How Javascript Works - Thread of execution and Memory

This is the first post in a series titled "How Javascript Works".

Thread of execution and memory:

The thread of execution describes how the javascript engine walks through the code. The context in which the code runs is called the execution context. An Execution context consist of two parts:

1. Memory for keeping track of label, value pairs.
2. Executing the code.

Javascript application begin in the global execution context. As functions are executed, a brand new execution context is created. This execution context is inside the one from which it was called. For now

Thread of execution example:

Global execution context

Line 1

Save a new constant in global memory with the label myFirstName which references the value 'Travers'

Line 2

Save a new constant in global memory with the label myLastName which references the value 'Pinkerton'

Line 4

Save a new function in global memory with the label fullName which refrences the function value (including the parameters and function body)

Line 9

(Breaking Line 9 into two parts: Left Hand Side of the equal sign, and right Hand Side of the equal sign)

Left Hand Side: Save a new constant in global memory with the labelmyFullName in global memory

Right Hand Side:

Look for constructFullName in the current execution context (in this case global) and we find constructFullName references a function

As indicated by the ( ... ) after constructFullName, call the function (which constructFullName references) which creates a execution context

Lookup myFirstName in the current execution context, where we find it references the value 'Travers'

Lookup myLastName in the current execution context, where we find it references the value 'Pinkerton'

Execute constructFullName('Travers', 'Pinkerton')

Create and enter into execution context for evaluation of constructFullName('Travers', 'Pinkerton')

constructFullName('Travers', 'Pinkerton') execution context

firstName parameter: Save in this execution context memory, the label firstName which references the argument 'Travers'

lastName parameter: Save in this execution context memory, the labellastNamewhich references the argument'Pinkerton'

Line 5:

Left Hand Side: Save a new constant with the label fullName

Right Hand Side: result of myFirstName + ' ' + myLastName

Lookup firstName in the current execution context, find references to value Travers

Lookup lastName in the current execution context, find references to value Pinkerton

Right Hand Side evaluates to 'Travers Pinkerton'

result of line 5 is a constant in the current execution context with label fullName and value 'Travers Pinkerton'

Line 6: lookup fullName in the current execution context, find fullName which references 'Travers Pinkerton'

return the value 'Travers Pinkerton' out of the current execution context

the constant myFullName in global memory references the returned value 'Travers Pinkerton'

Something to note is that when the 'Travers Pinkerton' is returned out of the result of the function call constructFullName( ... ), the execution context that was created is gone along with its memory including fullName : 'Travers Pinkerton'