IES: JS-deck 1

Question 1

JS background & history

Answer 1

• JavaScript
• Object-based scripting language
• the language interpreter is embedded inside web browser software such as:
  1. Google Chrome
  2. Microsoft Edge
  3. Firefox
  4. Opera
  5. Safari
• JavaScript cannot read or write files for security (exception: cookies (store minimal data))
• JS also named:
  1. “LiveScript” (initial name)
  2. “JavaScript” (possibly due to the popularity of Sun Microsystem’s Java(little resemblance)) (name widely used)
  3. “ECMAScript” (June, 1997: Ecma international standards organization standardization) (name not widely used)
  4. Not “JScript” (different language altogether)

Question 2

Brendan Eich

Answer 2

• created JavaScript at Netscape
• First introduced JS in December of 1995
• Co-founded Mozilla project
• Helped launch Firefox

Question 3

JSON

Answer 3

• JavaScript Object Notation
  *

Question 4

IES JavaScript examples-3 key ingredients

Answer 4

1. Language basics-illustrates language mechanics:
2. Syntax
3. Keywords
4. Operators
5. Structure
6. Built-in objects
7. Web page functionality
   * Illustrates the use of the browser’s DOM to provide user interaction
8. Web applications
   * Illustrates responsive web-based apps
   * Illustrates JSON techniques

Question 5

JS in HTML doc

Answer 5

• included directly: must be between <script> / </script>
• Doc can include multiple scripts
• Scripts may be in head or body of document
• (recommended) place scripts at the body end
• Place immediately before the </body>: so browser renders webpage before script interpretation

Question 6

JS don’ts

Answer 6

• type="text/JavaScript"in a <script> is NO LONGER NECESSARY as JS is HTML default scripting language
• DO NOT Include <script> and </script> in an external JS file

Question 7

JS external files

Answer 7

• Written in plain text files given a .js extension
• Allows several different pages to call upon same script
• To include in HTML doc, file name is assigned to src of the <script>
• Script source can be placed in doc head or body
• browser will treat script as if written directly where script source is placed
• can make code maintenance easier

Question 8

locating .js file to assign as script source

Answer 8

• Requires script file be in same folder (directory) as HTML doc
  Ex. <script src="external_script.js"> </script>
• script in adjacent folder: Assign relative path address of file
  Ex. <script src="js/external_script.js"> </script>
• Script located elsewhere?: assign absolute path file address
  Ex. <script src="https://www.example.com/js/external_script.js"> </script>

Question 9

disable JS

Answer 9

• some content will only appear if JS is disabled in the web browser
• JS is disabled by including a <noscript> in HTML doc body
  Ex. <noscript>JavaScript is Not Enabled!</noscript>

Question 10

document.getElementbyId( ‘message’ ).innerText = ‘Hello World!’

Answer 10

• JS displays output dynamically writing content into HTML
• element identified by id attribute
• innerText specifies text

Question 11

window.alert( ‘Hello World!’ )

Answer 11

• JS displays content in a pop-up dialog box
• Calls the alert() method
• Of the window object
• Displays content specified within the parentheses in a dialog box
  Ex. (This page says
  Hello World!
  OK button (bottom, right-hand corner))

Question 12

” . “

Answer 12

• operator
• Describes properties or methods of an object
• “dot notation”

Question 13

console

Answer 13

• provide helpful messages when code error occurs
• great for debugging code
• All leading browsers have JS console within developer tools
• Developer Tools feature-typically accessed: F12
• Displays output as well as HTML Doc name and line number of source JS code

Question 14

console.log( ‘Hello World!’ )

Answer 14

• displays output in browser’s JS console
• (when developing in JS and learning the language, displaying here is initially better)
• Calls the log() method
• Of the console object
• Displays content within () in console window

Question 15

document.write()

Answer 15

• Use is generally considered bad practice
• method that replaces the entire header and body of the web page

Question 16

JS statements

Answer 16

• a series of instructions that compose JS code
• Generally executed in top-to-bottom order as browser’s JS engine proceeds
• JS statements often grouped within braces in function blocks
• Function blocks can be repeatedly called to execute
• may contain the following:
  1. Keywords words of special significance in JS language
  2. OperatorsSpecial characters -perform an operation on one or more operands
  3. Values text strings come on numbers, boolean (true/false), undefined, and null
  4. Expressions code units-produce single value
  ( JS expression produces a value; JS statement performs an action)
• Earlier, JS-terminated by semicolon (some coders still prefer this): now, optional except with semicolon-separated list of multiple statements on a single line

Question 17

JS whitespace (tabs & spaces):

Answer 17

• JS interpreter ignores white space
• use spacing to make code more readable
• Indent function block statements- improves readability
• (use space bar to indent (2 spaces) statements) because (tab spacing may be treated differently in text editors)

Question 18

JS syntax

Answer 18

• the rules that govern JS
• Recognizes two types of values
  1. fixed
  2. variable
• literals

Question 19

JS fixed values

Answer 19

• a constant: a keyword that defines a variable or pointer as unchangeable
• Static and cannot change
• These are often called literals
• Include:
  1. null
  2. undefined
  3. number literals
  4. string literals
  the booleans:
  5. true
  (or)
  6. false

Question 20

JS variable values

Answer 20

• called variables
• Used to store data within a script
• Can be created using the JS “let” keyword
  Ex. “let total” creates a variable named “total”
• variable can be set value to store using JS assignment operator “=”
  Ex. let total= 300