Scope & Scope Chains

The Global Environment & Global Window Object

The Global Environement

• The base context that all functions and components in your program has access to.
• "global" - code accessible to every line of code
  • The global environment can be determined lexically (the physical placement of code in your program)
• When your program executes, the global execution context is created. 
  • The Global Window Object
  • "this"
  • Created by the javascript engine

Global Execution Context

Let's take a look at an example!

Execution Context

• A wrapper that assists and manages the code that is running.

• The lexical environments running are managed by execution contexts.  Execution contexts contain values beyond what is written in your code.

• Two Phases
  • Creation Phase
  • Execution Phase

Creation Phase

• Global Execution Context is created
• "this" is created
• Outer Environment
• Sets up all variables and functions in memory

Execution Context (Creation Phase)

Execution Phase

• Each line is executed line by line

What is "scope"?

"scope"

function grace(){
}



function hopper(){
    grace();
}


hopper();

Global Execution Context

(created and executed)

function grace(){
}



function hopper(){
    grace();
}


hopper();

Global Execution Context

(created and executed)

Global Scope

function hopper(){
    var myNum;    
}

function grace(){
   var myNum = 2;
   hopper();
   
}

var myNum = 1;
grace();

Global Execution Context

(created and executed)

myNum

1

function hopper(){
    var myNum;    
}

function grace(){
   var myNum = 2;
   hopper();
   
}

var myNum = 1;
grace();

Global Execution Context

(created and executed)

myNum

1

function hopper(){
    var myNum;    
}

function grace(){
   var myNum = 2;
   hopper();
   
}

var myNum = 1;
grace();

Global Execution Context

(created and executed)

myNum

1

function hopper(){
    console.log(myNum);    
}

function grace(){
   var myNum = 2;
   hopper();
   
}

var myNum = 1;
grace();

Global Execution Context

(created and executed)

myNum

1

"closure"

• "closure" is when a function remembers and accesses its lexical scope even when that function is executing outside its lexical scope