1. A box, container, or a wrapper which store variables and in which a piece of our code is evaluated and executed.
2. global execution context
(1) Code that is not inside any function
(2) Associated with the global object
(3) In the browser, that is the window object
3. Execution context object
(1) Variable Object
(2) Scope chain
(3) "This" variable
4. The execution context in detail
(1) Creation phase
a. creation of the VO
- The argument onject is created, containing all the arguments that were passed into the function
- Code is scanned for function declarations: for each function, a property is created in the VO, pointing to the function
- Code is sacnned for variable declarations: for each variable, a property is created in the VO, and set to undefined.
b. creation of scope chain
- each new function creates a scope: the space/ environment, in which the variables it defines are accessible.
- Lexical scoping: afunction that is lexically within another function gets access to the scope of the outer function.
c. determine value of the 'this'
- Regular function call: the this keyword points at the global object, (the window object , in the browser). (function declaration and call)
- Method call: the this variable points to the object that is calling the method.
- The this keyword is not assigned a value until a function where it is defined actually called.
(2) Execution phase
The code of the function that generated the current execution context is ran line by line.