An Introduction to Javascript

• Created in the 1990's by Mosaic
  • ECMA (European Computer Manufactures Association) specifies the standard.
  • ECMAScript - the standard.
• Used by as high as 98% of web sites as client side programming language. Reference.
• Supported by nearly all, if not all browsers.
• The language supports
  • just in time compiling
  • dynamic typing
  • event driven programming
• Strange things to c++ programmers:
  • dynamic typing
  • shallow copies
  • arrow functions
  • no i/o
  • tied to browsers but with NodeJS it can move to a runtime environment.
• This is a client side programming language
  • The browser downloads and parses/compiles the code
  • It then runs on an "engine" inside of the browser.
  • Therefore it is running on the client's computer.
  • For that reason, reading and writing files on the local machine is not directly supported.
• By default JavaScript allows some less than desired behavior
  • You can read about it here if you wish.
  • The biggest is allowing non-declared variables.
  • To eliminate this, at the top of every file put

  • "use strict"

  • This places the compiler in strict mode.
• Variables
  • In strict mode, all variables must be declared before they are used.
  • Two types
    • var identifier [ = value]
      • This declares variables in the global scope
      • This is not the preferred method
    • let identifier [ = value]
      • Declares variables within the block scope.
      • {}
      • This is preferable.
  • We are working in an event driven paradigm
    • This means we will not always have control over the calling of our functions.
    • The environment may do this for us.
    • Which means we might not have control over our parameter lists.
    • So we will use global variables.
    • Don't go nuts.
• A side note on Event Driven
  • Your typical program in our sequence is

    main {
       initializations
       while not done processing
          process one bit of data
          If you need input prompt the user for input
             carry out he appropriate action
    }

    • Note in this case the program is "driving" the action.
    • Input is performed at the programs discression.
    • This is called imperative/procedural or polling driven
  • In event driven programming this is reversed
    • An event is anything that can happen
      • A keypress, mouse press, mouse movement
      • Data arriving in a queue
      • An internally generated event
      • When the program encounters an event it calls a function called an event handler which deals with the event.
    • The program, perhaps through a library keeps a queue of these events
      • Called an event queue
    • The main program is then usually

       Set up the event handlers for the events the program is dealing with
      while not done
         if events.size() > 0
             currentEvent = events.dequeue()
             call the appropriate event handler for currentEvent
         else
             call the idle event handler

  • We will begin dealing with event handling here
  • But we will do much more later