CS50

by => Jordan Hayashi

Lecture 0 : Diving Into Javascript

Interpretor

Syntax

var fisrtname = "art";
var lastname = "dvp";
var arr = ['student' , 23,true , function(){
  console.log('hi')
}];

//hi 
for(var i = 0 ; i < arr.length; i++){
  console.log(arr[i]);
}

Types

• Dynamic typing
• Primitive tpyes(no methods)
  • undefined
• null
• boolean
• number
• string
• (symbol)

Typecasting ? Coercion

• Explicit vs. Implicit coercion

• var x = 42;

• var explicit = String(x); // explicit === "42"

• var implicit = x + "" ; // implicit === "42"

• == vs. ===

• == coerces the type

• === requires equivalent types

Coercion, cont.

• Which values are falsy?
• undefind
• null
• false
• +0 , -0 , NaN

• Which values are truthy?
• {}
• []
• Everthing esle

Objects, Arrays ,Functions,Objects

• ^ did i put Object twice?

• Nope,I put it 4 time

• Everything else is an object

• Prototypical Inheritance

var o = new Object();
o.firstname = "art";
o.lastname = "dvp";
o.isStudent = true;
o.age = 23;
o.greeting = function() { console.log("Hey"); };

console.log(JSON.stringify(o));

var o2 = {};
o2['firstname'] = "art";
var a = 'lastname';
o2[a] = 'DVP';

var o3  = {
 firstName : "ART",
 lastName : "DvP",
 greet : function(){
   console.log('hi');
 },
 address : {
   street : 'Main st.',
   number : '111'
 }
};

Prototpical Inheritance

• Non-primitive types have a few properties/method associated with them

• Array.prototype.push()

• String.prototype.toUpperCase()

• Each object stores a reference to its prototype

• Properties/methods defined most tightly to the instance have priority

Prototypical Inheritance

• Most primitive types have object wrappers

• String()

• Number()

• Boolean()

• Object()

• (Symbol())

• JS will automatically "box" (wrap) primitive values so you have access to methods

42.toString() //Error
var x = 42;
x.toString() // "42"
x.__proto__ // [Number : 0]
x instanceof Number // false


var xx = new Number(32);

• Why use reference
• What's the alternative?
• What's the danger?

Scope

• Variable lifetime
• Variables live from when they're declared until when their function ends
• Hoisting
• Function definitions are hoisted, but not varible initializations

greeting();
doesThisWork();

function doesThisWork(){
    console.log('pls');
}

function greeting(){
    console.log('Hi');
}

• But/How why?

The Javascript Engine

• Before executing the code, the engine reads the entire file and will throw a syntax error if one is found
• Any function definitions will be saved in memory
• Variable initializations will not be run ,but variable names will be declared

console.log(i);
greeting();

var i = 42;

function greeting(){
   console.log('Hi');
}

var i 

The Global Object

• All variables and functions are actually parameters and methods on the global object
• Browser global object is the 'window' object
• Node.js global object is the 'global' object

Execution context

• Equivalent to a 'stack frame' in C
• Wrapper of varibles and functions local to a function's execution
• Collection of execution contexts is known as the execution stack

Lexical environment

• Determines how variable names are resolved, especially with nested functions
• Child functions contain the scope of the parent function,even if the parent has returned

var x = 50;
function test(){
   var x = 42;
   function printX(){
       console.log(x);
   }
}

Lecture 1 : Advanced JavaSript