
Java/JS/PHP: // comment
PHP: # comment

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// single-line comment
/* multi-line comment */ JS

Math object

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var rand1to10 = Math.floor(Math.random() * 10 + 1);
var three = Math.floor(Math.PI); JS

methods: abs, ceil, cos, floor, log, max, min, pow, random, round, sin, sqrt, tan
properties: E, PI

Special values: null and undefined

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var ned = null;
var benson = 9;
// at this point in the code,
// ned is null
// benson's 9
// caroline is undefined JS

undefined : has not been declared, does not exist
null : exists, but was specifically assigned an empty or null value
Why does JavaScript have both of these?

Logical operators

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> < >= <= && || ! == != === !==
most logical operators automatically convert types:
5 < "7" is true
42 == 42.0 is true
"5.0" == 5 is true
=== and !== are strict equality tests; checks both type and value
"5.0" === 5 is false

if/else statement (same as Java)

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if (condition) {
statements;
} else if (condition) {
statements;
} else {
statements;
}
JS

identical structure to Java's if/else statement
JavaScript allows almost anything as a condition

Boolean type

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var iLike190M = true;
var ieIsGood = "IE6" > 0; // false
if ("web devevelopment is great") { /* true */ }
if (0) { /* false */ } JS

any value can be used as a Boolean
"falsey" values: 0, 0.0, NaN, "", null, and undefined
"truthy" values: anything else
converting a value into a Boolean explicitly:
var boolValue = Boolean(otherValue);
var boolValue = !!(otherValue);

for loop (same as Java)

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var sum = 0;
for (var i = 0; i < 100; i++) {
sum = sum + i;
} JS

var s1 = "hello";
var s2 = "";
for (var i = 0; i < s.length; i++) {
s2 += s1.charAt(i) + s1.charAt(i);
}
// s2 stores "hheelllloo" JS

while loops (same as Java)

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while (condition) {
statements;
} JS

break and continue keywords also behave as in Java

do {
statements;
} while (condition); JS

Popup boxes

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alert("message"); // message
confirm("message"); // returns true or false
prompt("message"); // returns user input string JS

Arrays

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var name = []; // empty array
var name = [value, value, ..., value]; // pre-filled
name[index] = value; // store element JS

var ducks = ["Huey", "Dewey", "Louie"];
var stooges = []; // stooges.length is 0
stooges[0] = "Larry"; // stooges.length is 1
stooges[1] = "Moe"; // stooges.length is 2
stooges[4] = "Curly"; // stooges.length is 5
stooges[4] = "Shemp"; // stooges.length is 5 JS

Array methods

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var a = ["Stef", "Jason"]; // Stef, Jason
a.push("Brian"); // Stef, Jason, Brian
a.unshift("Kelly"); // Kelly, Stef, Jason, Brian
a.pop(); // Kelly, Stef, Jason
a.shift(); // Stef, Jason
a.sort(); // Jason, Stef JS

array serves as many data structures: list, queue, stack, ...
methods: concat, join, pop, push, reverse, shift, slice, sort, splice, toString, unshift
push and pop add / remove from back
unshift and shift add / remove from front
shift and pop return the element that is removed

String type

methods: charAt, charCodeAt, fromCharCode, indexOf, lastIndexOf, replace, split, substring, toLowerCase, toUpperCase
charAt returns a one-letter String (there is no char type)
length property (not a method as in Java)
Strings can be specified with "" or ''
concatenation with + :
1 + 1 is 2, but "1" + 1 is "11"

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var s = "Connie Client";
var fName = s.substring(0, s.indexOf(" ")); // "Connie"
var len = s.length; // 13
var s2 = 'Melvin Merchant'; JS

More about String

accessing the letters of a String:

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var count = 10;
var s1 = "" + count; // "10"
var s2 = count + " bananas, ah ah ah!"; // "10 bananas, ah ah ah!"
var n1 = parseInt("42 is the answer"); // 42
var n2 = parseFloat("booyah"); // NaN JS

escape sequences behave as in Java: \' \" \& \n \t \\
converting between numbers and Strings:

var firstLetter = s[0]; // fails in IE
var firstLetter = s.charAt(0); // does work in IE
var lastLetter = s.charAt(s.length - 1); JS

Splitting strings: split and join

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var s = "the quick brown fox";
var a = s.split(" "); // ["the", "quick", "brown", "fox"]
a.reverse(); // ["fox", "brown", "quick", "the"]
s = a.join("!"); // "fox!brown!quick!the" JS

split breaks apart a string into an array using a delimiter
can also be used with regular expressions (seen later)
join merges an array into a single string, placing a delimiter between them