WebFund 2013F Lecture 4

Audio from the lecture given on September 18, 2013 is available here

Readings

• Chapters 3 (Objects) and 4 (Functions), JavaScript: The Good Parts

Topics

• Array versus dot notation for objects
• scoping
  • var declarations
• prototype and delegation
• empty objects
• object literals
• reflection
  • typeof
  • hasOwnProperty
• for and object properties
• delete
• global abatement
• functions as objects
• function literals

notes

September 18

• empty object
  • foo={ };
• delegation : waiting for another object to do your work
  • similar to inheritance in java
• delegation of objects happens in the prototype property
• every object has a default prototype
• everything delegates to object
  • similar to inherits but there are no classes and no hierarchy
• object reflection
  • look inside an object and see what's there
• for loops in javascript

	for(p in foo){
	
	}

• • similar to a for each loop in java
  • will iterate through all the properties in foo, no fixed order
  • DO NOT DO THIS
  • follows prototype and looks at the properties in the entire prototype chain
• typeof(foo.x) can tell you if x is
  • number
  • string
  • object
• foo.hasOwnProperty('x') returns true or false
• for loops that don't care about the prototype chain
  • create an array that has the properties you want and iterate through them
• vector array uses [] instead of {}
  • ie. [5, hello, etc];
  • use an array because they're ordered!
• foo.x is equivalent to foo["x"]
  • objects are like associative arrays where you index by letters/strings

FUNCTIONS

• functions are an object
• scope!
  • scoping rules : visibility of identifiers
    • variables
    • constants
    • functions
    • objects etc

2 ways for scoping

• dynamic
  • symbols you can access and the meaning of them depends on what code has been run before hand
  • x=7;
  • x=3;
  • x is what part came first
  • global variables are inheritently global scoped
• lexical/textual
  • • variables it can see depends on what is around it
    • idea of local variables
• javascript has a very important global score
• create a namespace to keep global script from getting too big
  • but doesn't exist in a standardized form
• global abatement - minimize what you write in the global scope
• local variable, say var in front of it
• no block scope in javascript, function scope is not block scope

f = function(a){
	return function(b)[
		return a+b;
	}
}; // data encapsulation 
g = t(5); g(7); returns 12